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. 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 blades, sliding into two parallel slots in the wall.
What about a traditional lightbulb? It doesn't have two male connectors. 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, 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; 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, 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. Of course the home owner had to run appliances by day, and lighting at night, swapping adapters and lightbulbs in a diurnal cycle.
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, even a lamp.
With the advent of alternating current and long distance transmission lines, a power station could be far from its customers. The Hoover Dam, near the border of Arizona and Nevada, supplies power to several cities in southern California. The cables that serve these cities can 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. 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 could reduce these losses to 3%, accounting for the energy required to maintain the sheath of liquid nitrogen.
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, 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 blade 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 blade 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 standard by the 1930's, but the system is not perfect. As mentioned above, one slot is hot, carrying the voltage, and the other slot is neutral, connected to the ground. However, the actual grounding may occur at a nearby distribution board or substation. Thus the neutral line could still carry a small charge. Furthermore, the hot wire is not always on the right, as the standard suggests. 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. The toaster, for instance, 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. So 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, assuming it is on the right. However, if the outlet is wired improperly, and hot is on the left, 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 you dare 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. I lived in a house built in 1975, decades after the standard was established, and still one of the sockets was wired backwards. To circumvent this problem, a new standard was established. The standard outlet now has two parallel slots and a third opening, just below the slots, that is guaranteed ground. 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, since you can put your hand on it, though perhaps not as well grounded as a modern 3-wire outlet.
That's not the end of the story. For additional safety, some appliances differentiate between neutral and line, so that the on-off switch disconnects the hot wire, and there is no voltage in the device when it is turned off. To ensure that the hot wire is indeed hot, the neutral blade 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, line is line, 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 discards the polarity, so that your skill saw could have voltage inside even when it is off. This relatively small risk is mitigated if the adapter connects the grounding pin of your skill saw to the grounded opening in the outlet, so that the device is properly grounded. As long as you don't open it up and fish around inside, you're ok.
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.
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 extremely 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 more than once.) 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.
Once upon a time, in a kingdom not far from here, a king summoned two of his advisors for a test. He showed them both a shiny metal box with two slots in the top, a control knob, and a lever. "What do you think this is?"
One advisor, an engineer, answered first. "It is a toaster," he said. The king asked, "How would you design an embedded computer for it?" The engineer replied, "Using a four-bit microcontroller, I would write a simple program that reads the darkness knob and quantizes its position to one of 16 shades of darkness, from snow white to coal black. The program would use that darkness level as the index to a 16-element table of initial timer values. Then it would turn on the heating elements and start the timer with the initial value selected from the table. At the end of the time delay, it would turn off the heat and pop up the toast. Come back next week and I'll show you a working prototype."
The second advisor, a computer scientist, immediately recognized the danger of such short-sighted thinking. He said, "Toasters don't just turn bread into toast, they are also used to warm frozen waffles. What you see before you is really a breakfast food cooker. As the subjects of your kingdom become more sophisticated, they will demand more capabilities. They will need a breakfast food cooker that can also cook sausage, fry bacon, and make scrambled eggs. A toaster that only makes toast will soon be obsolete. If we don't look to the future, we will have to completely redesign the toaster in just a few years."
"With this in mind, we can formulate a more intelligent solution to the problem. First, create a class of breakfast foods. Specialize this class into subclasses: grains, pork, and poultry. The specialization process should be repeated with grains divided into toast, muffins, pancakes, and waffles; pork divided into sausage, links, and bacon; and poultry divided into scrambled eggs, hard-boiled eggs, poached eggs, fried eggs, and various omelet classes."
"The ham and cheese omelet class is worth special attention because it must inherit characteristics from the pork, dairy, and poultry classes. Thus, we see that the problem cannot be properly solved without multiple inheritance. At run time, the program must create the proper object and send a message to the object that says, 'Cook yourself.' The semantics of this message depend, of course, on the kind of object, so they have a different meaning to a piece of toast than to scrambled eggs."
"Reviewing the process so far, we see that the analysis phase has revealed that the primary requirement is to cook any kind of breakfast food. In the design phase, we have discovered some derived requirements. Specifically, we need an object-oriented language with multiple inheritance. Of course, users don't want the eggs to get cold while the bacon is frying, so concurrent processing is required, too."
"We must not forget the user interface. The lever that lowers the food lacks versatility, and the darkness knob is confusing. Users won't buy the product unless it has a user-friendly, graphical interface. When the breakfast cooker is plugged in, users should see a cowboy boot on the screen. Users click on it, and the message 'Booting UNIX v. 8.3' appears on the screen. (UNIX 8.3 should be out by the time the product gets to market.) Users can pull down a menu and click on the foods they want to cook."
"Having made the wise decision of specifying the software first in the design phase, all that remains is to pick an adequate hardware platform for the implementation phase. An Intel 80386 with 8MB of memory, a 30MB hard disk, and a VGA monitor should be sufficient. If you select a multitasking, object oriented language that supports multiple inheritance and has a built-in GUI, writing the program will be a snap. Imagine the difficulty we would have had if we had foolishly allowed a hardware-first design strategy to lock us into a four-bit microcontroller!"
The wise king had the computer scientist thrown in the moat, and they all lived happily ever after.