Grade 11 Physical Sciences · Electricity and Magnetism

Calculate electrical power and energy

Calculate electrical power with P = VI, P = I²R or P = V²/R, or energy used and its cost in kWh (a 2 kW kettle for 1 hour).

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Grade 11 Electricity: Power (electrical power)Miss Martins Maths and Science

Cost Electricity Grade 11Kevinmathscience

Cost of Electricity Grade 11 Physics (Electric circuits)Miss Martins Maths and Science

Electric Power and The Cost of Electricity Using kWhThe Organic Chemistry Tutor

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Question 1.8 · November 2024 · Paper 1 · National (DBE) · 2 marks

1.8 The kilowatt-hour (kWh) is a unit of ...(2)

A power.
B electric current.
C electrical energy.
D potential difference.

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

More past paper questions: calculate electrical power and energy

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3. Question 8.3.1 · 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.
8.3 Switch $S_{1}$ is now closed and switch $S_{2}$ is opened. The power dissipated by each headlight is $15$ W.

Answer the questions that follow.
8.3.1 Calculate the current passing through $L_{1}$ .(3)
Show the memo (3 ticks)Memo
1. ✓
  METHOD 1: Formula $P = I^{2} R$
  1 mark · Calculate electrical power and energy
2. ✓
  METHOD 1: Substitution $15 = I^{2} (12)$
  1 mark · Calculate electrical power and energy
3. ✓
  METHOD 1: Answer with unit $I = 1, 12$ A
  1 mark · Calculate electrical power and energy
4. Question 8.2.1 · November 2024 · Paper 1 · National (DBE) · 3 marks
$L_{1}$ and $L_{2}$ are two light bulbs that have the following ratings:

$L_{1}$ : $36$ W ; $20$ V and $L_{2}$ : $48$ W ; $32$ V

The two bulbs are connected as shown in the circuit diagram below. The battery has an internal resistance of $0, 6 Ω$ while the conducting wires and the ammeters have negligible resistance. $R_{1}$ and $R_{2}$ are resistors.
8.2 If both light bulbs operate as RATED, calculate the:
8.2.1 Reading on ammeter $A_{2}$ (3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $P = V I$
  1 mark · Calculate electrical power and energy
2. ✓
  Substitution $48 = 32 I$
  1 mark · Calculate electrical power and energy
3. ✓
  Answer $I = 1, 5$ A
  1 mark · Calculate electrical power and energy
5. Question 9.2.3 · November 2023 · Paper 1 · National (DBE) · 3 marks
9.2 The circuit diagram below shows an electric kettle and a toaster connected to an AC source with an rms voltage of $220$ V. The ammeter, connecting wires and switches S $_{1}$ and S $_{2}$ have negligible resistance.

Answer the questions that follow.
When switch S $_{1}$ is OPEN and switch S $_{2}$ is CLOSED, the root mean square current in the circuit is $2, 62$ A.
9.2.3 Calculate the energy consumed by the toaster in two minutes.(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $W = V I Δ t$
  1 mark · Calculate electrical power and energy
2. ✓
  Substitution $= (220) (2, 62) (120)$
  1 mark · Calculate electrical power and energy
3. ✓
  Answer with unit $= 69168$ J ( $6, 92 \times 1 0^{4}$ J)
  1 mark · Calculate electrical power and energy
6. Question 8.2.3 · 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:
8.2.3 Power dissipated in the $3 Ω$ resistor(3)
Show the memo (3 ticks)Memo
1. ✓
  Formula $P = \frac{V ^{2}}{R}$
  1 mark · Calculate electrical power and energy
2. ✓
  Substitution $= \frac{10 , 5 ^{2}}{3}$
  1 mark · Calculate electrical power and energy
3. ✓
  Answer with unit $= 36, 75$ W
  1 mark · Calculate electrical power and energy

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