From power lines to computer chips, electrical circuits are ubiquitous in the modern world, and yet, because we don't actually see the electricity moving through them, most people have only a vague idea of what the electricity is actually doing in a circuit. This lack of intuition, of the "common sense" about a subject that comes from everyday familiarity - can be a problem when students first begin to try to understand electricity and circuits. The extended analogy presented here should help, by comparing the flow of electricity to the more-familiar flow of water.

The analogy includes a discussion of the following concepts: conductor, non-conductor, voltage, current, power, battery, transistor, capacitor, resistor, and diode.

Electricity and water are very different things, and they flow for different reasons, but there are enough similarities that comparing the two can help you imagine, understand, and remember what is happening in an electrical circuit. For example, water will flow from a high place to a low place, if there's nothing blocking it from doing so. It flows for the same reason - gravity - that a rock in a high place will fall to a low place, as long as there's nothing in the way that blocks it from falling.

You may remember, from basic mechanics in physics class, that when the water (or rock) is the high place, ready to fall, it is said to have potential energy. This just means that it is capable of releasing energy, or doing work, simply because of its position. As the water (or rock) moves down through the gravitational field, the potential energy changes to kinetic energy, the energy of the movement of the water (or rock). When it reaches the bottom the potential energy is gone, "used up" by the movement; but meanwhile, you might have been able to use it to do work, having the falling water turn a water wheel, for example (or using the falling rock to break open a nut).

Electrical charge also flows when it can move from someplace with a higher potential energy to someplace where it will have a lower potential energy. This change in potential energy is due to an electrical field, instead of a gravitational field. The fact that electrical charges will move, if they can, to a place with a lower electrical potential energy is one of those basic laws of the universe, like gravity. And, just as with the flowing water, we can use the flow of charge - electricity - to do work as it moves.

But, just as many things can block water from flowing, many things can block electricity from flowing, too. Water flows well through air, but not through solid materials such as metal. Electricity does not flow well through air, or through many solid materials, such as rubber and plastic. Picture these non-conductors as high river banks or walls that block the flow. Electricity does flow well through metal, so picture such conductors as open spaces that the electrons can flow freely in. A long, thin, metal wire, then, surrounded by a rubber coating, is like a water channel with high banks; if it's "higher" at one end than the other, the water (electrons) will flow freely from one end of the channel (wire) to the other with very little leakage.

Figure 1: A wire surrounded by a non-conducting coating is analogous to a river flowing between high banks.

(a) (b)

What if there's a breach in the banks? Remember, air is not a good conductor, so simply removing a section of the rubber coating may not hurt anything. But if a good conductor does touch the wire at the bare spot (and remember, you are a good conductor), the result is about the same as digging a hole in the riverbank; if the hole gives the water (electrons) a chance to flow to a "lower" spot, the result can be a catastrophic flood! You can consider any connection that allows the electricity to spread out over a large area (a grounding wire, for example) the same as a direct outlet to the sea; both water and electricity will "prefer" any short-cut to the "lowest" place.

Figure 2: Any breach in the banks, or the non-conducting coating, allows the flow to "escape". The escaped flow is no longer available to do work in the circuit. But before trying to repair any breaches, turn off the flow! You don't want to be in the way when it tries to go to a "lower" place. Remember, for electricity, "lower" does not mean "closer to the ground"; it means any place (including the ground) where it will have a lower potential energy.

(a) (b)

We make electrical circuits in order to do something with the electricity flowing through them, so one of the questions you might have about a circuit is: how much work can it do? The answer depends on two different things: voltage and current.

When you measure the current of a river, you are measuring how much water is flowing past a certain point in a certain amount of time. If you watch a small brook, for example, you may see only a few cubic feet of water go by every second. A big river, on the other hand, can move thousands of cubic feet of water past you in one second. Similarly, the current in a wire is a measure of how much electricity is going through it per second. Electrical current is measured in amperes; one ampere is one coulomb of electrical charge going through any particular point every second. That's a lot of electrons (more than a billion billion) going by every second, just as a cubic foot of water is an awful lot of drops of water going by.

Obviously, a bigger current can do more work. Picture the current of a big river pushing barges from one town to another, or generating a lot of electricity by turning some big turbines in a dam. You might get a little work out of the small brook, by putting a water wheel in it, but you're not going to light up an entire city that way. But current is not the only issue. Picture two power-generating dams on that big river. In one dam, the water only falls ten feet from one side of the dam to the other; in the other dam, it falls one hundred feet. Same river, same current, but the second dam can generate more power because the water falls through a much greater height. There is a greater "difference" in the potential energy of the water entering and leaving the higher dam. Voltage is the difference in electrical potential energy. So a 12-volt battery comes with a bigger built-in "height" than a 9-volt battery.

Volts are just a measure of the difference in the potential energy, not a direct measure of potential energy, rather like measuring the height from the base to the top of a waterfall. It would be much harder to measure the absolute height of the top of the waterfall above sea level, or from the center of the earth, and it wouldn't give you any more useful information about power, anyway. Being high in the mountains doesn't make a waterfall more powerful; what matters is the change in potential energy (height) from the beginning to the end of the waterfall. In electricity, even this change is difficult to measure directly, so a volt is measured and defined in terms of how much work it can do.

A volt can be defined as "the amount of change in potential that will cause a one-ampere current to give one watt of power. Another way of saying this is: Power is voltage times current. So as the current gets bigger, so does the power, and when the voltage gets higher the power also increases.

It's useful to understand what it means physically that "power is voltage times current". For example, will touching a particular wire hurt you? If it has a very high voltage but a very low current, it might hurt you about as much as standing under a very high waterfall (high voltage) that has almost no water in it (low current). The small sparks caused by static electricity in cool dry weather, are high-voltage, low-current flows. The voltage has to be pretty high to get the electricity to jump through the air, since air is not a good conductor, but the small current involved means the "shock" it will give you will be pretty small.

A circuit with a very high current but a low voltage, on the other had, is about as dangerous as a fairly flat section of a big river. The river has a lot of current, but its height (voltage) from one end to the other is small. Try standing in such a river, and it may have enough power to sweep you far away; but a trained swimmer can negotiate it safely. A welding machine uses a high-current, low-voltage flow. It has enough current to melt the metal it touches, but its voltage is low enough that it can be safely handled by people trained to do so. Car batteries also have a lot of current but a low voltage.

Electricity - like water - is most dangerous when both the current and the voltage are high. Consider how quickly things can change with multiplication: if either the voltage or the current is "one", and the other is "ten", then the power is only "ten". But if both are "ten", the power is "one hundred"! You don't want to stand under Niagara Falls - lots of current, big height (potential difference) - and you don't want to touch a power line or stick your finger in a socket - also high current, and high voltage (potential difference). But that high power also means lots of energy is available to do work.

So to get things moving in your circuit, you're going to need to create that electrical potential difference. One easy way to do this is with a battery. You can picture the battery as two water reservoirs, one higher than the other. A high battery voltage represents a top reservoir that's much higher than the bottom reservoir. If the battery voltage is low, the top reservoir is not that much higher than the bottom one. If the battery is charged, there is plenty of water behind a dam in the upper reservoir. If the battery is "dead", too much of the water has run down to the low reservoir already. Some batteries you can recharge, just as you can refill the upper reservoir, but of course it takes energy to do so. (You can wait for rain to fill the upper reservoir, but that is just relying on the sun's energy to move the water.) Inside the battery, the two reservoirs are kept separate, so the only way for the electricity to move is for you to provide a channel (a wire) from the upper reservoir (the anode) to the lower reservoir (the cathode).

Now, if you just connect the two reservoirs with a wire made out of a good conductor, you're just opening the floodgates and letting the water run unimpeded - whoosh - in a waterfall from high to low. This is not useful. In order to begin to make it useful, you may want to add some resistance to the flow. A resistor slows down the flow without stopping it. It's used to adjust the current and voltage in the circuit. Instead of the water pouring straight down, you've added an obstacle - maybe a platform filled with rocks, that the water has to pass before it can continue to fall. The water keeps falling, but it has slowed down. Note that this also means it has lost energy; a resistor does use up electrical energy. This can be necessary so that other objects in the circuit aren't hurt by a current or voltage that are too high. You might want the waterfall to lose some energy if you're going to stand underneath it to take a shower, for example. It's also possible to use the energy loss from the resistor in a useful way; this is how filament light-bulbs work. In this type of bulb, part of the circuit is a length of wire that is not such a good conductor. Its resistance to the electricity moving through it heats it up to the point of glowing brightly. It is as if you put those rocks in the way of the waterfall because you wanted to use the moving water to clean them.

Sometimes you may want to be able to control the current precisely, as if you have captured the waterfall in a large pipe with a faucet at the lower end. When you add a transistor to your circuit, you have added a faucet that can control the flow. The transistor is controlled, in turn, by the voltage or current in a different wire in your circuit. It is as if the handle of the faucet is inside another pipe. The handle moves with the flow in the other pipe, so that the stronger the flow, the more it opens, allowing a bigger current to flow through the waterfall pipe, but the handle is attached to a spring that closes it when it is not being forced open, so if the flow in the second pipe slows, your waterfall pipe also slows.

Another way to control the flow is by using a capacitor. Putting a capacitor into your circuit is like building an extra reservoir - a water tower, for example - to capture the flow. You can then let the flow out of your water tower (capacitor) at a very steady rate, or all at once, in a burst. This extra reservoir can be useful in a couple of different ways. For example, it might be important that the machine being powered by your current (whether a water wheel or light bulb) receives a very constant current that is not too big or too small. Again, too big might hurt the machine; too small and it will stop working. If your upper lake is prone to droughts and floods (or the source of your electricity gives an alternating or variable current), your water tower (capacitor) can capture the floods and release them in a slow steady stream, even during dry spells. On the other hand, there are some machines that work best when given bursts of power of a certain size. Camera flashes, for example, and defibrillators, work by suddenly discharging a specific amount of electricity. It is as if you have built a machine that works best if you suddenly dump exactly 10 gallons of water onto it from 10 feet up. (Perhaps this is what it takes to move a paddle that does work in the machine.) If you dump less than that amount, your machine won't work; but if you dump more, you would be wasting water energy, and might break the machine, too. What you want, in this case, is to design your water tower so that it releases the water in 10-gallon bursts, suddenly pouring all ten gallons onto your machine. A capacitor can also release bursts of electricity of a specific size. When using capacitors, however, you may want to remember: if you're using simply the force of gravity, you can't fill your water tower any higher than your original reservoir. To fill a tower above that height, you'd have to actually use energy to pump the water "uphill". In the same way, you can't charge a capacitor to a higher voltage than the voltage that is being supplied to the circuit.

So, a faucet or water tower (transistor or capacitor) can regulate the forward flow of the current, allowing you to set up some sort of machine to use the current to do work. But what if water flowing the wrong way through the pipe would break your machine (perhaps because of the way the paddles are attached)? You'll want to be very careful to attach your pipes so that water running from the upper to the lower reservoir always goes through your machine in the right direction. One possible safety measure is to add a valve that only opens in one direction. Like the valves in your veins, if liquid tries to come from the other direction, the pressure of the liquid just closes the valve tightly, so that nothing gets through. In an electrical circuit, a diode allows the electricity to flow through it only in one direction, so that if you attach the battery the wrong way, the elements on your circuit won't be hurt by electricity flowing through them in the wrong direction.