Electric circuits
Circuit diagrams, series and parallel circuits, combined resistance, and action of components.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 4.3.1Draw and interpret circuit diagrams using standard symbols.
- 4.3.2Describe how current and potential difference are distributed in series and parallel circuits.
- 4.3.3Calculate the combined resistance of resistors in series and in parallel.
- 4.3.4Describe the action of a thermistor and a light-dependent resistor in a sensing circuit.Supplement
8 minute read
Series and parallel circuits
Circuit diagrams
Circuits are drawn using standard symbols: a cell (long and short line), a resistor (rectangle or zigzag), an ammeter (A in a circle, always in series), a voltmeter (V in a circle, always in parallel with the component it measures), a switch, a lamp, a diode, a thermistor and an LDR.
Series circuits
In a series circuit, there is only one path for current, so:
- The current is the same at every point in the circuit.
- The supply potential difference is shared between the components, in proportion to their resistance.
- Total resistance:
R = R₁ + R₂ + …— adding a resistor in series always increases total resistance.
Parallel circuits
In a parallel circuit, there are multiple paths, so:
- The potential difference is the same across every branch.
- The current splits between the branches, and the branch currents add up to the total current.
- Total resistance:
1/R = 1/R₁ + 1/R₂ + …— adding a resistor in parallel always decreases total resistance, and the total is always less than the smallest individual resistor.
Sensing components
A thermistor's resistance falls as temperature rises, so it is used in temperature-sensing circuits (e.g. thermostats). An LDR (light-dependent resistor)'s resistance falls as light intensity increases, so it is used in light-sensing circuits (e.g. automatic street lights).
Think of it like this
A series circuit is like a single-lane road — everyone travels at the same rate (current) because there is nowhere to overtake. A parallel circuit is like several separate lanes to the same destination — traffic (current) splits between them, but every lane experiences the same road conditions (potential difference).
Worked examples
Method, step by step
Two resistors of 4.0 Ω and 12 Ω are connected in parallel. Calculate the combined resistance.
- 11/R = 1/4.0 + 1/12
- 21/R = 3/12 + 1/12 = 4/12 = 1/3
R = 3.0 Ω
Common misconceptions
- Believing current is "used up" as it passes around a series circuit. Current is the same at every point in a single loop; it is energy, not current, that is transferred to the components.
- Thinking adding a resistor in parallel increases total resistance, by analogy with series. It is the opposite — an extra parallel path always reduces total resistance.
- Placing an ammeter in parallel or a voltmeter in series by mistake. An ammeter must be in series with the component; a voltmeter must be in parallel with it.
In the exam
- When calculating parallel resistance, always check your final answer is smaller than the smallest individual resistor — this catches the common mistake of forgetting to invert 1/R at the end.
- For sensing-circuit questions, explain the resistance change first, then the consequence for current or voltage in the circuit — both parts are usually needed for full marks.