Electrical measurements: circuits, meters and components

Drawing the circuit

• Use standard circuit symbols; labelled boxes are not accepted.
• Ammeter in series with the component; voltmeter in parallel across exactly the component you are studying (not across a protective resistor too).
• Include the supply, a switch and a means of varying the current (a variable resistor or variable supply).
• Do not add components the question does not need: an extra component in the wrong place can stop the circuit working and lose the mark.

Resistors with a job

• A fixed protective resistor in series limits the maximum current, protecting the cell or component from overheating or a short circuit.
• A variable resistor lets you vary the current. In some experiments it is used to keep a p.d. constant while something else changes.
• Check the power rating: the power dissipated, P = I^2R = V²/R = IV, must be less than the rating, or the resistor overheats.

Good practice

• Digital meters: the resolution is the last digit; choose the smallest range that fits.
• Switch off between readings so components do not heat up (resistance changes with temperature; a cell's e.m.f. and internal resistance drift).
• Check meters read zero with no current (zero error).
• Use clean contacts: contact resistance adds a constant to resistance readings.

Worked example

A 6.0 V supply is to drive at most 50 mA through a component of negligible resistance. Choose a protective resistor.

1. Minimum resistance: R = V / I = 6.0 / 0.050 = 120 Ω
2. Power dissipated at that current: P = IV = 0.050 x 6.0 = 0.30 W
3. So choose at least 120 Ω with a power rating above 0.30 W (for example 0.5 W).

Answer: A resistor of 120 Ω or more, rated above 0.30 W

Check yourself

1. Where should the voltmeter be connected to find the resistance of a component in series with a protective resistor?
   • Across the component and the resistor together
   • In series with the component
   • Across the supply
   • Across the component only
   Show answer

   Across the component only. The voltmeter must measure the p.d. across the component being studied.

2. A 9.0 V supply must not drive more than 0.030 A through a circuit. What is the minimum series resistance in Ω?
   Show answer

   300 Ω. R = V / I = 9.0 / 0.030 = 300 Ω.

3. A 47 Ω resistor carries a current of 0.12 A. Find the power dissipated in W, to 2 s.f.
   Show answer

   0.68 W. P = I^2R = 0.0144 x 47 = 0.677 W.

4. A resistor will dissipate 0.68 W. Which power rating should it have?
   • 1 W
   • 0.25 W
   • 0.5 W
   • 0.6 W
   Show answer

   1 W. The rating must exceed the power dissipated; only 1 W does.

5. What is the purpose of a fixed resistor in series with a cell in core practical 8?
   • To keep the p.d. constant
   • To measure the current
   • To limit the current so the cell is not damaged or overheated
   • To increase the e.m.f.
   Show answer

   To limit the current so the cell is not damaged or overheated. Without it, reducing the variable resistance to zero would short-circuit the cell.

6. A voltmeter reads 2.46 V across a wire and an ammeter reads 0.315 A. Find the resistance in Ω, to 3 s.f.
   Show answer

   7.81 Ω. R = 2.46 / 0.315 = 7.8095, so 7.81 Ω.

7. Why might an examiner give no credit for a circuit drawn with labelled boxes for each component?
   • Boxes are too small
   • Labels are not allowed in exams
   • Standard circuit symbols are required
   • Boxes imply the components are in parallel
   Show answer

   Standard circuit symbols are required. The mark scheme requires recognised circuit symbols.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Pearson examiners' report: IAL Physics WPH13/01, January 2024
• Pearson examiners' report: IAL Physics WPH13/01, October 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.