Electrical terms and units of measure

Power

Power is the rate at which energy is produced or used. The unit of measurement for power is the watt (W).

If you examine a light bulb carefully you will see that it has maybe "45, 60 or 100 watts" written on it. If it has "60 watts" written on it this means that it uses 60 joules of energy for every second it is on.

If a power station has an output of 600 megawatts (MW) of electrical power, it is producing 600 000 000 joules of energy every second. Imagine how many light bulbs that would keep going!

Voltage

The volt is named in honour of Alessandro Volta, an Italian scientist.

Voltage is the difference of electrical potential between two points of an electrical or electronic circuit, expressed in volts. It measures the potential energy of an electric field to cause an electric current in an electrical conductor. Depending on the difference of electrical potential it is called extra low voltage, low voltage, high voltage or extra high voltage.

Voltage is the unit of measure for electro-motive force required to pass one ampere through a resistance of one ohm (see "Resistance" below).

A simple way to understand voltage is to compare electricity flowing through a wire to water flowing through a pipe. If you pump water through a 100 metre long pipe with a small diameter, the water will barely flow out the other end. This is because there is a certain amount of friction which increases relative to the length of the pipe. Two things can increase the flow of water:

1. Making the pipe diameter larger (thus decreasing resistance);
2. Increase the pressure of the water (or voltage).

If electricity is to be transported over long distances, the resistance in the conductor (known as ohms) must be accounted for. Therefore, the diameter of the conductors and the voltage ("pressure") must be increased.

Electricity is generated at a comparatively low voltage at a generating (power) station. One of the voltages used by Queensland power generators is 15 750 volts. In order to transport this electricity over long distances, the voltage must be increased to as high as 275 000 volts by transformers.

Ampere

The ampere (or "amp"), named after the French scientist Andre Maria Ampere, is a measure applied to the flow of electrons. An "amp" is a unit of measurement for electrical current.

One ampere represents the rate of 1 coulomb of charge per second. A coulomb is equivalent to 6.24 X 1018 electron charges, hence a current of one amp means a flow of 6.24 X 1018 electron charges passes any given cross section of the conductor per second.

Resistance ohm

The German scientist Georg Simon Ohm discovered that in any conductor, electrons encounter a resistance whilst flowing through a circuit. He proved experimentally that the current in a metallic conductor is directly proportional to the voltage between its ends.

Therefore,

The unit of resistance is the volt per ampere. A resistance of one volt per ampere is called one ohm. That is, the resistance of a conductor is one ohm if the voltage between the ends of the conductor is one volt when the current in the conductor is one ampere.

Power

A watt is the unit of electric power. One watt is a measure of the power consumed when a current of one "ampere" flows under a pressure of one "volt".

Electrical arithmetic

On all electrical appliances you will find the "rating" marked in watts (for example, iron - 600 watts, heater - 1 000 watts, lamps - 100 watts. The power taken by any appliance (such as a toaster, heater, kettle etc.) is found by multiplying the voltage by the current:

watts = volts X amperes (Amps)

and

1000 watts = 1 Kilowatt (kW)

1,000,000 watts = 1 Megawatt (MW)

A watt-hour is the amount of energy consumed by an appliance in one hour if it operates at a power of one watt. A watt-hour is too small a unit for most purposes, so we use the "kilowatt-hour" (kWh), which is equal to 1000 watt-hours. Kilowatt-hours are the units measured by the electricity meter in your home.

1 kilowatt hour (kWh) = 1000 watt hours

To find the number of kilowatt-hours used by an appliance in any period, multiply the rating in watts by the number of hours in operation and divide by 1000:

The number of hours required for an appliance to use one kilowatt-hour is found by dividing 1000 by the rating in watts.

Thus, a 1000 watt radiator would take one hour to consume 1 kWh, but a 100 watt lamp would take 10 hours.

An average home (three bedroom home occupied by two adults and two children) uses an average of 17kWh per day.

When working out how much electricity is used by each appliance in your home, remember that some things like electric irons are only using power intermittently, as they are switched on and off either automatically or by hand.