IGCSE Physics / Current and voltage in circuits
Voltage: energy per unit charge revision notes
Edexcel International GCSE Physics (4PH1), chapter 7: Current and voltage in circuits, specification point 2.20, 2.21. Free lesson.
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Voltage is the energy transferred per unit charge passed. This definition helps you understand what a voltage measures and calculate energy transferred using charge, current and time.
What voltage means
Voltage is the energy transferred per unit charge passed. One volt is one joule transferred for every coulomb of charge, so 1 V = 1 J/C. Learn the definition in words as well as the equation. A 6 V battery gives each coulomb of charge 6 J of energy. The charge carries energy round the circuit and transfers it in the lamp. The charge is not used up, only its energy is passed on. The charge returns to the battery and is given more energy.
How to calculate energy transferred
Use E = Q × V to find energy transferred. Rearranging gives V = E/Q, which means voltage is energy transferred divided by charge. This is the definition of voltage written as an equation. E is energy transferred, measured in joules (J). Q is charge, measured in coulombs (C). V is voltage, measured in volts (V).
How current and time link to energy
When you know the current and the time, first find the charge using Q = I × t. Current is measured in amperes (A), and time must be in seconds (s). You can then use E = Q × V. Combining the equations gives E = I × V × t.
What counts as a definition
The definition that earns the mark is: voltage is the energy transferred per unit charge passed. V = I × R lets you calculate a voltage, but it does not define what voltage is. Use the words “energy transferred per unit charge passed” when asked to define voltage.