Why must gravity exist?
To begin with, we accept (gratefully) that the laws of physics are the same everywhere in the universe. This didn't have to be the case, but it appears that it is the case. A person throws a ball into the air and it falls back to the Earth. An observer here in Austin would describe the motion of the ball in the same way, using the same equations, as an observer in Boston, Berlin or Bangkok. I push on a rock, and it pushes back on me (with an equal but oppositely directed force), both on Earth and on the Moon. Even on Pluto. A proton is attracted to an electron with a force determined according to a certain equation, valid here in the Milky Way galaxy, but also in other galaxies. An experiment repeated in many different locations may yield different answers, but each time the answer will be calculated by using the same equation. Gravity is weaker at the top of Mount Everest, far from the Earth's center, but a ball dropped from that height falls according to the same equation of motion as a ball dropped from my apartment balcony.
Taking things a step further, the laws of physics are the same whether you are standing still or moving at a constant velocity. (Is standing still even possible? I may be at rest on the Earth, but the Earth is hurtling around the sun, so I'm moving relative to the sun. If anything is moving, everything is moving. It's all relative!) Flying in an airplane, if you were to toss a coin up and forward, it would trace out a parabola on its way to the ground, just as it would do if you were standing firmly on the Earth when you tossed it. If the aisles were wide enough and you didn't fear arrest, you could play catch with another passenger, tossing a baseball back and forth. It would be no different than if you were doing so in Central Park, New York. The ball wouldn't behave differently. Unless the plane hit an air pocket, that is. Or slowed down suddenly. Or turned. Then the ball would appear to move in some arbitrary way. But we're assuming a constant velocity. Moving at a constant speed in a straight line. If the ride was especially smooth, and the all of the windows of the airplane were closed, you wouldn't even be able to tell if you were moving. Just like we can't tell that we (and the Earth) are moving through space right now, orbiting the sun. If everything around you is moving at the same speed you are, then as far as you're concerned, nothing is moving. So in summary, regardless of how fast I'm moving, as long as I'm moving at a constant velocity, I can describe some phenomena using the same laws of physics as someone else that is moving at a different constant velocity. I, standing in the middle of a basketball court, will describe the path of a falling ball using the same equation as a boy sailing across the floor on his skateboard, traveling a constant 5 mph. The universe was just built that way.
Okay, so we're getting closer to answering the original question: why does gravity exist? Hey, if it only took a few sentences to explain it, you'd probably already know it!
Our goal: a world in which the laws of physics are the same to all observers. Are we there, yet? No, not quite. Now we come to acceleration. Say gravity didn't exist. There would be nothing pressing you against your seat. You might float right off the chair as you pressed on your keyboard and it pushed back on you. But what if a rocket was affixed to the bottom of the chair? Upon ignition, it would propel your chair, and your chair would propel you, up towards the ceiling. If you closed your eyes and plugged your ears, all you'd notice was the sensation of the chair pressing against your butt. It would feel no different than if gravity was pulling you into the seat. Einstein hit upon this idea. Gravity and acceleration are the same thing! As far as scientific experiments designed to test this idea are concerned, if you were in a steadily accelerating windowless elevator, accelerating at the same rate forever, you wouldn't be able to tell if you were accelerating or standing still on the surface of a planet (with gravity pulling you to the ground). Were it not for gravity, you'd be able to tell the difference. Without gravity, if you were standing, with feet pressed against a surface, you would know that the surface was accelerating upwards against your feet. Acceleration would be absolute, not relative. (But we want it to be relative. It shouldn't matter whether or not you are accelerating when you choose a set of equations to model some phenomenon. Just like it shouldn't matter where you are or what your relative velocity is compared to someone or something else.) Without gravity, if you held a ball in front of you and released it, you would witness one of two outcomes. The ball might hover there in front of you or it might "fall" towards the ground. Since there's no gravity, the only way to explain its "fall" would be to assert that the ground (and you) were accelerating upwards while the ball was at rest. Once it came in contact with the ground, it would start accelerating upwards with you, resting there at your feet. You see, the ball would behave differently based on whether or not you were accelerating. A person accelerating would describe the motion of the ball using different equations than would a person not accelerating. The laws of physics would not be the same for the two observers. To ensure that the laws of physics are indeed the same for all observers, gravity must exist! With gravity, we can use the same set of equations to describe that ball's path, regardless of whether we are standing on the surface of a planet or steadily accelerating through space in a windowless elevator.
So that's why gravity must exist in a universe in which the laws of physics are the same to all observers, no matter where those observers are, whether they are standing still, moving at a steady velocity, or accelerating.
Wednesday, February 3, 2010
Wednesday, January 20, 2010
Glass: Liquid or Solid?
Is glass a liquid or a solid? Actually, the answer is not straightforward.
Solids, made of atoms or molecules that lack the thermal energy to bump past one another and so get locked into place by electric attraction, are generally classified as crystalline or amorphous. In a crystalline solid, the atoms or molecules lie in an orderly array (i.e. they have an orderly internal structure). Not so in amorphous solids, in which the atoms or molecules lie in a random jumble.
Glass is an amorphous solid. However, some chemists prefer to label glass as a supercooled liquid. Normally, the cooling of a substance from a very hot liquid state results in the crystallization of the substance. Its atoms or molecules line up in an orderly manner as they settle into place. Glass is produced by cooling a molten liquid fast enough that crystallization doesn't occur. As the glass cools, the time needed for it to exhibit liquid behavior, such as flowing, increases and reaches extremes. It becomes like a very thick syrup, so viscous (or resistant to flow) that liquid behavior becomes noticeable only on a geologic timescale (as in billions of years).
Another defining characteristic of glasses, which differentiates them from normal solids, is that they lack a latent heat of fusion. The latent heat of fusion is the amount of thermal energy which must be absorbed or lost for a substance to change states (from solid to liquid or vice versa). In other words, if you were to put a container of liquid in a room set to a temperature well below the liquid's freezing point, the liquid would begin giving off heat and decrease in temperature. When it reached its freezing point, however, it would remain at a constant temperature (the freezing point) as it continued to give off heat. This would be the actual phase transition, and the heat given off during this transition is termed the "latent heat of fusion". After the liquid solidified, it would start dropping in temperature again as it continued releasing heat. It would stop releasing heat when its temperature matched the temperature of the room. Water, for example, has a specific amount of heat it must release, per gram, as it transitions from liquid water to ice. While it's releasing this heat of fusion, its temperature remains steady at 0 degrees C. Glass, on the other hand, never enters a stage during its production in which its temperature stops decreasing as it continues to give off heat. There is no defined phase transition between molten glass and "solid" glass.
As a side note, some people claim to have seen evidence of flow in very old glass (perhaps very old windows in a historic building). These claims are not valid. Because of the amount of time needed for glass to start exhibiting liquid behavior, no human will ever witness such a thing.
Solids, made of atoms or molecules that lack the thermal energy to bump past one another and so get locked into place by electric attraction, are generally classified as crystalline or amorphous. In a crystalline solid, the atoms or molecules lie in an orderly array (i.e. they have an orderly internal structure). Not so in amorphous solids, in which the atoms or molecules lie in a random jumble.
Glass is an amorphous solid. However, some chemists prefer to label glass as a supercooled liquid. Normally, the cooling of a substance from a very hot liquid state results in the crystallization of the substance. Its atoms or molecules line up in an orderly manner as they settle into place. Glass is produced by cooling a molten liquid fast enough that crystallization doesn't occur. As the glass cools, the time needed for it to exhibit liquid behavior, such as flowing, increases and reaches extremes. It becomes like a very thick syrup, so viscous (or resistant to flow) that liquid behavior becomes noticeable only on a geologic timescale (as in billions of years).
Another defining characteristic of glasses, which differentiates them from normal solids, is that they lack a latent heat of fusion. The latent heat of fusion is the amount of thermal energy which must be absorbed or lost for a substance to change states (from solid to liquid or vice versa). In other words, if you were to put a container of liquid in a room set to a temperature well below the liquid's freezing point, the liquid would begin giving off heat and decrease in temperature. When it reached its freezing point, however, it would remain at a constant temperature (the freezing point) as it continued to give off heat. This would be the actual phase transition, and the heat given off during this transition is termed the "latent heat of fusion". After the liquid solidified, it would start dropping in temperature again as it continued releasing heat. It would stop releasing heat when its temperature matched the temperature of the room. Water, for example, has a specific amount of heat it must release, per gram, as it transitions from liquid water to ice. While it's releasing this heat of fusion, its temperature remains steady at 0 degrees C. Glass, on the other hand, never enters a stage during its production in which its temperature stops decreasing as it continues to give off heat. There is no defined phase transition between molten glass and "solid" glass.
As a side note, some people claim to have seen evidence of flow in very old glass (perhaps very old windows in a historic building). These claims are not valid. Because of the amount of time needed for glass to start exhibiting liquid behavior, no human will ever witness such a thing.
Saturday, January 2, 2010
Entropy and Ice
There are many more ways to be messy than organized. Things tend toward disorder because, statistically, there's just so many more ways to be disordered than ordered. This is the central idea behind the Second Law of Thermodynamics. Exhale and the carbon dioxide that comes from your mouth is not likely to float there in front of your face, bound up like a little ball of gas. It's not likely to stay so organized. The molecules are going to randomly move about and distance themselves from one another. There's no force pushing them apart; it's random motion. Divide the room you're in into a billion equal-sized cubes. The cubes right in front of your mouth have a large number of carbon dioxide (CO2) molecules in them, right after you exhale. The other cubes around the room have some CO2 molecules scattered about them. With all these molecules whizzing about at large velocities, what are the odds that as many CO2 molecules will enter the cubes (the space; the volume) in front of your mouth as will leave that volume? Practically zero. The universe naturally tends towards disorder. In fact, nothing will happen spontaneously unless it (i.e. the process, the reaction) increases the disorder of the universe. Entropy is a property of a system that measures the system's disorder. When a system becomes more disordered, we can say its entropy has increased. While we can talk of entropy in a qualitative sense, it's a number. (Its units are joules per Kelvin; energy is measured in joules, while temperature is measured in Kelvin, so we have amount of energy per unit (think degree) of temperature.) We see some process occur spontaneously and we think, entropy must have increased.
Go outside on a winter day and you may see ice -- solid water. And solids are more organized than liquids. In a piece of ice the water molecules are arranged in a certain pattern, and the ice itself takes up a small amount of space and doesn't spread out like liquid water. How could water suddenly become more organized in its structure? The ice you see formed spontaneously. Does this violate the Second Law of Thermodynamics, which states that the entropy of an isolated system must increase for any spontaneous process?
It's true that the entropy of the water decreased when the ice froze, but what about the entropy of the "isolated system"? Does the water/ice constitute an isolated system? No. It's in direct contact with the ground and with the air. We realize the Second Law holds only when we realize that the entropy of something else increased when the entropy of the water decreased. And not only did the entropy of something else increase, but it increased by more than the entropy of the water decreased. What became more disordered in this process? The air around the water-turned-ice. When ice freezes, it releases heat, and that heat goes into the air. The heat warms the air and gets some air molecules to speed up a bit. It increases the air's local temperature ever so slightly.
I need to mention that our intuitive notion of temperature is a result of atoms and molecules in motion (i.e. thermal energy). When something is heated up, its atoms/molecules move and vibrate more rapidly. When something is cooled down, its atoms/molecules move and vibrate less rapidly. Our skin can sense these motions, in the aggregate, and our brains interpret these sensations as temperature. With increased thermal energy comes increased entropy (or disorder). So hot air is more disordered than cold air. And, in our example, the air around the water-turned-ice is slightly more disordered than before the water froze, because the air has just been heated up a bit.
What dictates when the increase in the entropy of the air around a pool of water is enough to allow the water to become more organized (i.e. to turn to ice)? We now know that the increase in the entropy of the air must be greater than the decrease in the entropy of the water for such a process to occur spontaneously. (Thanks to the Second Law of Thermodynamics.) When is this the case? Why doesn't a pool of water, out on the sidewalk, turn to ice on a summer day? Again, what determines when the entropy of the air will increase by a greater amount than the entropy of the water will decrease? ... The initial temperature of the air, of course. But why?
Scenario A: I'm in the mood to dance. I go to a night club and make my way to the dance floor. Some hundred people are already shaking their bodies to the beat. I join in.
Scenario B: I'm in the mood to dance. I head over to the campus library and walk directly to the large reading room. Some hundred students are spread about the room, at tables, quietly reading. I start dancing in the middle of the room. They stare.
In which scenario have I contributed most to the energy in the room? Definitely scenario B. At the night club, there's already a scene. A hundred people dancing. I go relatively unnoticed. At the library, however, I'm really stirring things up. I'm definitely noticed. With it so calm and quiet prior to my arrival, I significantly contribute to the energy in the room once I start dancing. Likewise, adding heat to a cool system differs from adding heat to a hot system. "[T]he arriving [thermal] energy more noticeably stirs up the molecules of a cool system, which have little thermal motion, than those of a hot system, in which the molecules are already moving vigorously." (Chemical Principles, 3rd edition, Atkins & Jones, p. 247)
We can now reason that the heat released by freezing water really stirs things up on a cold day, when the air molecules have little thermal motion. And the heat that would be released by the water on a warm day would be an insignificant contribution to the thermal motion of the warm air. It would add little to the entropy of the air. So for a given amount of heat, introduced into a system, entropy will increase more when the system is at a low temperature than when it is at a high temperature. What's the magic temperature below which a bit of released heat (from water) will increase the entropy of the air more than the entropy of the water will decrease? Zero degrees centigrade, of course. Water, we know, freezes at 0 degrees C. When the outside air temperature is greater than 0 degrees C, the heat that would come from the freezing of water is not capable of increasing the entropy of the air enough to warrant the freezing. Only when the temperature is 0 degrees or less is the entropy of the entire system increased by the freezing of water, so only then is freezing spontaneous.
Sources: Chemical Principles, 3rd edition, by Atkins & Jones;
Principles of Chemistry, by Michael Munowitz
Go outside on a winter day and you may see ice -- solid water. And solids are more organized than liquids. In a piece of ice the water molecules are arranged in a certain pattern, and the ice itself takes up a small amount of space and doesn't spread out like liquid water. How could water suddenly become more organized in its structure? The ice you see formed spontaneously. Does this violate the Second Law of Thermodynamics, which states that the entropy of an isolated system must increase for any spontaneous process?
It's true that the entropy of the water decreased when the ice froze, but what about the entropy of the "isolated system"? Does the water/ice constitute an isolated system? No. It's in direct contact with the ground and with the air. We realize the Second Law holds only when we realize that the entropy of something else increased when the entropy of the water decreased. And not only did the entropy of something else increase, but it increased by more than the entropy of the water decreased. What became more disordered in this process? The air around the water-turned-ice. When ice freezes, it releases heat, and that heat goes into the air. The heat warms the air and gets some air molecules to speed up a bit. It increases the air's local temperature ever so slightly.
I need to mention that our intuitive notion of temperature is a result of atoms and molecules in motion (i.e. thermal energy). When something is heated up, its atoms/molecules move and vibrate more rapidly. When something is cooled down, its atoms/molecules move and vibrate less rapidly. Our skin can sense these motions, in the aggregate, and our brains interpret these sensations as temperature. With increased thermal energy comes increased entropy (or disorder). So hot air is more disordered than cold air. And, in our example, the air around the water-turned-ice is slightly more disordered than before the water froze, because the air has just been heated up a bit.
What dictates when the increase in the entropy of the air around a pool of water is enough to allow the water to become more organized (i.e. to turn to ice)? We now know that the increase in the entropy of the air must be greater than the decrease in the entropy of the water for such a process to occur spontaneously. (Thanks to the Second Law of Thermodynamics.) When is this the case? Why doesn't a pool of water, out on the sidewalk, turn to ice on a summer day? Again, what determines when the entropy of the air will increase by a greater amount than the entropy of the water will decrease? ... The initial temperature of the air, of course. But why?
Scenario A: I'm in the mood to dance. I go to a night club and make my way to the dance floor. Some hundred people are already shaking their bodies to the beat. I join in.
Scenario B: I'm in the mood to dance. I head over to the campus library and walk directly to the large reading room. Some hundred students are spread about the room, at tables, quietly reading. I start dancing in the middle of the room. They stare.
In which scenario have I contributed most to the energy in the room? Definitely scenario B. At the night club, there's already a scene. A hundred people dancing. I go relatively unnoticed. At the library, however, I'm really stirring things up. I'm definitely noticed. With it so calm and quiet prior to my arrival, I significantly contribute to the energy in the room once I start dancing. Likewise, adding heat to a cool system differs from adding heat to a hot system. "[T]he arriving [thermal] energy more noticeably stirs up the molecules of a cool system, which have little thermal motion, than those of a hot system, in which the molecules are already moving vigorously." (Chemical Principles, 3rd edition, Atkins & Jones, p. 247)
We can now reason that the heat released by freezing water really stirs things up on a cold day, when the air molecules have little thermal motion. And the heat that would be released by the water on a warm day would be an insignificant contribution to the thermal motion of the warm air. It would add little to the entropy of the air. So for a given amount of heat, introduced into a system, entropy will increase more when the system is at a low temperature than when it is at a high temperature. What's the magic temperature below which a bit of released heat (from water) will increase the entropy of the air more than the entropy of the water will decrease? Zero degrees centigrade, of course. Water, we know, freezes at 0 degrees C. When the outside air temperature is greater than 0 degrees C, the heat that would come from the freezing of water is not capable of increasing the entropy of the air enough to warrant the freezing. Only when the temperature is 0 degrees or less is the entropy of the entire system increased by the freezing of water, so only then is freezing spontaneous.
Sources: Chemical Principles, 3rd edition, by Atkins & Jones;
Principles of Chemistry, by Michael Munowitz
Friday, December 18, 2009
The Nature of Time
The passage of time is probably an illusion.
For one, time is relative. Or perhaps I should say, simultaneity is relative. If I stand midway between two poles, with a right-angled mirror that allows me to observe both, and if a lightning bolt hits each pole "simultaneously", I will indeed observe the simultaneity of the two lightning strikes. Light from each event will reach my eyes at the same time. For another observer, moving rapidly past me just as the lightning strikes, the events will not be simultaneous. As he's moving towards one of the poles and away from the other, the light from the pole he's moving towards will reach his eyes before the light from the pole he's moving away from. This is due, of course, to the shorter distance traveled by the light from the pole he's moving towards. But neither observer can claim absolute authority as to the simultaneity of the events; neither person is more special than the other. Time, here, is relative. If asked to signal the arrival of the lightning bolt on the pole toward which the moving observer is approaching, the moving observer will signal before the stationary observer will. The moving observer's "present" is the stationary observer's "future". Therefore, it doesn't make sense to confer special status on the present moment, because whose "present" would that moment refer to?
Another argument against the passage of time is the fact that nothing in known physics corresponds to its passage. The equations upon which physics rest work equally well whether time runs forwards or backwards. The present moment has no special significance. It seems that time is laid out in its entirety, with all times equally real. Our perception of the past, present, and future is not a result of time passing over us but instead of the way our brains work.
Why do we perceive time to move in one direction: from past to present to future? We're confusing the passage of time with the "arrow of time." The arrow of time points towards an asymmetry between past and future. (We, by convention, label the direction in which the arrow points as toward the "future".) A drinking glass dropped on the floor shatters, but a shattered glass never automatically reassembles itself and returns to your hand. If you were to see such a thing in a movie, you'd think that the film was being played in reverse. This forward-pointing arrow of time seems to be related to the second law of thermodynamics, which basically states that the entropy (or, roughly, disorder) of a closed system will increase in time. A shattered glass is definitely less ordered than an intact glass. But the fact that the arrow is pointing forward does not mean that it is moving forward. Some physicists speculate that the unidirectionality inherent in the formation of memories - new memories add information and raise the entropy of the brain - might lead to our perception of the flow of time. Others speculate that this perception may have something to do with quantum mechanics.
Sean Carroll (2006) proposes that the reason our arrow of time points towards the "future" and not the "past" is just a quirk of chance. That is, there's nothing fundamental about it. It could have just as well pointed in the opposite direction. Our universe just happens to be moving from a low-entropy state to a high-entropy state, but other universes may be moving in the reverse direction. Supposedly, though, on an ultra-large-scale, the entirety of all universes would be moving towards increased entropy, for the simple reason that there are more ways to be high entropy (disordered) than low entropy (ordered).
And another thing about time. Recent studies suggest that our perception of the world may not be continuous but might instead be a series of discrete snapshots like frames in a film. Actually, "it seems that each separate neural process that governs our perception might be recorded in its own stream of discrete frames" (Fox, 2009). And these streams (which need not all progress at the same rate) are then fit together in a separate process within the brain that produces a consistent picture of the world.
Not everyone agrees on the ideas presented above.
Sources:
"That Mysterious Flow" by Paul Davies, Scientific American, Volume 16, Number 1, 2006.
"The Time Before Time" by Sean Carroll, Seed, Volume 2, Number 6, September 2006.
"The time machine in your head" by Douglas Fox, NewScientist, Volume 204, Number 2731, 24 October 2009.
For one, time is relative. Or perhaps I should say, simultaneity is relative. If I stand midway between two poles, with a right-angled mirror that allows me to observe both, and if a lightning bolt hits each pole "simultaneously", I will indeed observe the simultaneity of the two lightning strikes. Light from each event will reach my eyes at the same time. For another observer, moving rapidly past me just as the lightning strikes, the events will not be simultaneous. As he's moving towards one of the poles and away from the other, the light from the pole he's moving towards will reach his eyes before the light from the pole he's moving away from. This is due, of course, to the shorter distance traveled by the light from the pole he's moving towards. But neither observer can claim absolute authority as to the simultaneity of the events; neither person is more special than the other. Time, here, is relative. If asked to signal the arrival of the lightning bolt on the pole toward which the moving observer is approaching, the moving observer will signal before the stationary observer will. The moving observer's "present" is the stationary observer's "future". Therefore, it doesn't make sense to confer special status on the present moment, because whose "present" would that moment refer to?
Another argument against the passage of time is the fact that nothing in known physics corresponds to its passage. The equations upon which physics rest work equally well whether time runs forwards or backwards. The present moment has no special significance. It seems that time is laid out in its entirety, with all times equally real. Our perception of the past, present, and future is not a result of time passing over us but instead of the way our brains work.
Why do we perceive time to move in one direction: from past to present to future? We're confusing the passage of time with the "arrow of time." The arrow of time points towards an asymmetry between past and future. (We, by convention, label the direction in which the arrow points as toward the "future".) A drinking glass dropped on the floor shatters, but a shattered glass never automatically reassembles itself and returns to your hand. If you were to see such a thing in a movie, you'd think that the film was being played in reverse. This forward-pointing arrow of time seems to be related to the second law of thermodynamics, which basically states that the entropy (or, roughly, disorder) of a closed system will increase in time. A shattered glass is definitely less ordered than an intact glass. But the fact that the arrow is pointing forward does not mean that it is moving forward. Some physicists speculate that the unidirectionality inherent in the formation of memories - new memories add information and raise the entropy of the brain - might lead to our perception of the flow of time. Others speculate that this perception may have something to do with quantum mechanics.
Sean Carroll (2006) proposes that the reason our arrow of time points towards the "future" and not the "past" is just a quirk of chance. That is, there's nothing fundamental about it. It could have just as well pointed in the opposite direction. Our universe just happens to be moving from a low-entropy state to a high-entropy state, but other universes may be moving in the reverse direction. Supposedly, though, on an ultra-large-scale, the entirety of all universes would be moving towards increased entropy, for the simple reason that there are more ways to be high entropy (disordered) than low entropy (ordered).
And another thing about time. Recent studies suggest that our perception of the world may not be continuous but might instead be a series of discrete snapshots like frames in a film. Actually, "it seems that each separate neural process that governs our perception might be recorded in its own stream of discrete frames" (Fox, 2009). And these streams (which need not all progress at the same rate) are then fit together in a separate process within the brain that produces a consistent picture of the world.
Not everyone agrees on the ideas presented above.
Sources:
"That Mysterious Flow" by Paul Davies, Scientific American, Volume 16, Number 1, 2006.
"The Time Before Time" by Sean Carroll, Seed, Volume 2, Number 6, September 2006.
"The time machine in your head" by Douglas Fox, NewScientist, Volume 204, Number 2731, 24 October 2009.
Sunday, November 22, 2009
Weightlessness
Are astronauts, in Earth orbit, without weight?
First, I should mention that there are two types of weight: actual weight and apparent weight. An Earth-bound object's actual weight is the downward force exerted upon it by the Earth's gravity. The object's apparent weight is the upward force, typically transmitted through the ground, that opposes gravity and prevents the object from falling through the floor or ground (towards the center of the Earth). When you stand on your bathroom scale, it's measuring your apparent weight (i.e. how hard it's having to push up on you to prevent you from accelerating downwards through the scale, crushing it). This doesn't necessarily have to be equal to your actual weight. In your bathroom, your apparent weight is equal to your actual weight, but if you carry the scale into an elevator and weigh yourself while accelerating upwards, the scale will register an apparent weight that is greater than your actual weight. Since a cable is pulling the elevator up rapidly, the elevator's floor is pushing up on the scale, and the scale is, in turn, pushing up on your feet. In order to force you upwards, against gravity, the scale is having to push on you harder than if it (and you) were stationary. Your actual weight doesn't change here, because it's dependent on your mass (which isn't changing) and your distance from the center of the Earth (which is changing only a negligible amount). But to the scale, it feels like you're growing heavier, because it's having to not only support you but push you (accelerate you) upwards. So it registers a heavier "weight", which we now know to be your apparent weight. What about when the elevator accelerates downwards? Your apparent weight becomes less than your actual weight. And now, what if the elevator's cable were to break and the elevator, scale, and you were all to freefall towards the ground below? The scale would be falling at the same rate you were falling, and so it wouldn't be supporting any of your weight. It would indicate a weight of zero. That is, your apparent weight would be zero. This is the definition of weightlessness. Weightlessness means without apparent weight; it has nothing to do with your actual weight. So ... an astronaut in orbit, in a constant state of freefall, kept to a near circular path around the Earth by gravity, is weightless, but only in the sense that he or she has no apparent weight. Astronauts most definitely do have actual weight!
Apparent weight can change fairly easily, we see. All you have to do is take a ride on an elevator, or rollercoaster, or some other device that accelerates you in a vertical direction. Does actual weight ever change (ignoring the effects of food)? Yes, it does. The force of gravity on you (which determines your actual weight) is dependent on your mass, the mass of the Earth, the gravitational constant G, and the distance between you and the center of the Earth. Assuming your mass is held constant, you can reduce your actual weight by increasing your distance from the center of the Earth. So astronauts have actual weight, but an actual weight that is slightly less than their actual weight back on Earth. How much less? In orbit around 300 km (185 miles) above the surface of the Earth, astronauts' actual weight will be about 8.8% less than back on Earth.
What makes you feel weightless when you're falling, even though you still have an actual weight? Or one could ask, what makes you feel heavy (or with weight) when you're standing on the ground? It's not gravity. It's the force of the surface you're standing on, pushing against you. If you're standing on a sidewalk, the concrete is pressing against your feet, which are in turn pressing against your ankles, which are pressing against your lower legs, which are pressing against your upper legs, and on and on. The feet are supporting your entire mass. Your chest, for example, only supports the mass of the body above the chest. You don't feel pressure evenly distributed throughout your body. (Well, you're used to the feeling of standing on a surface, and so you may have a hard time sensing this uneven pressure distribution, but it's there.) Your sense of weight also comes from your arms pulling down on your shoulders. When in freefall, this pressure gradient (or change over space) disappears. Each section of your body, each cell, is falling at the same rate. Therefore, your upper body isn't pushing on your lower body. Your ankles aren't pushing on your feet. There is no pushing at all. Neither are your arms pulling down on your shoulders. The absence of these sensations is what one equates to feeling weightless.
How does NASA simulate a weightless environment for astronaut training? They could put their astronauts in an elevator, take it to the top of a tall building, cut the supporting cable, and allow the elevator and its inhabitants to freefall for several seconds. But the impact upon hitting the ground would be extreme and most unpleasant. Instead, NASA sends its astronauts up in an airplane, and the airplane flies in the parabolic trajectories of freely falling objects. Soaring over the Gulf of Mexico, pilots level off at about 26,000 feet. They then shoot the plane upward at about a 45-degree angle. At this point, the apparent weight of the people inside the nearly empty, padded fuselage increases to about 1.8 times their actual weight. Half a minute later, pilots push the aircraft's nose over the top of this "parabola", and the plane falls some 8,000 feet or so until its pointing downward at about 30 degrees. During this freefall, the aircraft's acceleration matches Earth's acceleration of gravity, making everything inside weightless for 17 to 25 seconds. (Parts of the movie Apollo 13 were filmed on this aircraft.) Over a two-hour flight, the aircraft may fly through some 40 of these parabolas. NASA used two KC-135 Stratotanker aircraft for these sessions from 1973 until 2005, when they were retired and replaced with a McDonnell Douglas C-9. The plane, not too surprisingly, earned the nickname "Vomit Comet."
First, I should mention that there are two types of weight: actual weight and apparent weight. An Earth-bound object's actual weight is the downward force exerted upon it by the Earth's gravity. The object's apparent weight is the upward force, typically transmitted through the ground, that opposes gravity and prevents the object from falling through the floor or ground (towards the center of the Earth). When you stand on your bathroom scale, it's measuring your apparent weight (i.e. how hard it's having to push up on you to prevent you from accelerating downwards through the scale, crushing it). This doesn't necessarily have to be equal to your actual weight. In your bathroom, your apparent weight is equal to your actual weight, but if you carry the scale into an elevator and weigh yourself while accelerating upwards, the scale will register an apparent weight that is greater than your actual weight. Since a cable is pulling the elevator up rapidly, the elevator's floor is pushing up on the scale, and the scale is, in turn, pushing up on your feet. In order to force you upwards, against gravity, the scale is having to push on you harder than if it (and you) were stationary. Your actual weight doesn't change here, because it's dependent on your mass (which isn't changing) and your distance from the center of the Earth (which is changing only a negligible amount). But to the scale, it feels like you're growing heavier, because it's having to not only support you but push you (accelerate you) upwards. So it registers a heavier "weight", which we now know to be your apparent weight. What about when the elevator accelerates downwards? Your apparent weight becomes less than your actual weight. And now, what if the elevator's cable were to break and the elevator, scale, and you were all to freefall towards the ground below? The scale would be falling at the same rate you were falling, and so it wouldn't be supporting any of your weight. It would indicate a weight of zero. That is, your apparent weight would be zero. This is the definition of weightlessness. Weightlessness means without apparent weight; it has nothing to do with your actual weight. So ... an astronaut in orbit, in a constant state of freefall, kept to a near circular path around the Earth by gravity, is weightless, but only in the sense that he or she has no apparent weight. Astronauts most definitely do have actual weight!
Apparent weight can change fairly easily, we see. All you have to do is take a ride on an elevator, or rollercoaster, or some other device that accelerates you in a vertical direction. Does actual weight ever change (ignoring the effects of food)? Yes, it does. The force of gravity on you (which determines your actual weight) is dependent on your mass, the mass of the Earth, the gravitational constant G, and the distance between you and the center of the Earth. Assuming your mass is held constant, you can reduce your actual weight by increasing your distance from the center of the Earth. So astronauts have actual weight, but an actual weight that is slightly less than their actual weight back on Earth. How much less? In orbit around 300 km (185 miles) above the surface of the Earth, astronauts' actual weight will be about 8.8% less than back on Earth.
What makes you feel weightless when you're falling, even though you still have an actual weight? Or one could ask, what makes you feel heavy (or with weight) when you're standing on the ground? It's not gravity. It's the force of the surface you're standing on, pushing against you. If you're standing on a sidewalk, the concrete is pressing against your feet, which are in turn pressing against your ankles, which are pressing against your lower legs, which are pressing against your upper legs, and on and on. The feet are supporting your entire mass. Your chest, for example, only supports the mass of the body above the chest. You don't feel pressure evenly distributed throughout your body. (Well, you're used to the feeling of standing on a surface, and so you may have a hard time sensing this uneven pressure distribution, but it's there.) Your sense of weight also comes from your arms pulling down on your shoulders. When in freefall, this pressure gradient (or change over space) disappears. Each section of your body, each cell, is falling at the same rate. Therefore, your upper body isn't pushing on your lower body. Your ankles aren't pushing on your feet. There is no pushing at all. Neither are your arms pulling down on your shoulders. The absence of these sensations is what one equates to feeling weightless.
How does NASA simulate a weightless environment for astronaut training? They could put their astronauts in an elevator, take it to the top of a tall building, cut the supporting cable, and allow the elevator and its inhabitants to freefall for several seconds. But the impact upon hitting the ground would be extreme and most unpleasant. Instead, NASA sends its astronauts up in an airplane, and the airplane flies in the parabolic trajectories of freely falling objects. Soaring over the Gulf of Mexico, pilots level off at about 26,000 feet. They then shoot the plane upward at about a 45-degree angle. At this point, the apparent weight of the people inside the nearly empty, padded fuselage increases to about 1.8 times their actual weight. Half a minute later, pilots push the aircraft's nose over the top of this "parabola", and the plane falls some 8,000 feet or so until its pointing downward at about 30 degrees. During this freefall, the aircraft's acceleration matches Earth's acceleration of gravity, making everything inside weightless for 17 to 25 seconds. (Parts of the movie Apollo 13 were filmed on this aircraft.) Over a two-hour flight, the aircraft may fly through some 40 of these parabolas. NASA used two KC-135 Stratotanker aircraft for these sessions from 1973 until 2005, when they were retired and replaced with a McDonnell Douglas C-9. The plane, not too surprisingly, earned the nickname "Vomit Comet."
Monday, November 2, 2009
Why Do Golf Balls Have Dimples?
Why do golf balls have dimples? The dimples enable the ball to fly much farther through the air. A swing, driving a smooth golf ball 70 yards, could drive a dimpled ball perhaps 250 yards. Why?
First, air pressure is the force exerted by air molecules divided by the area on which the force is exerted. That is, force per unit area. The force comes from the countless collisions of the air molecules (i.e. nitrogen and oxygen molecules, as well as a very small number of carbon dioxide molecules and argon atoms) against the surface in question. Keep in mind that a net force on an object causes that object to accelerate (or decelerate). If the net force acts in the direction of the object's motion, it accelerates the object; acting in the direction opposite the object's motion, it decelerates the object.
Daniel Bernoulli was born in the Dutch Republic (now known as The Netherlands) in the year 1700. He's perhaps best known for discovering a relationship between the pressure, velocity (speed in a certain direction), and height (above some arbitrary reference level) of an incompressible fluid in perfect steady-state flow. Water being pumped through a pipe can fit this description. It's virtually incompressible, and the pump can keep it moving at a steady rate through the pipe. Air, while not incompressible, is close enough to an incompressible fluid in steady-state flow under certain conditions (velocity less than 300 km/h and no pressure differences of more than one tenth of an atmosphere) that we can use Bernoulli's equation to understand its behavior. So what's the relationship? For a fluid as described above, the pressure, plus one-half times the density times the velocity squared, plus the density times the acceleration due to gravity times the height above some arbitrary reference level, is constant. In equation form, P + 1/2 dv2 + dgh = constant. So what happens if I increase the velocity (v) of the fluid? Either the pressure (P) must decrease or the height (h) must decrease, so that the left side of the equation remains equal to the constant. You should take from this equation the following: for an incompressible, steady-state flow liquid, of a particular density (d), and at a set height (that doesn't change), pressure and velocity always move in opposite directions. If pressure decreases, velocity increases. If pressure increases, velocity decreases.
When the path of a fluid in steady-state flow bends, the pressure on the outside of the bend is always higher than the pressure on the inside of the bend. It's this pressure imbalance that causes the fluid to bend. This pressure change indicates a change in the fluid's velocity. So does the fluid on the outside of the bend speed up or slow down? It slows down. And the fluid on the inside of the bend? It speeds up, of course.
When a ball is hurtling through the air, the air it encounters is forced to flow around it. Some of the air flows over the top of the ball, some flows beneath the ball, and some air flows around each side. Air pressure above, beneath, and aside the ball is not everywhere the same. As the air encounters the front of the ball, it bends away from the ball, moving out of the way. (The ball is on the outside of the bend.) This creates a high-pressure zone in front of the ball. And the air here slows down. The air then curves back towards the ball, on all sides of the ball, hugging its surface as it moves towards the back of the ball. This puts the ball on the inside of many curved paths (or bends). Therefore, the air around the ball's middle is at low pressure and high speed. As the air reaches the back of the ball, it peels away from the ball and straightens back out. This bending of the air away from the ball creates a high-pressure zone behind the ball. Low speed air. Now you ask, how can the low-pressure air along the sides of the ball move into the high-pressure zone behind the ball? Doesn't air always move from a high-pressure zone into a low-pressure zone? Normally, yes. Here, the low-pressure air is definitely moving against the tide, so to speak. It's fighting its way into the high-pressure zone, slowing down (decelerating) as the high-pressure air pushes on it. But it has enough energy to successfully make the trip. It does reach the back of the ball. Now, these pressure imbalances are symmetric about the ball; they balance one another and produce no net force on the ball. They don't accelerate or decelerate the ball itself. Air resistance does exist, but it's a result of air near the ball's surface rubbing against the surface, producing a type of friction. Viscous drag, it's called. The air resistance is not a result of the pressure variations just described. Okay, now for a qualifier! The behavior of the air about the ball, as described in this paragraph, applies to balls traveling at slow speeds. This is important. The air behaves differently when it encounters a ball moving at high speed.
To describe the path of air flowing around a fast-moving ball, I must introduce the term boundary layer. A thin layer of air moving very close to the surface of the ball is called the boundary layer, and it behaves differently from air farther from the surface. It moves more slowly and has less total energy than the freely flowing air farther out. Why? Because friction with the ball's surface (i.e. viscous drag) slows it down and robs it of energy.
Hmmm. So you're thinking, it's hard for the air along the sides of the ball to push into the high-pressure zone behind the ball. Okay. But it sounds like it can do it anyways. Guess it has enough energy to do so. And that boundary layer. It has less energy than the air just a bit farther out. But, well, it seems that it, too, is able to push into the high-pressure zone. At least when the ball is moving slowly. (Good. You're right so far.) And so does this change when the ball is moving rapidly? Yes. When the ball is moving rapidly, this lower-energy boundary layer of air is no longer able to push into the high-pressure zone behind the ball. In fact, it is pushed back towards the sides of the ball by the adverse pressure gradient, cutting like a wedge between the ball and the freely flowing air outside this boundary layer. No longer does the air curve around behind the ball. This leaves us with an air pocket behind the ball; a turbulent wake, in other words. In this wake, the air pressure is roughly atmospheric. There goes the symmetry of pressure forces on the ball. Now there is no high-pressure zone behind the ball to cancel the high-pressure zone in front of the ball. There is a large pressure drag, a force on the ball in the direction of downwind, slowing the ball down. Decelerating it. This pressure drag is what limits the range of a smooth golf ball. Yes, there is also viscous drag, but it's not nearly as significant as the large pressure drag caused by the turbulent wake.
(turbulent wake behind ball, which is moving to the left)
So dimpled golf balls travel farther than smooth golf balls. Do the dimples somehow reduce the size (and severity) of this turbulent wake, reducing the pressure drag on the ball, preventing the ball from slowing so much as it arcs through the air? Yes, indeed. The dimples, or surface irregularities, cause the air in the boundary layer to tumble about. This tumbling about gives the boundary-layer air more energy, and more forward momentum. It now has a much better chance of pushing around to the back side of the ball, into the high-pressure zone. Alas, it still doesn't make it, but it comes much closer. It travels partially around the back of the ball before its progress is stopped and it separates from the surface. The air outside the boundary layer, following along, hugs the ball for a longer time, as well. It separates from the ball at the same spot where the boundary layer separates, this being a fair ways down the backside of the ball. The result is a smaller air pocket. A small turbulent wake. A less dramatic variation in air pressure between the front of the ball and the back of the ball. A more modest force of pressure drag. And this reduction in pressure drag is what enables the dimpled ball to soar some 200 yards farther than a smooth ball.
First, air pressure is the force exerted by air molecules divided by the area on which the force is exerted. That is, force per unit area. The force comes from the countless collisions of the air molecules (i.e. nitrogen and oxygen molecules, as well as a very small number of carbon dioxide molecules and argon atoms) against the surface in question. Keep in mind that a net force on an object causes that object to accelerate (or decelerate). If the net force acts in the direction of the object's motion, it accelerates the object; acting in the direction opposite the object's motion, it decelerates the object.
Daniel Bernoulli was born in the Dutch Republic (now known as The Netherlands) in the year 1700. He's perhaps best known for discovering a relationship between the pressure, velocity (speed in a certain direction), and height (above some arbitrary reference level) of an incompressible fluid in perfect steady-state flow. Water being pumped through a pipe can fit this description. It's virtually incompressible, and the pump can keep it moving at a steady rate through the pipe. Air, while not incompressible, is close enough to an incompressible fluid in steady-state flow under certain conditions (velocity less than 300 km/h and no pressure differences of more than one tenth of an atmosphere) that we can use Bernoulli's equation to understand its behavior. So what's the relationship? For a fluid as described above, the pressure, plus one-half times the density times the velocity squared, plus the density times the acceleration due to gravity times the height above some arbitrary reference level, is constant. In equation form, P + 1/2 dv2 + dgh = constant. So what happens if I increase the velocity (v) of the fluid? Either the pressure (P) must decrease or the height (h) must decrease, so that the left side of the equation remains equal to the constant. You should take from this equation the following: for an incompressible, steady-state flow liquid, of a particular density (d), and at a set height (that doesn't change), pressure and velocity always move in opposite directions. If pressure decreases, velocity increases. If pressure increases, velocity decreases.
When the path of a fluid in steady-state flow bends, the pressure on the outside of the bend is always higher than the pressure on the inside of the bend. It's this pressure imbalance that causes the fluid to bend. This pressure change indicates a change in the fluid's velocity. So does the fluid on the outside of the bend speed up or slow down? It slows down. And the fluid on the inside of the bend? It speeds up, of course.
When a ball is hurtling through the air, the air it encounters is forced to flow around it. Some of the air flows over the top of the ball, some flows beneath the ball, and some air flows around each side. Air pressure above, beneath, and aside the ball is not everywhere the same. As the air encounters the front of the ball, it bends away from the ball, moving out of the way. (The ball is on the outside of the bend.) This creates a high-pressure zone in front of the ball. And the air here slows down. The air then curves back towards the ball, on all sides of the ball, hugging its surface as it moves towards the back of the ball. This puts the ball on the inside of many curved paths (or bends). Therefore, the air around the ball's middle is at low pressure and high speed. As the air reaches the back of the ball, it peels away from the ball and straightens back out. This bending of the air away from the ball creates a high-pressure zone behind the ball. Low speed air. Now you ask, how can the low-pressure air along the sides of the ball move into the high-pressure zone behind the ball? Doesn't air always move from a high-pressure zone into a low-pressure zone? Normally, yes. Here, the low-pressure air is definitely moving against the tide, so to speak. It's fighting its way into the high-pressure zone, slowing down (decelerating) as the high-pressure air pushes on it. But it has enough energy to successfully make the trip. It does reach the back of the ball. Now, these pressure imbalances are symmetric about the ball; they balance one another and produce no net force on the ball. They don't accelerate or decelerate the ball itself. Air resistance does exist, but it's a result of air near the ball's surface rubbing against the surface, producing a type of friction. Viscous drag, it's called. The air resistance is not a result of the pressure variations just described. Okay, now for a qualifier! The behavior of the air about the ball, as described in this paragraph, applies to balls traveling at slow speeds. This is important. The air behaves differently when it encounters a ball moving at high speed.
To describe the path of air flowing around a fast-moving ball, I must introduce the term boundary layer. A thin layer of air moving very close to the surface of the ball is called the boundary layer, and it behaves differently from air farther from the surface. It moves more slowly and has less total energy than the freely flowing air farther out. Why? Because friction with the ball's surface (i.e. viscous drag) slows it down and robs it of energy.
Hmmm. So you're thinking, it's hard for the air along the sides of the ball to push into the high-pressure zone behind the ball. Okay. But it sounds like it can do it anyways. Guess it has enough energy to do so. And that boundary layer. It has less energy than the air just a bit farther out. But, well, it seems that it, too, is able to push into the high-pressure zone. At least when the ball is moving slowly. (Good. You're right so far.) And so does this change when the ball is moving rapidly? Yes. When the ball is moving rapidly, this lower-energy boundary layer of air is no longer able to push into the high-pressure zone behind the ball. In fact, it is pushed back towards the sides of the ball by the adverse pressure gradient, cutting like a wedge between the ball and the freely flowing air outside this boundary layer. No longer does the air curve around behind the ball. This leaves us with an air pocket behind the ball; a turbulent wake, in other words. In this wake, the air pressure is roughly atmospheric. There goes the symmetry of pressure forces on the ball. Now there is no high-pressure zone behind the ball to cancel the high-pressure zone in front of the ball. There is a large pressure drag, a force on the ball in the direction of downwind, slowing the ball down. Decelerating it. This pressure drag is what limits the range of a smooth golf ball. Yes, there is also viscous drag, but it's not nearly as significant as the large pressure drag caused by the turbulent wake.
(turbulent wake behind ball, which is moving to the left)So dimpled golf balls travel farther than smooth golf balls. Do the dimples somehow reduce the size (and severity) of this turbulent wake, reducing the pressure drag on the ball, preventing the ball from slowing so much as it arcs through the air? Yes, indeed. The dimples, or surface irregularities, cause the air in the boundary layer to tumble about. This tumbling about gives the boundary-layer air more energy, and more forward momentum. It now has a much better chance of pushing around to the back side of the ball, into the high-pressure zone. Alas, it still doesn't make it, but it comes much closer. It travels partially around the back of the ball before its progress is stopped and it separates from the surface. The air outside the boundary layer, following along, hugs the ball for a longer time, as well. It separates from the ball at the same spot where the boundary layer separates, this being a fair ways down the backside of the ball. The result is a smaller air pocket. A small turbulent wake. A less dramatic variation in air pressure between the front of the ball and the back of the ball. A more modest force of pressure drag. And this reduction in pressure drag is what enables the dimpled ball to soar some 200 yards farther than a smooth ball.
Wednesday, October 14, 2009
Superconductivity
When Thomas Edison went about providing electric power to New York City in the late 1800s, he knew that energy was dissipated as heat in the wires that delivered electric current to his customers. This reduced the amount of power that made it to the homes of his customers, and presented Edison with the problem of trying to minimize this power loss. (I talked about this in a previous blog entry.)
The issue faced by Edison was one of electrical resistance, which is a measure of the degree to which an object opposes an electric current through it. When current flows through an object with resistance, electrical energy is converted to heat at a rate equal to the square of the current times the resistance. This rate is a measure of power loss.
While Edison had means to lessen this loss of power, he couldn't escape it completely. That's because conductors (i.e. materials that conduct electricity) naturally heat up as an electric current moves through them. The electrons that comprise this current, as they snake forward through the material, are constantly bumping into the atoms (ions) of the conductor. At each collision, an electron loses a bit of kinetic energy to an ion, increasing the kinetic energy of the ion, generating heat and increasing the temperature of the conductor. While conductors exhibit less resistance at lower temperatures, ordinary conductors can never be cooled enough to achieve zero resistance.
It was in 1911 that a scientist, Heike Kamerlingh Onnes, discovered that certain unordinary conductors, under certain conditions, do possess zero electrical resistance. That is, passing an electric current through these materials does not result in the heating of the materials and, therefore, no power is lost in them. The reason why no one had seen such behavior before: it only takes place in certain materials, and these materials have to be unimaginably cold. It was only just prior to 1911 that such cold temperatures were achieved in the laboratory (by Onnes). Onnes had taken helium gas and got it so cold (down to 4.2 degrees above absolute zero) that it condensed into a liquid. Using this liquid helium as a refrigerant, he tested the electrical resistance of mercury and was amazed to find that it actually dropped to ZERO! Such behavior was a completely new phenomenon, never before witnessed. Onnes labeled it "superconductivity."
Onnes didn't understand what was going on inside the superconducting material. How could the electrons avoid bumping into the material's ions, passing kinetic energy to them? Why did such behavior occur only below a certain temperature, labeled the critical temperature? Twenty-two years later, in 1933, the answer was still unknown. But in this year, Walter Meissner and Robert Ochsenfeld made an important new discovery about superconducting materials (which, as a class, had expanded to include materials other than mercury). They found that superconductors expelled applied magnetic fields. Magnetic field lines that passed through a sample of material were, in a sense, pushed out of the material (or more accurately, cancelled within the material) when the material was cooled below its critical temperature. This finding, now known as the Meissner effect, provided evidence that superconductivity was, most fundamentally, a magnetic phenomenon. Such a finding also changed the mindset that the fundamental property of a superconductor was zero resistance.
A theory explaining the phenomenon of superconductivity was proposed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer. It became known as the BCS Theory, after their initials. It had to do with phonons (not photons) and Cooper pairs. Phonons are quantized crystal lattice vibrations. What does this mean? Certain materials exist as crystals, which means "the constituent atoms, molecules or ions [which are atoms or molecules with a net electric charge] are packed in a regularly ordered, repeating pattern in all three spatial dimensions." (Wikipedia) The graphic below is an example of a unit cell, which is periodically repeated in three dimensions to form a crystal. Each sphere represents an atom and the tubes represent bonds between atoms.
A lattice is a sort of framework upon which, at each point, there exists a unit cell like you see pictured above. So the crystal looks the same when viewed from any lattice point. As an electron moves through a crystal, it exerts a force (i.e. it pulls) on the positively charged lattice ions, distorting them towards its (the electron's) path. As the electron then moves away from that point on the lattice, the lattice ions return to their original position. Because all atoms in a crystal are connected, "the displacement of one or more atoms from their equilibrium positions will give rise to a set of vibration waves propagating through the lattice." (Wikipedia) Finally, these vibration waves are quantized, which means they can't possess just any amount of energy but only certain discrete numerical values.
What happens as an electron moves through a crystal, generating a phonon? Let's picture an electric current flowing through the material. One electron after another. An electron zips past a point in the crystal lattice, distorting the lattice through the creation of a phonon. The lattice is pulled inward towards the negatively-charged electron, but the electron quickly moves away, faster than the lattice can relax back to its original position. This creates a region of positive charge, as the lattice ions that are pulled inward are positively charged. Here's the cool part. A second electron can be attracted to the region of positive charge along the path of the first electron. And these two electrons, which would normally repel one another (because they are both negatively charged), can become bound to one another. "If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance." (Wikipedia) These electron pairs are called Cooper pairs, and they lie at the heart of the BCS Theory. They are what allow for superconductivity; they carry the superconducting current. But, as noted just above, the temperature has to be low. Above a critical temperature, the atoms in the crystal are jostling around too much, bumping into the electron pairs with enough force to knock them apart. This breaking apart of the Cooper pairs destroys superconductivity in the material, and the material becomes "normal." What's the highest temperature at which a known material will superconduct? A special ceramic material comprised of many different atoms has been observed to superconduct at -135 degrees C. Notice the negative sign. The holy grail of those working in the field is to find a material that superconducts at room temperature. (Obviously, no material yet identified would have helped Edison ... although there are techniques, which I addressed in a previous blog entry, that lessen the problem.)
The material that superconducts at -135 degrees C (or 138 K), like all materials that superconduct above around -243 degrees C (or 30 K), is called a "high-temperature" superconductor. This is obviously a relative term. Such materials are not consistent with the BCS Theory and there is no good theory to describe how these high-temperature superconductors work.
The issue faced by Edison was one of electrical resistance, which is a measure of the degree to which an object opposes an electric current through it. When current flows through an object with resistance, electrical energy is converted to heat at a rate equal to the square of the current times the resistance. This rate is a measure of power loss.
While Edison had means to lessen this loss of power, he couldn't escape it completely. That's because conductors (i.e. materials that conduct electricity) naturally heat up as an electric current moves through them. The electrons that comprise this current, as they snake forward through the material, are constantly bumping into the atoms (ions) of the conductor. At each collision, an electron loses a bit of kinetic energy to an ion, increasing the kinetic energy of the ion, generating heat and increasing the temperature of the conductor. While conductors exhibit less resistance at lower temperatures, ordinary conductors can never be cooled enough to achieve zero resistance.
It was in 1911 that a scientist, Heike Kamerlingh Onnes, discovered that certain unordinary conductors, under certain conditions, do possess zero electrical resistance. That is, passing an electric current through these materials does not result in the heating of the materials and, therefore, no power is lost in them. The reason why no one had seen such behavior before: it only takes place in certain materials, and these materials have to be unimaginably cold. It was only just prior to 1911 that such cold temperatures were achieved in the laboratory (by Onnes). Onnes had taken helium gas and got it so cold (down to 4.2 degrees above absolute zero) that it condensed into a liquid. Using this liquid helium as a refrigerant, he tested the electrical resistance of mercury and was amazed to find that it actually dropped to ZERO! Such behavior was a completely new phenomenon, never before witnessed. Onnes labeled it "superconductivity."
Onnes didn't understand what was going on inside the superconducting material. How could the electrons avoid bumping into the material's ions, passing kinetic energy to them? Why did such behavior occur only below a certain temperature, labeled the critical temperature? Twenty-two years later, in 1933, the answer was still unknown. But in this year, Walter Meissner and Robert Ochsenfeld made an important new discovery about superconducting materials (which, as a class, had expanded to include materials other than mercury). They found that superconductors expelled applied magnetic fields. Magnetic field lines that passed through a sample of material were, in a sense, pushed out of the material (or more accurately, cancelled within the material) when the material was cooled below its critical temperature. This finding, now known as the Meissner effect, provided evidence that superconductivity was, most fundamentally, a magnetic phenomenon. Such a finding also changed the mindset that the fundamental property of a superconductor was zero resistance.
A theory explaining the phenomenon of superconductivity was proposed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer. It became known as the BCS Theory, after their initials. It had to do with phonons (not photons) and Cooper pairs. Phonons are quantized crystal lattice vibrations. What does this mean? Certain materials exist as crystals, which means "the constituent atoms, molecules or ions [which are atoms or molecules with a net electric charge] are packed in a regularly ordered, repeating pattern in all three spatial dimensions." (Wikipedia) The graphic below is an example of a unit cell, which is periodically repeated in three dimensions to form a crystal. Each sphere represents an atom and the tubes represent bonds between atoms.
A lattice is a sort of framework upon which, at each point, there exists a unit cell like you see pictured above. So the crystal looks the same when viewed from any lattice point. As an electron moves through a crystal, it exerts a force (i.e. it pulls) on the positively charged lattice ions, distorting them towards its (the electron's) path. As the electron then moves away from that point on the lattice, the lattice ions return to their original position. Because all atoms in a crystal are connected, "the displacement of one or more atoms from their equilibrium positions will give rise to a set of vibration waves propagating through the lattice." (Wikipedia) Finally, these vibration waves are quantized, which means they can't possess just any amount of energy but only certain discrete numerical values.
What happens as an electron moves through a crystal, generating a phonon? Let's picture an electric current flowing through the material. One electron after another. An electron zips past a point in the crystal lattice, distorting the lattice through the creation of a phonon. The lattice is pulled inward towards the negatively-charged electron, but the electron quickly moves away, faster than the lattice can relax back to its original position. This creates a region of positive charge, as the lattice ions that are pulled inward are positively charged. Here's the cool part. A second electron can be attracted to the region of positive charge along the path of the first electron. And these two electrons, which would normally repel one another (because they are both negatively charged), can become bound to one another. "If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance." (Wikipedia) These electron pairs are called Cooper pairs, and they lie at the heart of the BCS Theory. They are what allow for superconductivity; they carry the superconducting current. But, as noted just above, the temperature has to be low. Above a critical temperature, the atoms in the crystal are jostling around too much, bumping into the electron pairs with enough force to knock them apart. This breaking apart of the Cooper pairs destroys superconductivity in the material, and the material becomes "normal." What's the highest temperature at which a known material will superconduct? A special ceramic material comprised of many different atoms has been observed to superconduct at -135 degrees C. Notice the negative sign. The holy grail of those working in the field is to find a material that superconducts at room temperature. (Obviously, no material yet identified would have helped Edison ... although there are techniques, which I addressed in a previous blog entry, that lessen the problem.)
The material that superconducts at -135 degrees C (or 138 K), like all materials that superconduct above around -243 degrees C (or 30 K), is called a "high-temperature" superconductor. This is obviously a relative term. Such materials are not consistent with the BCS Theory and there is no good theory to describe how these high-temperature superconductors work.
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