Power: potential difference, V, in volts, V

The idea

Potential difference, symbol V, is the energy transferred per unit charge as charge moves between two points in a circuit. It is measured in volts, symbol V, so the quantity and its unit share the same letter. One volt means one joule of energy is transferred per coulomb of charge, linking V = W ÷ Q to the power equation P = V I. In a series circuit the supply potential difference is shared between components; in parallel, components across the same supply have equal potential differences. A voltmeter is connected in parallel across the component being measured, because it must compare the energy at two points.

For example: A 12 V supply driving 2 A through a lamp transfers 24 J each second, since P = V I = 12 V × 2 A = 24 W.

What earns the marks

• State that potential difference is the energy transferred per unit charge between two points in a circuit.
• Recall that the unit of potential difference is the volt, symbol V, and that one volt equals one joule per coulomb.
• Use the equation V = W ÷ Q, or P = V I, correctly when potential difference, energy, charge or power values are given.
• Describe connecting a voltmeter in parallel across a component, and explain that this is needed to measure the difference in energy per charge between two points.
• Interpret circuit values: in series the supply potential difference is shared, while in parallel components across the same supply have the same potential difference.

Common mistakes and how to put them right

• Writing the unit as 'v' or confusing the quantity symbol V with the unit symbol V.

  Put it right: Both are capital V, but the quantity is potential difference and the unit is the volt.

• Connecting a voltmeter in series with a component.

  Put it right: A voltmeter is connected in parallel across the component so it compares two points.

• Treating potential difference and current as the same quantity.

  Put it right: Current is the rate of flow of charge in amperes, while potential difference is energy transferred per unit charge in volts.

Exam tips

• Always write the unit after a calculated value, for example 6.0 V, and check that the unit matches the quantity asked for.
• When using P = V I, rearrange before substituting so the unknown is isolated, then substitute values with units.
• In circuit questions, sketch the circuit and mark where the voltmeter connects before choosing a calculation method.