Series and parallel circuits: There are two ways of joining electrical components, in series and in parallel. Some circuits include both series and parallel parts.

The idea

Components can be connected in two distinct arrangements. In a series connection, components sit one after another along a single loop, so the same current passes through each in turn. In a parallel connection, components sit on separate branches between the same two points, so the current splits between the branches. Many real circuits, such as a car lighting circuit or a household ring with a lamp spur, mix the two: a parallel pair may itself be wired in series with another component. To classify a circuit, trace each loop from the cell and note where paths divide and rejoin. A junction is any point where three or more wires meet; branches between the same pair of junctions are parallel, while components on one unbranched path are in series.

What earns the marks

• States that in a series connection components form a single loop and the same current flows through each component in turn.
• States that in a parallel connection components are connected across the same two points, forming separate branches, so the current divides between the branches.
• Recognises that a circuit may contain a series part and a parallel part, for example a resistor in series with a parallel pair of lamps.
• Identifies parallel branches as paths between the same pair of junctions, and series components as those on one unbranched path.
• Uses correct vocabulary such as branch, junction, loop and component when describing a given circuit diagram.

Common mistakes and how to put them right

• Assuming any circuit drawn with more than one loop is entirely parallel.

  Put it right by tracing from the cell and checking whether some components lie on a shared unbranched path before the loops divide.

• Thinking current is used up as it passes through series components.

  Put it right by stating that current is conserved around a series loop, so the same current passes through each component.

• Treating a component drawn beside another as automatically parallel.

  Put it right by checking whether the two components connect between the same pair of junctions.

Exam tips

• Trace the circuit with your finger from the cell, marking each junction, before deciding whether parts are series, parallel or mixed.
• When asked to describe a circuit, name the arrangement of each part in turn, for example 'the two lamps are in parallel, and this pair is in series with the resistor'.
• Sketch a simple mixed circuit and label one series part and one parallel part to practise the vocabulary the examiner expects.