ALTERNATING CURRENT AND DIRECT CURRENT
3 marks · NormalThe graph below shows how the voltage changes with time for an alternating current (AC) supply and a direct current (DC) supply.
Describe the main difference between the two graphs.
State which graph represents the supply from a cell.
Give one reason why the mains supply uses AC rather than DC.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) AC voltage changes with time and reverses direction; DC voltage is constant.
(b) DC graph represents the supply from a cell.
(c) Because AC can be transformed to different voltages using transformers / is easier to transmit over long distances.
Must show: AC: wave crossing zero, DC: flat horizontal line
Also accept: Part a: AC varies with time, DC remains steady.; Part b: DC.; Part c: Accept 'AC can be easily transformed to different voltages' or 'easier to transmit over long distances'
Common mistakes: Mixing up AC and DC graph identification.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Describes AC voltage as changing/reversing and DC as constant/steady.
B2 (b) · 1 mark · Correctly identifies DC as the supply from a cell.
B3 (c) · 1 mark · Gives plausible reason for AC use (e.g., can be easily transformed, easier transmission).
Worked solution
a) The AC graph is a wave and crosses zero, showing reversal and varying voltage. The DC graph is a horizontal line, showing a constant voltage.
b) The DC graph represents the cell supply.
c) AC is used for mains supply because it is easier to transform to higher or lower voltages for efficient transmission.