Physics

How is electricity measured?

electric

Three units (volt, ampere and ohm) describe the ‘flow’ of electricity which is described by analogy with the flow of water along a pipe under gravity. For water flows:

  • The ‘current’ is the amount of water passing one point in the pipe per second. This is analogous to the amount of electrical charge passing a point in a wire per second
  • Tilting the pipe gives the water more energy for each extra distance the water flows. The extra energy per unit mass is analogous to electrical voltage, which is related to the energy per unit charge.
  • The resistance of a pipe makes it hard for water to flow.

Similarly the electrical resistance of a wire describes how hard it is to pass a given electric current

Coulomb: The unit of electrical charge is called a Coulomb. One Coulomb is a massive amount of electricity – the experiments with the balloons typically involved a few nano-coulombs and an individual electron has a tiny charge 1.6 x 10-19 coulomb.

Ampere (or ‘amp’ for short): When one coulomb of electrical charge per second flows along a wire then we say a current of 1 ampere is flowing. Because the charge on each electron is so tiny, this involves the motion of massive numbers of electrons.

Volt: When some electrical charge has flowed far enough so that its energy has changed by 1 joule for every coulomb of flowing charge, we say it has flowed through a ‘potential difference’ of 1 volt. So in a 9 V battery, the electrons leaving the – terminal flow ‘downhill’ to the positive terminal and for every coulomb of charge that moves along the wire it gains 9 joules of energy
Ohm: The electrical resistance of wire is measured in ohms. If it takes a voltage of 10 volts to drive 2 amperes of current we say the electrical resistance is 10÷2 = 5 ‘volts per ampere’ or 5 ohms.

Watt: A watt is a unit of power (not necessarily electrical) which specifies how much energy (joules) per second a process uses. If a 240 volt mains source passes 4 amperes through a resistance then it will dissipate V x I = 240 x 4 = 960 watts or just under 1 kilowatt.