Series and parallel circuits: the total potential difference of the power supply is shared between the components

The idea

In a series circuit, the total potential difference (p.d.) of the power supply is divided among the components. The sum of the potential differences across each component equals the potential difference of the supply. This follows from conservation of energy: the energy transferred per unit charge by the source is shared as energy is transferred by each component. For example, if a 6.0 V battery is connected in series with two identical lamps, each lamp has a p.d. of 3.0 V. If the lamps have different resistances, the p.d.s differ but still sum to 6.0 V. The p.d. across a component is measured with a voltmeter connected in parallel. This rule allows you to calculate an unknown p.d. by subtracting known values from the supply p.d.

What earns the marks

• State that in a series circuit the total potential difference of the supply is equal to the sum of the potential differences across each component.
• Explain that energy is conserved: the energy transferred per unit charge by the supply is shared between the components.
• Use the equation V_total = V₁ + V₂ + ... for series circuits.
• Apply the rule to calculate an unknown potential difference when the supply p.d. and other component p.d.s are known.
• Recognise that the potential difference across each component depends on its resistance, but the sum remains equal to the supply p.d.

Common mistakes and how to put them right

• Thinking that the potential difference is the same across all components in series.

  Put it right: In series, the p.d. is shared; it is only the same across identical components if they have equal resistance.

• Confusing potential difference with current.

  Put it right: Current is the same through all components in series, but potential difference is shared.

• Forgetting to include all components when summing potential differences.

  Put it right: The sum must include every component in the series loop, and the total equals the supply p.d.

Exam tips

• When calculating an unknown p.d. in a series circuit, write down the sum of the known p.d.s and subtract from the supply p.d.
• Remember that voltmeters are connected in parallel across a component to measure its potential difference.
• Use the correct unit, the volt (V), and show your working clearly in calculations.