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Voltage

Electricity and magnetism · level 4, lesson 1 of 6

A question to start

An AA battery says 1.5 V. A car battery says 12 V. The socket in the wall gives 120 V. What does the V measure?

The idea

Voltage measures how hard a battery or power supply pushes charge round a circuit. More exactly, it is the energy each unit of charge carries: 1 volt (V) means 1 joule of energy for every coulomb of charge. (A coulomb, C, is a fixed amount of charge, the charge of about \(6.25 \times 10^{18}\) electrons.)

Source Voltage
AA or AAA cell 1.5 V
Phone battery about 3.85 V
Car battery 12 V
Wall socket in North America 120 V
Wall socket in much of the world 230 V

Cells joined in a line, + to −, add their voltages. Two AA cells give 3 V; four give 6 V.

1.5 V1.5 V1.5 V1.5 V together: 6 V

Worked examples

1. Energy per charge. In a 12 V circuit, each coulomb that goes round carries 12 J of energy to the parts.

2. A torch. Three AA cells in a line: \(3 \times 1.5 = 4.5\) V.

3. One backwards. Four cells with one turned round: \(1.5 + 1.5 + 1.5 - 1.5 = 3\) V.

Try it

  1. What does a voltage of 1 V mean?
  2. How many AA cells in a line give 9 V?
  3. How much energy do 5 coulombs carry from a 12 V battery?
  4. Why is a wall socket dangerous when an AA cell is not?
Answers
  1. Each coulomb of charge carries 1 joule of energy.
  2. \(9/1.5 = 6\) cells.
  3. \(5 \times 12 = 60\) J.
  4. It pushes about 80 times as hard (120 V against 1.5 V), enough to drive a dangerous current through a body.

Common mistake

"Voltage flows round the circuit." Charge flows. The voltage is the push, or the energy each coulomb carries.

Where it leads