Series and parallel circuits: describe the difference between series and parallel circuits

The idea

The difference lies in how components are connected and how current and potential difference behave. In a series circuit the components form one loop, so the same current passes through each component in turn; the cell potential difference is shared between them, and adding a component increases the total resistance, reducing the current. In a parallel circuit the components sit on separate branches connected across the same cell terminals; the current from the cell splits between the branches and recombines, while the potential difference across each branch equals the supply potential difference. Adding a branch therefore increases the total current drawn. A useful check is to remove one lamp: in series the whole circuit stops, whereas in parallel the other branches keep working.

What earns the marks

• States that a series circuit has a single loop, so the current is the same at every point.
• States that in a series circuit the supply potential difference is shared between the components.
• States that in a parallel circuit the current splits between branches and recombines at the cell terminals.
• States that the potential difference across each parallel branch equals the supply potential difference.
• Explains that adding a component in series increases total resistance and reduces the current.
• Explains that adding a branch in parallel reduces the total resistance and increases the total current.
• Uses the removal of one component to distinguish the circuits: the series circuit stops, while parallel branches continue to work.

Common mistakes and how to put them right

• Saying current is used up as it passes through components in a series circuit.

  Put it right: Current is conserved around the loop and the same current passes through each component.

• Claiming the potential difference across each branch of a parallel circuit is different.

  Put it right: Each branch has the full supply potential difference across it.

• Believing that adding a parallel branch increases the total resistance.

  Put it right: Adding a branch provides another path, so the total resistance falls and the total current rises.

Exam tips

• Compare the two circuits directly, using the words current and potential difference in each sentence.
• Support each statement with a simple example, such as two lamps in series versus two lamps in parallel.
• Use the removal test to justify your answer when asked how the circuits behave differently.