Copyright © Karl Dahlke, 2023
Is the flow of electricity through a wire like the flow of water through a hose? In some respects yes, thus the analogy can prove helpful in the classroom. The amount of water passing through a hose, so many liters per second, is like the flow of electricity through a wire, which is measured in amps. And the force of the water pushing through the hose is like the force of the electricity, which is measured in volts. Suppose the water turns a wheel, perhaps to mill grain, as we have done for thousands of years. How much work can the water do? More water per second turns the wheel faster, and more force can turn a heavier wheel. The work is the flow times the force. This is intuitive with respect to water, and it is also true for electricity. The power of a circuit, measured in watts, is volts times amps. In fact one amp times one volt is one watt. If your microwave oven runs at 660 watts, and the wall socket delivers 110 volts, the appliance pulls 6 amps. In other words, 6 amps times 110 volts = 660 watts. In Europe, household power runs at 220 volts, thus a European microwave oven of the same power pulls 3 amps. Appliances pull as many amps as they need to do the job, since the voltage at the wall socket is fixed. However, if you draw more than 1,500 watts you'll blow a fuse - wall circuits cannot deliver that much power. My little kitchenette has a microwave oven, a toaster, and a coffee maker all on the same power strip, hence I can only run one at a time. When the coffee is done percolating, it's time to make the toast.
The water electricity analogy is helpful, to a point, but it soon breaks down. For instance, water can exit a hose and turn a wheel and do work, and then splash out all over the lawn. In contrast, electricity has to run around in a complete circle, as though a second hose, downstream from the wheel, captured the water and returned it to the municipal water supply. If an electrical circuit is not complete all the way around, the electrons do not flow, and no work is done. You need two wires, one to carry the electrons to the toaster or light or television, and one to bring the electrons back. That's why a plug has two prongs, sliding into two parallel slots in the wall.
What about a traditional lightbulb? How does it become part of a circuit? It doesn't have two metal prongs. It does however have a circular metal ring sporting the threads of the screw, and a metal tip at the bottom of the lightbulb. When the bulb is screwed all the way into its socket, the tip touches a metal plate at the base of the socket, which completes the circuit. Electricity enters the lightbulb through the base, heats the filament white hot, and leaves the lightbulb through the ring of threads, on its way back to the power station that generated the electricity in the first place. This standardized connector is called the Edison Screw, or the lampholder socket.
Electricity and lighting were almost synonymous in the 1880's, thus residential wiring ran directly to an Edison screw in the wall, ready to receive its lightbulb. However, people soon realized that electricity could do more, it could run a vacuum cleaner, or a fan. The earliest models had to plug into a lampholder socket; because the houses didn't have anything else. Conforming to this standard was rather problematic. You can easily turn a lightbulb in your hand, screwing it into and out of its lampholder socket, but you can't pick up a vacuum cleaner and spin it around, again and again, until it screws into the aforementioned receptacle. There were only two options at the time. Either the end of the cord turned freely, so it could screw into the lampholder socket, maintaining electrical contact all the while, or, employing a simpler design, an adapter screwed into the lampholder socket and presented two slots, similar to the wall outlets we see today. The appliance then plugged into this adapter in the now-familiar fashion.
If the house had one socket, which was not uncommon, the home owner had to run appliances by day, and lighting at night.
Q: How many people does it take to turn on a fan?
A: Two. One to remove the lightbulb and one to plug the fan into the lampholder socket.
Ok, it was funnier in 1884.
The need for a superior plug and socket design was clear, and by the late 1880's some homes were equipped with universal wall outlets that could power any appliance, including a lamp.
With the advent of alternating current and long distance transmission lines, a power station could be far from its customers. For example, the Hoover Dam, near the border of Arizona and Nevada, supplies power to several cities in southern California. The cables that serve these cities carry 200,000 volts. These cables are expensive to build and maintain, primarily because they are elevated dozens of meters off the ground for safety considerations. There is no insulation on these cables - if you touch one it's game over. Fortunately you can reach that high, unless you have a long metal pole, but don't do that!
As per the earlier paragraphs, you need two such cables to provide power to a city, one to send the electricity out and one to bring it back. But aha, you can do it with one cable, and cut the cost in half. The secret is to let the electricity return to the power station through the ground. In other words, the ground completes the circuit. At first this seems like it wouldn't work. Rocks and dirt are terrible conductors of electricity. But electricity is opportunistic. It takes every possible path from your home, through the ground, back to the power station, and it runs all these paths simultaneously. How many such paths are there? Millions, billions, trillions. Electrons can flow around this tree to the left, or around that rock to the right, or seven miles over to the river and back, or through the center of the earth. Each path only needs to carry the tiniest trickle of an amp to account for all the electricity used by a city. Dirt and rocks, poor conductors that they are, can accommodate these feeble currents. Furthermore, the resistance, i.e. the energy wasted on the return trip, is almost nil. Each path through the ground has such a tiny current, that there is virtually no heating of the rocks. In contrast, energy is indeed lost as electricity travels through the high voltage cable to the city. The wire runs hot, proportional to the square of the current it is carrying. Losses of 6% are typical. Superconducting wire, (if such could be developed), could reduce these losses to 3%, accounting for the energy required to maintain the sheath of liquid nitrogen.
You may at times see birds perched on residential power lines, especially in winter, to warm their feet. This causes no trouble, because the bird does not simultaneously touch the ground. If he had a 30 foot beak, and touched the ground, it would be game over.
Let's review. A power plant generates electricity at high voltage and pushes it along an overhead cable to your town. (Actually it pushes and pulls electrons back and forth, 60 times a second, or 50 times a second in Europe, using a system called alternating current, but let's keep things simple and say it pushes the electrons along the cable.) At the edge of your city, the voltage is decreased, and wires carry power to each neighborhood, and then to each house. Electricity enters your home through a panel of circuit breakers, then travels along wires embedded in the walls. It reaches the right slot of your electrical outlet and just sits there until you plug something in. If an appliance, such as a toaster, is present and turned on, electricity flows into the right prong of the plug, down the right wire of the cord, and through the nichrome wires, heating up your piece of toast. From there it travels back along the left wire of the cord, out the left prong of the plug, into the left slot in the outlet, along another wire in the wall, into the ground, and back through the earth to the power station. A power plant several hundred miles away is cooking your breakfast - how cool is that‽
These wall outlets were common by the 1930's. As mentioned above, one slot is hot, carrying the voltage, and the other slot is neutral, connected to the ground. There is a standard here. The hot wire is black, and the neutral wire is white. You can remember it by thinking of the black death. In theory you can touch the white wire, that is, the copper wire inside the white insulation, at any time, even when an appliance is running. It is connected to ground, and so are you. However, people do make mistakes, especially do-it-yourself home-owners who aren't familiar with the standard, so I don't recommend touching the white wire, or any wires, unless you know the breaker is off and the electricity is interrupted.
As a consumer, or as the designer of an appliance, you can't know for sure whether power comes in on the right or the left. Consider the toaster, as an example. It contains exposed wires; you can see them glowing red when the toaster is on. These can't be insulated, because the insulation would simply burn away. You can, without much difficulty, put your hand in the slot and touch these thin wires. You would never do so when the toaster is on - the heat is too intense - but when the unit is off you can definitely touch the wires with your hand or with (shudder) an inserted metal implement. The simplest design, from an engineering standpoint, uses the on-off switch to interrupt the hot line, i.e. the black wire. However, if the outlet is wired improperly, then those thin nichrome wires carry voltage all the time, even when the toaster is off. If you touch them with your hand, you are completing the circuit with your body - you are providing the path back to ground. In other words, zap! Since the nichrome wires are unavoidably exposed, the handle of the toaster disconnects both lines when the unit is off, and reconnects both lines when you press down on the handle to make toast.
When I was a kid, did I stick my hand in the (cold) toaster and touch the nichrome wires, assuming that off equals safe? Chuckle - of course I did - and I got lucky - nothing happened. Don't try it though; I can't guarantee every toaster cuts both lines in the off position.
The shortcomings of the two-slot outlet go beyond the toaster. The metal shell of a power tool, such as a skill saw, should really be connected to ground, so if there is a short circuit, your hand will not receive the shock. But there is no guarantee which slot is hot and which slot is neutral. You don't know which slot to connect to the metal shell. To circumvent this problem, another standard was established. The standard outlet now has two parallel slots and a third opening, just below the slots, that is guaranteed ground. It is simply called ground, but I like to think of it as "guaranteed ground", because neutral is ground as well. A slightly longer, round pin slides into this opening, so that the appliance is grounded before it receives any power. The shell of the device is safely wired to ground, whereupon accidental shock is nearly impossible. All new houses contain these three wire outlets, but in 2014 I lived in an older home, built in 1938, that still had the older 2-slot outlets. I had to use adapters to plug in modern equipment such as computers and microwave ovens. These adapters connect the ground pin to the metal plate in front of the electrical outlet, which is well grounded, though perhaps not as well grounded as a modern 3-wire outlet.
The wire that connects to ground is often uninsulated, to save money. Of course all three wires, black white and ground, are wrapped in an outer sheath, but there is no pressing need to insulate the ground wire within, since it connects directly to ground. You could touch it, even when the appliance is running.
That's not the end of the story however. For various reasons, some appliances differentiate between hot and neutral. To ensure this distinction, the neutral prong on the plug is somewhat taller. This is a polarized plug, and it requires a polarized outlet, wherein one slot is slightly taller than the other. This asymmetry is nothing more than a guarantee that the outlet is wired correctly. Neutral is neutral, hot is hot, and ground is ground. Of course it is just another source of annoyance if your appliance is polarized and your socket is not. Once again an adapter can come to the rescue, although it no longer assures the distinction between hot and neutral. It only pretends to.
When referring to a car, ground is no longer the ground under your feet. In fact that ground is electrically isolated from the car by the rubber tires. Ground is the metal chassis or frame of the car. Once again we only need one wire to connect the battery to each device - since the current returns through the chassis and back to the negative terminal of the battery. (The same is true in a plane, or the Space Station, or any other self-contained electrical system.) This cuts the number of wires, and the resistive losses, in half. You only need one wire running to the left tail light, etc.
Suppose the battery in car B is dead, whence the engine will not start. Car A has a full charge, and you want to use this battery to "jump" car B. Using proper jumper cables, you could connect plus to plus and minus to minus from battery A to battery B. Battery A takes over the function of battery B and starts the car. However, there are other, safer ways to make the connection. The second cable, that connects minus to minus, could instead connect ground to ground. Clip one end to the metal frame of car A and the other end to the metal frame of car B. Each frame is wired directly to the negative terminal of its battery, so the circuit is essentially the same. The front bumper is often used for this purpose, assuming it is made out of metal and not plastic. Thus the second jumper cable could connect the two bumpers of the cars. With this in mind, the recommended connection is: plus to plus, minus to ground. What does this mean? The first cable joins plus on battery A to plus on battery B. The second cable connects minus on battery A to the chassis on car B, which is ground for the second car. Why not connect minus to minus directly? A dead battery sometimes releases hydrogen gas, which is flammable. If the second connection is made at the negative terminal, there is bound to be a spark, and that could lead to an explosion. It's not likely, but just to be safe we connect the second cable to the front bumper, far from the dead battery.
After making these connections you may find that car B still won't start. The dead battery is pulling all the charge from the live battery, and there isn't enough to start the motor. (I have experienced this before.) Start car A and let it run. This adds the electricity from the alternator into the mix. You may need to let car A run for five or ten minutes to partially charge battery B, so it is not pulling all the current. At this point car B should start.