4.2.3.1Direct and alternating potential difference
AQA GCSE Physics Foundation (8463), Foundation tier · Electricity › Domestic uses and safety
Practise Direct and alternating potential difference. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The difference between direct and alternating potential difference, and the values for the UK mains supply. Expect short recall questions (50 Hz, about 230 V) and 2-mark 'explain the difference' questions, sometimes with a graph of potential difference against time.
Key facts
dc: pd (and current) in one direction only
ac: pd (and current) keeps reversing direction
Cells and batteries supply dc
UK mains: ac, 50 Hz, about 230 V
Frequency = number of complete cycles per second, in hertz (Hz)
Notes
Direct potential difference (dc)
A direct potential difference always acts in the same direction, so the current flows in one direction only.
Cells and batteries supply a direct pd: the positive terminal stays positive and the negative terminal stays negative.
On a graph of pd against time, a steady dc supply is a horizontal straight line.
Alternating potential difference (ac)
diagram
An alternating potential difference keeps reversing direction, so the current keeps changing direction.
On a graph of pd against time, ac is a wave: the pd rises to a positive peak, falls through zero to a negative peak, and repeats.
dc: steady, one direction. ac: keeps reversing, repeating every cycle.
The frequency is the number of complete cycles each second, measured in hertz (Hz).
The UK mains supply
Mains electricity is an ac supply.
In the UK the domestic supply has a frequency of 50 Hz and a potential difference of about 230 V.
50 Hz means 50 complete cycles every second. Each cycle has two changes of direction, so the mains reverses direction 100 times a second.
How to answer each type of question
Recall the mains values
1 mark eachGrade 3
Learn the two numbers: 50 Hz and about 230 V.
Give the unit with each number.
Example. (a) State the frequency of the UK mains electricity supply. (b) State the potential difference of the UK mains electricity supply.
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(a) 50 Hz (1) (b) About 230 V (1)
Explain the difference between direct and alternating pd
2 marksGrade 5
Write one sentence about dc: acts in one direction only.
Write one sentence about ac: keeps reversing (changing) direction.
Link each to the current if you can: the current is one way, or keeps changing direction.
Example. A torch uses a battery. A kettle uses the mains supply. Explain the difference between the potential difference supplied by the battery and the potential difference supplied by the mains.
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The battery supplies a direct pd, which acts in one direction only, so the current is always in the same direction (1). The mains supplies an alternating pd, which keeps reversing direction, so the current keeps changing direction (1).
Don’t lose marks
Saying ac 'changes size' or 'turns on and off'. The key idea is that it changes direction.
Swapping the values: the mains is 230 V and 50 Hz, not 50 V and 230 Hz.
Saying batteries supply ac.
Reading the time for half a cycle from a graph and using it as the time period.
More tips
Memory tricks
AC = Alternating = keeps changing direction. DC = Direct = one Direction.
Mains numbers: '230 volts, 50 hertz'. The bigger number is the volts.
One cycle has two reversals, so 50 Hz means 100 changes of direction per second.
Exam technique
In 'explain the difference' questions, describe BOTH types, one sentence for each.
Use the word 'direction': examiners look for 'one direction' and 'changes / reverses direction'.
Always give units: V for potential difference and Hz for frequency.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that mains electricity is acThe mains supply is an alternating current (ac) supply.
Grade 3
Recall the UK mains frequency and pdThe UK domestic supply has a frequency of 50 Hz and is about 230 V.
Grade 4
Name a source of direct pdCells and batteries supply a direct potential difference.
Grade 5
Explain the difference between dc and acA direct pd acts in one direction only; an alternating pd keeps reversing direction.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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)