OxfordAQA International GCSE Physics: Mains electricity, safety, power and the cost of electricity (9203) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sections 3.6.3 (Using electricity in the home) and 3.6.5 (Transferring electrical energy) of the OxfordAQA International GCSE Physics (9203) specification, for exams from May/June 2018. The qualification is untiered. Take the mains pd as 230 V unless a question says otherwise. Read the study guide first if a topic is new, and use the revision notes for recall. See also the OxfordAQA International GCSE Physics hub, the printable checklist and the free diagnostics.

Questions

1. An oscilloscope trace of an ac supply shows 4 complete cycles across a time of 20 ms. Calculate the period and the frequency of the supply. [3]

2. A trace shows a 9.0 V battery and an ac supply on the same screen. The vertical scale is 3.0 V per division. The ac wave reaches 4.0 divisions above the centre line at its highest point.

(a) State two differences between the current from the battery and the current from the ac supply. [2] (b) Calculate the peak pd of the ac supply and state how much greater it is than the battery pd. [2]

3. A toaster has a metal case, a live wire, a neutral wire and an earth wire, with a fuse in the live wire. A fault makes the live wire touch the metal case. Explain how the earth wire and the fuse make the toaster safe to touch. [4]

4. A hair dryer has a plastic body and has no earth wire.

(a) Explain why this is safe. [2] (b) The household circuit also has a circuit breaker. Give two advantages of a circuit breaker over a fuse. [2]

5.

(a) State the main energy transfer in a loudspeaker and in an electric kettle. [2] (b) A drill transfers 54 000 J of energy in 90 s. Calculate its power. [3]

6. An electric iron is rated at 920 W and runs from the 230 V mains. Fuses of 3 A, 5 A and 13 A are available. Calculate the current in the iron and choose a fuse, giving a reason. [5]

7.

(a) A 9.0 V lamp has 3.0 C of charge pass through it. Calculate the energy transferred. [3] (b) A kettle on the 230 V mains transfers 138 000 J. Calculate the charge that passes through it. [3]

8. A 60 W lamp is left on for 5.0 hours every day for 30 days. Electricity costs 30 p per kWh. Calculate the energy transferred in kWh and the cost, in pounds. [5]

9. An electricity meter reads 62581 kWh at the start of a month and 62915 kWh at the end. Electricity costs 30.0 p per kWh. Calculate the cost for the month, in pounds. [4]

10. An electric shower is rated at 8.8 kW on the 230 V mains.

(a) Calculate the current in the shower. [2] (b) Explain why a 13 A fuse is not suitable for protecting the shower circuit. [1] (c) Calculate the energy transferred in joules during a shower lasting 8.0 minutes. [3] (d) Calculate the energy in kWh for the same shower and its cost at 30 p per kWh. [2]

11. A room heater is rated at 2.0 kW. It is used for 3.0 hours each day for 7 days. Electricity costs 28.0 p per kWh. Fuses of 3 A, 5 A and 13 A are available.

(a) Calculate the current in the heater on the 230 V mains. [2] (b) Choose a suitable fuse. [1] (c) Calculate the energy transferred in the week, in kWh. [2] (d) Calculate the cost for the week, in pounds. [2]

Answers

1. Time for one cycle = 20 ms / 4 = 5.0 ms [1]. Period = 5.0 ms = 0.0050 s [1]. Frequency = 1 / T = 1 / 0.0050 = 200 Hz [1]. [3] Examiner insight: Convert milliseconds to seconds before using f = 1 / T, and divide by the number of cycles before you quote a period.

2. (a) The battery current always flows in the same direction (direct current) [1]. The ac current repeatedly changes direction [1]. (b) Peak pd = 4.0 × 3.0 = 12 V [1]. Difference = 12 − 9.0 = 3.0 V greater than the battery [1]. [4] Examiner insight: A comparison needs both supplies mentioned. Saying only “ac changes direction” leaves the battery behaviour unstated.

3. The earth wire joins the metal case to earth [1]. When the live wire touches the case, a large current flows through the earth wire to earth instead of through the person [1]. The large current makes the fuse wire in the live wire overheat and melt [1]. This disconnects the toaster from the live supply, so the case is no longer live [1]. [4] Examiner insight: Give the sequence in order: fault, large current, fuse melts, supply disconnected. A list of safety words without that chain earns little.

4. (a) The plastic body and internal construction are double insulated, so the user cannot touch any live part [1]. Because the metal parts cannot become live, no earth wire is needed [1]. (b) It works faster than a fuse [1]. It can be reset instead of being replaced [1]. [4] Examiner insight: “Safer” on its own is too vague. Name the specific advantage, such as speed or resetting.

5. (a) Loudspeaker: electrical to sound energy [1]. Kettle: electrical to thermal energy [1]. (b) P = E / t [1]. P = 54 000 / 90 [1]. P = 600 W [1]. [5] Examiner insight: Write the energy forms in full. “Electricity to heat” is weaker than electrical to thermal. Include the unit in the final answer.

6. I = P / V [1]. I = 920 / 230 [1]. I = 4.0 A [1]. The 5 A fuse is suitable [1] because it is the smallest rating above the 4.0 A working current, so it will not melt in normal use but will protect the wiring [1]. [5] Examiner insight: A fuse choice needs a reason that mentions the working current. Naming 5 A alone does not show the link to the calculated 4.0 A.

7. (a) E = Q × V [1]. E = 3.0 × 9.0 [1]. E = 27 J [1]. (b) Q = E / V [1]. Q = 138 000 / 230 [1]. Q = 600 C [1]. [6] Examiner insight: Rearrange before substituting in part (b). Writing the rearranged form shows the method even if the arithmetic slips.

8. Power = 60 W = 0.060 kW [1]. Time = 5.0 × 30 = 150 h [1]. Energy = 0.060 × 150 = 9.0 kWh [1]. Cost = 9.0 × 30 = 270 p [1]. Cost = £2.70 [1]. [5] Examiner insight: Show the conversion from watts to kilowatts as its own line. A final answer in pence when pounds are requested loses the last step.

9. Units used = 62915 − 62581 = 334 kWh [1]. Cost = 334 × 30.0 [1] = 10 020 p [1]. Cost = £100.20 [1]. [4] Examiner insight: Subtract the earlier reading from the later one, then multiply. Giving a number of pence without converting to pounds does not answer the question.

10. (a) I = P / V = 8800 / 230 [1]. I = 38 A (38.3 A) [1]. (b) The current of about 38 A is far greater than 13 A, so a 13 A fuse would melt as soon as the shower was switched on [1]. (c) Time = 8.0 × 60 = 480 s [1]. E = P × t = 8800 × 480 [1]. E = 4 224 000 J (about 4.2 MJ) [1]. (d) E = 8.8 × (8.0 / 60) = 1.17 kWh [1]. Cost = 1.173 × 30 = 35 p (35.2 p) [1]. [8] Examiner insight: Pick the unit system first and stay in it. Use watts with seconds for joules, or kilowatts with hours for kWh, and do not mix them in one line.

11. (a) I = P / V = 2000 / 230 [1]. I = 8.7 A [1]. (b) 13 A fuse, because 8.7 A is above 5 A [1]. (c) Time = 3.0 × 7 = 21 h [1]. E = 2.0 × 21 = 42 kWh [1]. (d) Cost = 42 × 28.0 = 1176 p [1]. Cost = £11.76 [1]. [7] Examiner insight: Part (b) follows from your part (a) answer. State the working current in your reason so the link to the fuse rating is clear.

Where marks are usually lost

• Using trough-to-peak instead of the zero line for peak pd on an ac trace.
• Leaving the period in milliseconds when using f = 1 / T.
• Describing safety features as “stopping electricity” without the sequence of fault, large current, fuse melts, supply disconnected.
• Placing the fuse in the wrong wire, or forgetting that it goes in the live wire.
• Choosing a fuse that equals the working current, or giving no reason for the choice.
• Treating double insulation as having an earth wire hidden inside.
• Mixing kW with seconds or W with hours in the same calculation.
• Forgetting to multiply the hours by the number of days.
• Giving the cost in pence when the question asks for pounds.

Next steps

• Revise with the revision notes.
• Re-read the study guide.
• Visit the course hub and the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Physics (9203). For exams May/June 2018 onwards. Sections 3.6.3 and 3.6.5. Published by OxfordAQA.