Electricity - Electrical Circuits 01

Electrical current is a flow of electrons. Each electron carries a small amount of charge and if there are enough electrons on the move, the amount of charge is useful. The unit of electrical charge is the coulomb (C) and if you have one coulomb of charge passing a given point in an electrical circuit per second, then a current of one amp is flowing. Two coulombs per second is two amps, five coulombs in a second is a current of five amps and so on. If you have ten coulombs flowing past in five seconds, that boils down to two coulombs per second, so the current is two amps. The current is worked out from the quantity of charge flowing divided by the number of seconds it takes it to pass a defined point in a circuit.

Moving electrical charge through a circuit needs work to be done - the greater the resistance, the greater the energy required. The energy is supplied by the power source for the electrical circuit. As the charge moves through the circuit, some of the energy it carries is transferred. In the wires, it is transferred to heat and wasted. In a component such as an LED, it is transferred into heat and also directly into light, which is the useful energy. Electrical potential difference is a measure of the energy transferred as the charge passes through a component and has the units of volts (V). One volt of electrical potential difference represents one coulomb of charge gaining or losing one joule of energy.

When you plot a graph of the current flowing through a component against the electrical potential difference, you end up with a current-potential difference graph. When asked about electricity in the GCSE, you should be able to recognise the shapes of graphs for a resistor at constant temperature, a diode and a filament lamp. The examiners may put one in for a component for which you have never seen the graph, but you will be told what it is and be asked some simple questions about the information you can get from it.