A-LevelPhysicsD.C. circuits9702/22 · May/June 2017 · Q710 Marks

Define electromotive force (e.m.f.) of a cell.

(a)Define electromotive force (e.m.f.) of a cell.[1]
(b)(i)The resistance of Y is varied from 0 to 8.00Ω. State and explain the variation in the potential difference (p.d.) between points A and B (terminal p.d. across C). Numerical values are not required.[3]
(b)(ii) 1.the resistance of X,[2]
(b)(ii) 2.the p.d. between points A and B,[2]
(b)(ii) 3.the efficiency of the cell.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[B1] energy transformed from chemical to electrical/unit charge (driven around a complete circuit)
[M1] the current decreases (as resistance of Y increases)
[M1] lost volts go down (as resistance of Y increases)
[A1] p.d. AB increases (as resistance of Y increases)
[C1] 1.50 = 0.180 × (6.00 + 0.200 + Rx)
[A1] Rx = 2.1(3) Ω
[C1] p.d. AB = 1.5 – (0.180 × 0.200) or 0.18 × (2.13 + 6.00)
[A1] = 1.46(4) V
[C1] efficiency = (useful) power output / (total) power input or IV/ IE
[A1] (= 1.46/1.5) = 0.97 [0.98 if full figures used]

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