Series and parallel circuits: the total current through the whole circuit is the sum of the currents through the separate components

The idea

This statement describes current in parallel circuits. Charge is conserved, so the current arriving at a junction equals the total current leaving it. In a parallel circuit, the current from the supply splits between the branches, and the total supply current equals the sum of the branch currents. For example, if branches carry 0.3 A and 0.5 A, the supply current is 0.8 A. This is the junction rule applied at each split and rejoin point. It contrasts with a series circuit, where current is the same at every point. Branch currents depend on branch resistance: a lower-resistance branch takes a larger share of the total current. This rule explains why adding parallel branches increases the total current drawn from a supply.

What earns the marks

• State that at a junction in a parallel circuit, the current from the supply splits between the branches.
• Explain that charge is conserved, so the total current entering a junction equals the total current leaving it.
• Calculate the supply current by adding the currents in the separate parallel branches, for example 0.3 A + 0.5 A = 0.8 A.
• Contrast with a series circuit, where the current is the same through every component and does not split.
• Relate branch current to branch resistance, noting that a lower-resistance branch carries a larger current.
• Recognise that adding another parallel branch increases the total current drawn from the supply.

Common mistakes and how to put them right

• Adding branch currents in a series circuit.

  Put it right by stating that series current is the same everywhere, and the sum rule applies only to parallel branches.

• Assuming each parallel branch carries the same current.

  Put it right by explaining that branch current depends on branch resistance, so branches can differ.

• Forgetting to include every branch when finding the total.

  Put it right by identifying all branches at the junction and summing each branch current once.

Exam tips

• Mark the junction on a circuit diagram and label the current in each branch before adding them.
• Check that the total current is larger than any single branch current, which is a quick way to spot an arithmetic slip.
• When branch currents are not given, calculate each using potential difference and resistance before summing.