Electricity - Current, Charge and Power

Knowing how current, charge and power are related can help you decide when you are buying. It can also help you with safety in the home as you can work out the current that each one will draw when running at full power. So how is this useful? You can work out the correct size of fuse to use and also make sure that an extension lead or household circuit is not being overloaded.

Electricity is a flow of electrons through a material. Electrons carry a very small amount of electrical charge but since they are extremely tiny, there are massive numbers of them on the move in a working circuit. One coulomb equals the negative of the charge of 6.24 x 10¹⁸ electrons. Using the coulomb avoids needing to use this large number in calculations. In equations, charge is represented by q (or Q) and has the SI unit symbol C.

Electrical current is the rate of flow of this charge in a circuit. It has the symbol I in equations and is measured in amperes (usually shortened to 'amps') which has the SI unit symbol of A. The current flowing round a circuit is measured using an ammeter. One amp is equivalent to one coulomb of electricity passing a given point in a circuit in one second.

Power refers to how fast energy is transferred, so you can calculate the power consumption (in watts) of an electrical appliance by dividing the energy transferred (in joules) by the time taken to transfer that amount of energy in seconds. One of the common mistakes made in senior high school is to forget to convert time into seconds. Power is directly related to the current and the potential difference, if you multiply the curent in amps by the potential difference in volts, you have the power of an appliance. In the exams, when asked about electricity you will often need to rearrange these relationships (or even combine them) to calculate the figure demanded in the question, so make sure that you revise the basic equations related to current charge and power thoroughly.