Series and parallel circuits: there is the same current through each component

The idea

In a series circuit, the current is the same at every point because there is only one path for charge to flow. Charge is conserved: the rate of flow of charge (current) cannot change unless charge accumulates, which it does not in a simple series loop. For example, if a cell, lamp and resistor are connected in series and an ammeter reads 0.30 A between the cell and lamp, then an ammeter placed between the lamp and resistor also reads 0.30 A, and between the resistor and cell it again reads 0.30 A. This holds regardless of component resistance or potential difference. The current is determined by the total resistance and the supply potential difference, but its value is identical through each component. This principle allows you to calculate current in any part of a series circuit once you know it at one point.

What earns the marks

• State that in a series circuit there is only one path for charge to flow, so the current is the same at every point.
• Explain that charge is conserved, so the rate of flow of charge (current) cannot change around a series loop.
• Use the equation I = Q / t to support the idea that if charge does not accumulate, current is constant.
• Apply the principle to a practical example: an ammeter placed anywhere in a series circuit will read the same value.
• Recognise that the current through each component is the same even if the components have different resistances or potential differences.

Common mistakes and how to put them right

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

  Put it right: Current is not used up; charge is conserved, so the same current flows through each component in series.

• Believing that a component with higher resistance draws more current in series.

  Put it right: In series, the current is the same through all components; the potential difference across each component differs according to its resistance.

• Assuming that current splits in a series circuit.

  Put it right: Current only splits in parallel circuits where there are multiple paths; in a series circuit there is a single path.

Exam tips

• When asked to compare currents in a series circuit, state clearly that the current is the same at all points and give a reason based on conservation of charge.
• If a question gives the current at one point in a series circuit, you can immediately state the current at any other point without calculation.
• Use the correct unit, the ampere (A), and remember that ammeters must be connected in series to measure current.