A-LevelPhysicsAlternating currents9702/41 · Oct/Nov 2017 · Q106 Marks
The mean value of an alternating current is zero. Explain why heating occurs when there is an alternating current in a resistor.
- (a)The mean value of an alternating current is zero. Explain why heating occurs when there is an alternating current in a resistor.[2]
- (b)(i)Transmission of electrical energy is frequently achieved using alternating high voltages. Suggest why high voltages are used,[2]
- (b)(ii)the voltage is alternating.[2]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.
- [B1] heating depends on current²/I²
- [B1] and current²/I² is always positive
- [B1] a.c. changes direction (every half cycle)
- [B1] but heating effect is independent of current direction
- [B1] voltage and current are always in phase in a resistor
- [B1] so V × I is always positive
- [B1] sketch graph drawn showing power against time
- [B1] comment that power is always positive
- [B1] for same power (transmission, higher voltage) → lower current
- [B1] lower current → less power loss in (transmission) cables
- [B2] voltage can be (easily) stepped up/down
- [B2] transformers only work with a.c.
- [B2] generators produce a.c.
- [B2] easier to rectify than invert
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