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Edexcel IGCSE Physics revision

Energy and voltage in circuits questions

Trace current paths, compare potential differences, and link circuit measurements to resistance, power and energy transfer.

Edexcel IGCSE Physics Subtopic 2.c

Energy and voltage in circuits syllabus

Questions on energy and voltage in circuits can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

  • 2.7 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting
  • 2.8 understand how the current in a series circuit depends on the applied voltage and the number and nature of other components
  • 2.9 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how to investigate this experimentally
  • 2.10 describe the qualitative effect of changing resistance on the current in a circuit
  • 2.11 describe the qualitative variation of resistance of light-dependent resistors (LDRs) with illumination and thermistors with temperature
  • 2.12 know that lamps and LEDs can be used to indicate the presence of a current in a circuit
  • 2.13 know and use the relationship between voltage, current and resistance: voltage = current × resistance V = I × R
  • 2.14 know that current is the rate of flow of charge
  • 2.15 know and use the relationship between charge, current and time: charge = current × time Q = I × t
  • 2.16 know that electric current in solid metallic conductors is a flow of negatively charged electrons
  • 2.17 understand why current is conserved at a junction in a circuit
  • 2.18 know that the voltage across two components connected in parallel is the same
  • 2.19 calculate the currents, voltages and resistances of two resistive components connected in a series circuit
  • 2.20 know that: voltage is the energy transferred per unit charge passed the volt is a joule per coulomb
  • 2.21 know and use the relationship between energy transferred, charge and voltage: energy transferred = charge × voltage E = Q × V

How to answer energy and voltage in circuits questions

  1. Identify whether the circuit is series or parallel before combining currents, voltages or resistances.
  2. Draw standard circuit symbols cleanly and place ammeters in series and voltmeters in parallel.
  3. Show unit conversions, especially milliamps, kilohms, kilowatt-hours and seconds.
  4. In explanations, follow charge or energy through the circuit rather than only naming a rule.

Energy and voltage in circuits example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Completing a circuit-symbol table

June 2017 (R) · Paper 1P · Question 4(a)

Question 1

The table shows the names and circuit symbols of some electrical components.

Complete the table by giving the missing information.

Component Circuit symbol
fixed resistor
variable resistor
________
lamp
________

Final answer

The four missing entries are shown below. Either lamp symbol is acceptable.

ComponentCircuit symbol
variable resistor
cell
lamp

  • Alternative 1: circle containing a cross.

  • Alternative 2: circle containing a curved filament.
fuse or circuit breaker

Mark scheme points

  1. M1 Draw a resistor rectangle crossed by a diagonal arrow. The arrow may point in either direction.
  2. M2 Name the component with one long and one short parallel line: cell.
  3. M3 Draw the lamp as a circle containing a cross, or use the accepted curved-filament symbol.
  4. M4 Name the rectangle with a line through it: fuse or circuit breaker. Either name earns the mark.

Connecting wires at each side of the symbols are optional.

Explanation

  • For the variable resistor, the arrow indicates that its resistance can be adjusted. Check that the arrow crosses the rectangle diagonally.
  • A single pair of unequal parallel lines represents a cell. The longer line represents the positive terminal.
  • For the lamp, draw a clear circle with either a cross or the curved filament inside. Only one of these symbols is needed.
  • The horizontal line passing through the rectangle distinguishes the fuse symbol from the fixed-resistor symbol. A fuse protects a circuit by breaking it if the current becomes too large.

Common mistakes

  • Drawing a horizontal or vertical arrow on the variable resistor. The arrow must be diagonal and have an arrowhead.
  • Writing only “battery” for the single pair of unequal lines. This wording earns no naming mark here; write cell.
  • Calling the final symbol a fixed resistor. Look for the horizontal line running through the rectangle: this is the fuse symbol.

Example 2: Comparing series and parallel resistance

June 2022 (R) · Paper 1P · Question 6(c)

Question 2

Diagram 1 shows a simple loudspeaker.

The coil is connected to an alternating current (a.c.) supply.

Diagram 1 of a simple loudspeaker showing a magnet, a paper cone, a tube and a coil on the tube.

Diagram 3 shows two loudspeakers connected in series to a cell. Diagram 4 shows the same loudspeakers connected in parallel to the same cell.

Diagram 3: two loudspeakers in series with a 1.5 V cell and total current 0.30 A. Diagram 4: the same loudspeakers in parallel with a 1.5 V cell and total current 1.2 A.

Comment on how the total resistance of the loudspeakers in diagram 3 compares with the total resistance of the loudspeakers in diagram 4.

Final answer

The total resistance in the parallel arrangement (diagram 4) is one quarter of the total resistance in the series arrangement (diagram 3). In series, the cell voltage is shared between the two loudspeakers; in parallel, each loudspeaker has the full cell voltage, so the current in each loudspeaker doubles. The total current from the cell is therefore four times greater, from 0.30 A to 1.2 A.

R(series) = 1.5 V / 0.30 A = 5.0 Ω
R(parallel) = 1.5 V / 1.2 A = 1.25 Ω
1.25 / 5.0 = 0.25

Mark scheme points

Any FOUR of these five points are sufficient for full marks.

  1. M1 In diagram 4, the cell voltage is no longer shared between the loudspeakers; each has the full cell voltage.
  2. M2 The current through each loudspeaker doubles compared with the series arrangement.
  3. M3 The total current from the supply is four times greater.
  4. M4 Use total resistance = voltage ÷ current, or R = V/I.
  5. M5 The total resistance in parallel is one quarter of the total resistance in series.

Explanation

In the series circuit, the two identical loudspeakers share the 1.5 V supply. In the parallel circuit, each loudspeaker has the full 1.5 V across it. The voltage across each speaker therefore doubles, so the current in each speaker doubles. The two branch currents add to give the current from the cell, making the total current four times the series current.

For the whole circuit, resistance is voltage divided by total current. Since the supply voltage is the same but the total current is four times greater in parallel, the total resistance is one quarter as large.

Common mistakes

  • The examiner report noted that misconceptions about these circuits made this question challenging. Start with the given voltage and total current, then use R = V/I to compare the total resistances.
  • Do not use the current in just one parallel branch as the total current from the cell; the branch currents add together.

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