Question
A student sets up a circuit that includes a 12 V battery, an 800 Ω resistor, a voltmeter and a thermistor. Fig. 8.1 is an incomplete circuit diagram because the symbol for the thermistor is missing.
The thermistor is connected between terminals P and Q.
(a) Complete Fig. 8.1 by drawing the symbol for a thermistor between terminals P and Q.
(b) The 12 V battery consists of eight identical cells connected in series.
Calculate the electromotive force (e.m.f.) of each cell.
e.m.f. =
(c) The reading on the voltmeter is 8.0 V.
(i) Determine the resistance of the thermistor.
resistance = ……………………………………………………………..
(ii) A few hours later, the student notices that the reading on the voltmeter is greater.
Explain what can be deduced from this observation.
Answer/Explanation
Answer:
(a)
(b) 1.5 V c.a.o.
(c)(i) 1600 Ω
(\(V_{800 Ω}\) =) 4.0 (V)
(I =) V / R in any form or 4.0 / 800 or 0.0050 (A) or ( R =) V / I or 8.0 / 0.0050
OR
1600 Ω
\((V_{800 Ω}\) =) 4.0 (V)
(\(R_{Th}\) =) \(R_{800 Ω}\) × \(V_{Th}\) / \(V_{800 Ω}\) in any form or (\(R_{Th}\) =) 800 × 8.0 / 4.0 in any form
OR
1600Ω 
(c)(ii) larger proportion of the e.m.f. (across thermistor) or smaller voltage across 800Ω
temperature (of thermistor) is smaller / has decreased
resistance of thermistor / circuit is large(r)
Question
(a) The electrical energy produced by a power station is transmitted over long distances at a very high voltage.
Explain why a very high voltage is used.
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(b) Fig. 10.1 represents a transformer.
(i) The primary coil P has 4000 turns and an input of 120 V. The secondary coil S has an output of 9.0 V.
Calculate the number of turns in the secondary coil.
number =…………………………………………………………………………………………………………
(ii) State a suitable material for the core of the transformer.
……………………………………………………………………………………………………………………….
Answer/Explanation
Answer:
(a) If voltage is (very) high, current is (very) low NOT if resistance is low
(If current is low,)thermal energy generated / power loss is low
(If current is low:) thinner / lighter / cheaper transmission cables / cables with less resistance / cheaper pylons can be used / cheaper
(b)(i) \(V_p\) / \(V_s\) = \(N_p\) / \(N_s\) in any form OR (\(N_s\) =) \(N_p\) \(V_s\) / \(V_p\) OR 4000 × 9 / 120
(\(N_s\) = ) 300
(ii) Iron or soft iron