Practise Direct and alternating potential difference. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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State the frequency of the mains electricity supply in the UK.
50 Hz
State what is meant by a frequency of 50 Hz for the mains supply.
The potential difference goes through 50 complete cycles every second.
State whether a torch battery provides a direct or an alternating potential difference.
Direct
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Electrical appliances can be powered by batteries or by the mains supply.
(a) What type of potential difference does a battery provide? Tick (✓) one box.[1]
alternating
direct
mains
(b) State the frequency of the mains electricity supply in the UK.[1]
(c) State the potential difference of the mains electricity supply in the UK.[1]
(d) Describe the difference between a direct potential difference and an alternating potential difference.[2]
Show the answer and mark scheme
(a)Answer: direct
(b)Answer: 50 Hz
50 (Hz)
(c)Answer: 230 V
(about) 230 (V)
(d)Answer: A direct pd is always in the same direction, so the current flows in one direction only; an alternating pd keeps reversing direction, so the current keeps changing direction.
direct: (the pd / current is) in one direction only
alternating: (the pd / current) changes direction / reverses repeatedly
Question 2Medium6 marks
The mains electricity supply in the UK is an alternating supply with a frequency of 50 Hz.
(a) Explain what is meant by an alternating potential difference with a frequency of 50 Hz.[2]
(b) Calculate the time taken for one complete cycle of the mains supply. Give your answer in milliseconds. Use the Physics Equations Sheet.[2]
(c) Which two of these provide a direct potential difference? Tick (✓) two boxes.[2]
a battery
a mains socket
a solar cell
the National Grid
Show the answer and mark scheme
(a)Answer: The potential difference goes through 50 complete cycles every second, reversing direction twice in each cycle (100 times per second).
the potential difference completes 50 cycles every second
the potential difference / current reverses direction (twice every cycle / 100 times per second)
(b)Answer: 20 ms
T = 1 ÷ 50 (= 0.020 s)
T = 20 (ms)
(c)Answer: a battery; a solar cell
Question 3Hard6 marks
A torch is powered by a 4.5 V battery. A desk lamp is powered by the UK mains electricity supply.
Compare the potential difference supplied by the battery with the potential difference supplied by the mains. Your answer should include a sketch graph of potential difference against time for each supply.[6]
Show the answer and mark scheme
Answer: The battery gives a direct pd of about 4.5 V, always in the same direction, so its graph is a horizontal line and the current is always in one direction. The mains gives an alternating pd of about 230 V that keeps reversing direction, 50 cycles per second (period 0.02 s), so its graph is a wave above and below the time axis and the current keeps changing direction.
the battery supplies a direct potential difference: it is always in the same direction
the battery's potential difference is (almost) constant at about 4.5 V, so its graph is a horizontal straight line
the current from the battery is always in the same direction (direct current)
the mains supplies an alternating potential difference: its direction keeps reversing
the mains potential difference is about 230 V, much larger than the battery's
the mains has a frequency of 50 Hz, so one cycle takes 0.02 s
the graph for the mains is a repeating wave, going above and below the time axis
the current from the mains keeps changing direction (alternating current)
Marked with levels of response: the full level descriptors are in the app.