Grade 10 Physical Sciences · Electricity and Magnetism

Apply Ohm's law

Use R = V/I to find current, voltage or resistance (a 12 V battery across a 4 Ω resistor gives 3 A).

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Question 8.2 · November 2025 · Paper 1 · National (DBE) · 3 marks

Two identical headlights,

L_{1}

and

L_{2}

, and a starter motor, M, of a car are connected to a battery, as shown in the circuit diagram below. The resistance of each headlight is

12 Ω

, while the resistance of the starter motor is

0, 1 Ω

. The emf (

ε

) and internal resistance (r) of the battery are unknown.

The ammeters and the connecting wires have negligible resistance, while the voltmeter has a high resistance. Switches

S_{1}

and

S_{2}

are initially open. Diagram for this question

Switch

S_{1}

remains open while switch

S_{2}

is closed.

The reading on ammeter

A_{2}

120

8.2 Calculate the reading on the voltmeter.(3)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: apply ohm's law

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3. Question 9.2.3 · May/June 2025 · Paper 1 · National (DBE) · 3 marks
9.2 An AC generator produces a maximum output voltage of $340$ V. A $1200$ W electric kettle operates optimally when it is connected to this generator.

Answer the questions that follow.
9.2.3 Calculate the resistance of the kettle.(3)
Show the memo (3 ticks)Memo
1. ✓
  METHOD 1: Formula $R = \frac{V _{r m s}}{I _{r m s}}$
  1 mark · Apply Ohm's law
2. ✓
  METHOD 1: Substitution $= \frac{240 , 42}{4 , 99}$
  1 mark · Apply Ohm's law
3. ✓
  METHOD 1: Answer $R = 48, 18 Ω$
  1 mark · Apply Ohm's law
4. Question 8.2.1 · May/June 2025 · Paper 1 · National (DBE) · 3 marks
In the circuit diagram below, the battery has an emf of $24$ V and an internal resistance of $0, 3 Ω$ . Light bulb L has a resistance of $9, 5 Ω$ and R is a resistor of unknown resistance. Ignore the resistance of the ammeters and the connecting wires.
8.2 When both switches $S_{1}$ and $S_{2}$ are CLOSED, the reading on ammeter $A_{1}$ is $0, 8$ A.

Calculate the:
8.2.1 Current through light bulb L(3)
Show the memo (3 ticks)Memo
1. ✓
  METHOD 1: Potential difference across the 13 Ω resistor $V = I R$
  $V = 0, 8 (13) = 10, 4$ V
  1 mark · Apply Ohm's law
2. ✓
  METHOD 1: Substitution for L $V = I R$
  $10, 4 = I_{L} (9, 5)$
  1 mark · Apply Ohm's law
3. ✓
  METHOD 1: Answer $I_{L} = 1, 09$ A
  1 mark · Apply Ohm's law
5. Question 8.4 · May/June 2024 · Paper 1 · National (DBE) · 3 marks
A battery of emf $12$ V and internal resistance $0, 2 Ω$ is connected to three resistors, a high-resistance voltmeter and two switches, an ammeter and connecting wires of negligible resistance, as shown in the circuit diagram below. The three resistors have different and unknown resistances.

The resistance of $R_{Y}$ is TWICE the resistance of $R_{X}$ .

When both switches are CLOSED, the reading on the ammeter is $5, 5$ A.
When both switches are now OPENED, the reading on the ammeter is $1, 3$ A.
8.4 Calculate the reading on the voltmeter.(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $V = I R$
  1 mark · Apply Ohm's law
2. ✓
  Substitution $V = 1, 3 (5, 95)$
  1 mark · Apply Ohm's law
3. ✓
  Answer with unit $V = 7, 72$ V
  1 mark · Apply Ohm's law
6. Question 8.2.1 · May/June 2023 · Paper 1 · National (DBE) · 3 marks
A battery with unknown emf ( $ε$ ) and unknown internal resistance (r) is connected to three resistors, a high-resistance voltmeter, two switches and two ammeters of negligible resistance, as shown below.
8.2 Both switch S $_{1}$ and switch S $_{2}$ are CLOSED. The reading on ammeter A $_{1}$ is $1, 5$ A.

Calculate the:
8.2.1 Reading on the voltmeter(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $R = \frac{V}{I}$
  1 mark · Apply Ohm's law
2. ✓
  Substitution $7 = \frac{V}{1 , 5}$
  1 mark · Apply Ohm's law
3. ✓
  Answer with unit $V = 10, 5$ V
  1 mark · Apply Ohm's law

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