Voltage and Current

When you first started learning about circuits, either in primary school or in Y7, you should have been told about voltage and current. These two measurements were probably presented as separate ideas but in fact they both relate to one thing; electrical charge.

Voltage relates to the amount of energy given to the charge that flows around an electrical circuit.

Current relates to the rate at which the charge flows.

These two ideas are contained in two equations, both of which you are expected to memorise.

One equation states that the potential difference (voltage) is equal to energy per unit charge. This can be expressed in symbols as;

V = E / Q

The other equation states that current is equal to the rate of transfer of charge. This can be expressed in symbols as;

I = Q / t

You should know the meaning of all these symbols and their SI units. But in case you need a reminder, the meanings are;

• V is the potential difference (voltage) and is measured in volts (V)
• E is energy and is measured in joules (J)
• Q is electrical charge and is measured in coulombs (C)
• I is current and is measured in amperes, or amps, (A)
• t is time and is measured in seconds (s)

These equations tell you, for example, that a 1.5 V cell gives 1.5 J of energy to each coulomb of charge. If that cell is connected to a circuit that draws 2 A then you know that 2 C of charge flows through the circuit every second. And since each coulomb of charge is carrying 1.5 J of energy, the circuit is transferring 3 J of energy every second.

You should recall that the rate of energy transfer is known as power, measured in watts (W). So the power rating of our circuit that has a 1.5 V cell supplying 2 C of charge, and giving 3 J per second, is in fact 3 W.

If all these equations and numbers seem confusing then you should refer to your revision guide (or revision cards) for more information.