Intro to Electricity: Part I

Electricity is defined by the Microsoft Encarta dictionary as (1) the energy created by moving charged particles and (2) electric current. (These two definitions speak of different things, however. Electric current is not energy, although it can be related to energy by way of a potential difference and time.)

Nevertheless, electricity is most commonly experienced as the flow of electrons in a wire (i.e. an electric current). Prior to the discovery of electrons, scientists thought of this mysterious flow of charge as perhaps that of a fluid. Benjamin Franklin, who died around one hundred years prior to the discovery of the electron, thought of electricity as the flow of a single type of fluid. To him, an electrically neutral object contained just the right amount of this fluid, while a "positively" charged object contained an excess of the fluid and a "negatively" charged object contained a deficiency. Franklin himself arbitrarily chose the labels "positive" and "negative". Other scientists adopted this terminology. Unfortunately, Franklin got it backwards. The "positively" charged object is not positive because it has an excess of "fluid", but because it is deficient in negatively-charged electrons. When a conductor with excess "fluid" comes into contact with a "fluid"-deficient conductor, it is not a flow of positive charge that seeks to equalize the charge of the two conductors but is instead the flow of negatively-charged electrons in the opposite direction. Today, we still (theoretically) consider current to flow from the positive to the negative terminal of a battery, which assumes a positively charged current. But we know that the electrons, which are the primary charge carriers, actually travel in the reverse direction, from negative to positive.

In the United States, ordinary wall sockets provide a source of power to run our appliances and electronics. We plug a lamp into an outlet and turn the lamp on. A circuit is completed and electrons travel through the lamp and its light bulb. As they do so, they lose energy to the lamp, powering the lamp. (Power is the transfer of energy per unit time.) So what kind of energy are they losing? It's potential energy, or energy of position. And it comes about because, as we all know, like charges repel one another and unlike charges attract one another. Two electrons positioned next to one another try to move so as to distance themselves from one another. There's a certain energy associated with their proximity to one another, and it's this energy that enables them to start moving apart. As they accelerate away from one another, they trade this energy of position for the energy of motion (i.e. kinetic energy). Energy is, of course, conserved. How this creates a current is easy to understand in terms of a battery. A chemical reaction in the battery causes electrons to accumulate on the negative terminal of the battery. Connecting the two terminals with a wire allows the electrons to distance themselves from one another. They rush away from the negative terminal, heading down the wire towards the positive terminal with its deficiency of electrons. This flow is what we call an electric current. As the electrons do this, they lose their energy of position. (In this case, mostly to energy in the form of heat.)

So does the power plant that provides our electricity send these electrons to our wall sockets, where they wait for us? No.

The power plant sells us not electrons but the ability to move electrons. It sells us a force field that pushes electrons along. It sells us energy of position, or electric potential energy. The potential at a point in space is often called voltage. Say we have two points in space, one with a voltage of 10 volts and one with a voltage of 5 volts. An electron situated at the first point will have a different energy of position than an electron situated at the second point. Take away both electrons, then place one at the point in space that is associated with an energy of position of 5 volts. It will spontaneously move towards the point with a voltage of 10 volts, just like a ball spontaneously rolls down a hill. (A positive charge would move in the opposite direction.) This voltage difference or gradient has another name: electric field. An electric field is a voltage drop per unit distance. Take a 9 V battery, with a distance of 0.005 m between its positive and negative terminals. It's called a 9 volt battery because the potential energy of one terminal is 9 volts less than the potential energy of the other terminal. Dividing the 9 volt voltage drop by the distance between the terminals (0.005 m), we get an electric field of 1800 volts per meter in the space between the terminals. It's this electric field or voltage gradient that we pay the power plant for.

Back to the lamp. Electrons already exist in the wires that run through the lamp and down its cord. Plugging the lamp into the wall and turning it on simply gets these electrons moving. And not very fast. They drift along barely at walking speed (because they keep running in to the atoms of the wire). But electrons are small, and lots of them fit into a very small bit of wire. When one amp of current is flowing through a wire, 6 quintillion (a 6 with 18 zeros behind it) electrons pass a given point in one second. (An amp is the basic unit used in the measurement of current.)

We've talked of voltage gradients (or differences in electric potential energy) and currents. Both are needed to define power. Power is the product of the two: a voltage difference times a current. One amp of current dropping one volt is equal to one watt of power. So we see there are two ways to increase power to a device. We can increase the voltage gradient across it ... or increase the current that flows through it. In other words, we can increase the amount of energy each electron loses as it passes through the device ... or we can increase the number of electrons passing through that device. (Or both.) This leads us to Thomas Edison.

Edison began to electrify New York City in 1882. He built power plants in the city that would send out a current through one wire and return it to his generators through another. The current flowed in one direction around the loops of copper wire he had laid between his plant and the houses of his customers. That is, it was direct current. But Edison quickly became aware of a fundamental problem with the setup. Power being sent out along the copper wires was being lost as heat (heating the wires), reducing the amount of power reaching the homes of his customers. The longer the wires (i.e. the farther the customer was from the power plant), the more power loss. Edison needed a way to increase the power reaching these distant customers. As we've already seen, he had two options: increase the voltage gradient or increase the current. But both had drawbacks. It was known that the power loss was proportional to the square of the current passing through the wires, so increasing the current caused even more power to be lost. Doubling the current quadrupled the power wasted as heat. Edison did try increasing the current by using thicker wires, but he realized that increasing the voltage gradient that pushed the electrons along was a better option. And so he did this. But high voltages were quite dangerous. They tended to create sparks and nasty shocks. So Edison could only raise the voltage gradient to a certain level before it became just too dangerous to send through someone's home. Foiled on both accounts, Edison did the best he could: he used thick wires, used the highest voltages that safety would allow, and he built power plants near his customers so the transmission lines wouldn't need to be very long. But who wants to live next to a power plant?

to be continued ...