Current, resistance and potential difference: potential difference, V, in volts, V

The idea

Potential difference (p.d.) is the energy transferred per unit charge passing between two points in a circuit, measured in volts (V). One volt equals one joule per coulomb, so a 6 V battery transfers 6 J of energy to each coulomb of charge that passes through it. In circuit diagrams, p.d. is measured across a component using a voltmeter connected in parallel. For a fixed resistor, p.d. and current are directly proportional at constant temperature, giving a straight line through the origin on a V–I graph. The symbol V stands for both the quantity and its unit, so write V = 4.5 V. When two components are in series, the p.d.s across them add to the supply p.d.; in parallel, each branch has the same p.d.

What earns the marks

• State that potential difference is the energy transferred per unit charge between two points, with the volt defined as one joule per coulomb (1 V = 1 J/C).
• Identify the correct unit as the volt (V) and use the symbol V for the quantity, writing values such as V = 4.5 V.
• Describe how a voltmeter is connected in parallel across the component whose potential difference is being measured.
• Interpret a V–I graph for a fixed resistor as a straight line through the origin, showing p.d. is directly proportional to current at constant temperature.
• Apply the relationship V = IR to calculate an unknown p.d. when current and resistance are given, or to find current or resistance by rearrangement.
• Explain series and parallel p.d. behaviour: p.d.s across series components sum to the supply p.d., while parallel branches share the same p.d.

Common mistakes and how to put them right

• Writing the unit as 'v' or omitting it entirely, for example 'V = 4.5v'.

  Put it right: The unit symbol is an upper-case V, so write V = 4.5 V.

• Connecting the voltmeter in series with the component.

  Put it right: A voltmeter is connected in parallel across the component because it must compare the energy per unit charge at two points.

• Treating V as only a unit and not as the symbol for the quantity.

  Put it right: V represents potential difference in volts, so equations such as V = IR use V as the quantity symbol.

Exam tips

• Always write the unit after a calculated value and check it is V, not A or Ω, since p.d. is measured in volts.
• When asked to describe a circuit test, state that the voltmeter is placed in parallel with the component and that readings are taken while current is varied.
• Use the definition 'energy transferred per unit charge' when explaining what a volt measures, and link 1 V to 1 J/C to gain credit for the definition.