Click here to purchase the entire book in PDF format. Current and EMF (Voltage) I have a slight plumbing problem in my kitchen. I have two sinks, side by side, one for washing dishes and one for putting the washed dishes in to dry. The drain of the two sinks feed into a single drain which has built up some rust inside over the years (I live in an old building). When I fill up one sink with water and pull the plug, the water goes down the drain, but can't get down through the bottom drain as quickly as it should, so the result is that my second sink fills up with water coming up through its drain from the other sink. Why does this happen? Well - the first answer is ``gravity'' - but there's more to it than that. Think of the two sinks as two tanks joined by a pipe at their bottoms. We'll put different amounts of water in each sink. The water in the sink on the left weighs a lot - you can prove this by trying to lift the tank. So, the water is pushing down on the tank - but we also have to consider that the water at the top of the tank is pushing down on the water at the bottom. Thus there is more water pressure at the bottom than the top. Think of it as the molecules being squeezed together at the bottom of the tank - a result of the weight of all the water above it. Since there's more water in the tank on the left, there is more pressure at the bottom of the left tank than there is in the right tank. Now consider the pipe. On the left end, we have the water pressure trying to push the water through the pipe, on the right end, we also have pressure pushing against the water, but less so than on the left. The result is that the water flows through the pipe from left to right. This continues until the pressure at both ends of the pipe is the same - or, we have the same water level in each tank. We also have to think about how much water flows through the pipe in a given amount of time. If the difference in water pressure between the two ends is quite high, then the water will flow quite quickly though the pipe. If the difference in pressure is small, then only a small amount of water will flow. Thus the flow of the water (the volume which passes a point in the pipe per amount of time) is proportional on the pressure difference. If the pressure difference goes up, then the flow goes up. The same can be said of electricity, or the flow of electrons through a wire. If we connect two ``tanks'' of electrons, one at either end of the wire, and one ``tank'' has more electrons (or more pressure) than the other, then the electrons will flow through the wire, bumping from atom to atom, until the two tanks reach the same level. Normally we call the two tanks a battery. Batteries have two terminals - one is the opening to a tank full of too many electrons (the negative terminal - because electrons are negative) and the other the opening to a tank with too few electrons (the positive terminal). If we connect a wire between the two terminals (don't try this at home!) then the surplus electrons at the negative terminal will flow through to the positive terminal until the two terminals have the same number of electrons in them. The number of surplus electrons in the tank determines the ``pressure'' or voltage (abbreviated V and measured in volts) being put on the terminal. (Note: once upon a time, people used to call this electromotive force or EMF but as knowledge increases from generation to generation, so does laziness, apparently... So, most people today call it voltage instead of EMF.) The more electrons, the more voltage, or electrical pressure. The flow of electrons in the wire is called current (abbreviated I and measured in amperes or amps) and is actually a specific number of electrons passing a point in the wire every second (6,250,000,000,000,000,000 to be precise). (Note: some people call this ``amperage'' - but it's not common enough to be the standard... yet...) If we increase the voltage (pressure) difference between the two ends of the wire, then the current (flow) will increase, just as the water in our pipe between the two tanks. There's one important point to remember when you're talking about current. Due to a bad guess on the part of Benjamin Franklin, current flows in the opposite direction to the electrons in the wire, so while the electrons are flowing from the negative to the positive terminal, the current is flowing from positive to negative. This system is called conventional current theory. There are some books out there that follow the flow of electrons - and therefore say that current flows from negative to positive. It really doesn't matter which system you're using, so long as you know which is which. Let's now replace the two tanks by a pump with pipe connecting its output to its input - that way, we won't run out of water. When the pump is turned on, it acts just like the fan in chapter 1 - it decreases the water pressure at its input in order to increase the pressure at its output. The water in the pipe doesn't enjoy having different pressures at different points in the same pipe so it tries to equalize by moving some water molecules out of the high pressure area and into the low pressure area. This creates water flow through the pipe, and the process continues until the pump is switched off.