Intro to Electricity: Part II

(continues from part 1)...

What to do about the need to locate a power plant near the plant's customers, so that a sufficient amount of energy survives the journey by wire and arrives, ready for use, at the customer's house? Who wants to live near a power plant? Thomas Edison, in trying to electrify New York City, faced this problem. As mentioned in the previous entry, Edison knew that the power reaching the homes of his customers was a product of the current and the drop in voltage (or electric potential) experienced by that current. A small current (or flow of electric charges) subject to a large voltage drop could produce the same amount of power as a large current subject to a small voltage drop. Edison also knew that power was lost (as heat) in the wires in proportion to the square of the current, so power was best delivered using a low current (to minimize power loss during transmission) and a large voltage drop. But, as mentioned in the previous entry, large voltage drops were dangerous things. If a person was to come into contact with a large enough voltage, it could propel a current through the person's body and disrupt the heart's rhythm. Thus the dilemma.

Fortunately, a solution to this problem was found. It goes by the name of alternating current, as opposed to the direct current that Edison worked with, and Edison was quite aware of it. First, what is alternating current? Second, how is it a solution to the problem mentioned previously? And third, why didn't Edison embrace alternating current (AC) as the savior it was?

Alternating current is a current (or flow of electric charges) that periodically reverses direction. Electrons rush right, slow down, stop, then rush left, slow down, stop, then rush right, etc. Power plants get current to do this by alternating the voltage drop that propels the electrons on their way. (So, yes, AC is what power plants deliver today.)

Alternating current's saving grace: it makes use of a special property of electricity (and magnetism) that allows for easy transfer of power from one AC circuit to another. This means the power plant and the electrical outlet in a customer's home don't have to be a part of the same circuit. (Direct current requires everything to be a part of the same circuit. This is of fundamental importance.) Here's the benefit: the various circuits that comprise the AC power distribution system can operate at different voltages with different currents. We can divide the distribution system into three circuits: one that originates within the power plant and terminates just outside the facility, a second that starts where the first ends and stretches for miles, terminating outside a customer's home, and a third that picks up where the second ends and carries power inside the customer's home. Maintaining a near constant level of power throughout the system, we can send high-current, low-voltage power through the first circuit, low-current, high-voltage power through the long second circuit, and then switch back to high-current, low-voltage power for the third and final circuit. The low voltages inside the power plant and inside the home are safe to be around, and even though current is high in these two circuits, the circuits are short and so little power is lost to heating the wires. The long second circuit, carried in wires high off the ground (or beneath the ground), can operate at a very high voltage and, therefore, a very low current, and so loses little power to heating the wires. There's no need to locate a power plant near a customer's home if you can locate the plant far away and transmit power to the home without losing much power along the way.

The physical device that is used to link two AC circuits, and that has the ability to alter the voltage (and current) from one circuit to the next, is called a transformer. You see these on the side of the road all the time. Transformers that increase the voltage from one circuit to the next are called step-up transformers; those that decrease the voltage (and so increase the current) are called step-down transformers. I won't go into how a transformer works in detail, but I'll mention that it relies on a fundamental relationship between electricity and magnetism. In short, accelerating electric charges (like electrons) produce magnetic fields that change in time. And magnetic fields that change in time, in turn, produce electric fields that generate currents in wires. Because of the alternating nature of the current in AC, electrons are constantly accelerating. (Not so in direct current.) And as electrons enter a transformer and accelerate through a coil of wire, they create a time-varying magnetic field that propels a current in a nearby coil of wire, which is part of a second circuit that exits the transformer. When the number of turns (of wire) in the secondary coil is greater than the number of turns in the primary coil, the voltage in the second circuit is stepped-up (i.e. it becomes higher than that in the first, or primary, circuit). Likewise, if the secondary coil has fewer turns than the primary coil, the voltage will decrease from the primary to secondary circuit, and we'll have a step-down transformer. (Further discussion will make things less clear, I think. It's really complicated how this works exactly.)

So why didn't Thomas Edison embrace AC, which would have allowed him to distribute power over great distances with little power lost during the transmission? He viewed its fluctuating nature as exotic and dangerous. Furthermore, he noticed that as the current reverses direction, there is a brief moment in time in which it's stopped. That is, there is a brief moment, many times each second, in which there is no power being delivered. (Is it possible to change your car's motion from 5 mph forward to 5 mph reverse without momentarily stopping?) It also came down to money. Not only were AC transmission lines much more expensive than DC lines, but Edison (and his backers) already had a lot of money invested in DC technology. It, therefore, fell to other pioneers of the late 1800s to develop AC power transmission and make it available to customers.

So what of those moments in time in which no current is flowing (and no power is available)? This is a real problem for most electronic and some electric devices. Not only are many devices sensitive to the direction of electron flow, but many devices need constant power and can't handle moments without it. A simple lamp is not one of these devices. It doesn't matter in which direction current flows as it moves through the lamp's filament. And the lamp survives quite well during those moments when it is without power; it just briefly stops producing light. These moments come and go so quickly the human eye can't detect the flickering, and so it's inconsequential. A radio, on the other hand, is different. Its more-sophisticated interior requires constant power and a current that travels in one direction. That is, it needs direct current, which it can obtain from a power adaptor. Most electronics require power adaptors (either internal or external) for this very same reason: the need to change the AC to DC and, usually, to lower the voltage as well (via a transformer).