Voltage, Current and Resistance

Imagine that you plug a space heater into a wall outlet. You measure the amount of current flowing from the wall outlet to the heater, and it is 10 amps. That means that it is a 1,200-watt heater.

Volts * Amps = Watts

... so 120 volts * 10 amps = 1,200 watts.

This is the same for any electrical appliance. If you plug in a toaster and it draws 5 amps, it is a 600-watt toaster. If you plug in a light and it draws half an amp, it is a 60-watt light bulb.

Let's say that you turn on the space heater, you go outside and you look at the power meter. The purpose of the power meter is to measure the amount of electricity flowing into your house so that the power company can bill you for it. Let's assume that nothing else in the house is on, so the meter is measuring only the electricity used by the space heater.

Your space heater is using 1,200 watts. That is 1.2 kilowatts -- a kilowatt is 1,000 watts. If you leave the space heater on for one hour, you will use 1.2 kilowatt-hours of power. If your power company charges you 10 cents per kilowatt-hour, then the power company will charge you 12 cents for every hour that you leave your space heater on.

1.2 kilowatts * 1 hour = 1.2 kilowatt-hours

1.2 kilowatt-hours * 10 cents per kilowatt-hour = 12 cents

Similarly, if you have a 100-watt light and you leave it on for 10 hours, the light will consume 1 kilowatt-hour (100 watts * 10 hours = 1 kilowatt-hour).

If you have a 20,000-watt heat pump and you leave it on for five hours every day, you will consume 100 kilowatt-hours per day (20 kilowatts * 5 hours = 100 kilowatt-hours), or 10 dollars of power per day if a kilowatt-hour costs a dime. If you do that for a month, your heat pump costs you (30 * $10) $300 per month. That is why your electric bills can get so high when the temperature is very cold -- the heat pump runs a lot.

The three most basic units in electricity are voltage (V), current (I) and resistance (r). As discussed previously, voltage is measured in volts, and current is measured in amps. Resistance is measured in ohms.

We can extend the water analogy a bit further to understand resistance. The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.

There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal to the voltage divided by the resistance.

I = V/r

Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden. What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.

Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.

When you look at a normal incandescent light bulb, you can physically see this water analogy in action. The filament of a light bulb is an extremely thin wire. This thin wire resists the flow of electrons. You can calculate the resistance of the wire with the resistance equation.

Let's say you have a 120-watt light bulb plugged into a wall socket. The voltage is 120 volts, and a 120-watt bulb has 1 amp flowing through it. You can calculate the resistance of the filament by rearranging the equation: r=V/I. So the resistance is 120 ohms. If it is a 60-watt bulb, the resistance is 240 ohms.

Beyond these core electrical concepts, there is a practical distinction that happens in the area of current. Some current is direct, and some current is alternating -- and this is a very important distinction.