Question
Fig. 9.1 shows a graph of current against potential difference (p.d.) for a filament lamp
(a) State what happens to the resistance of the filament of the lamp as the p.d. changes
(i) from 0 V to 1.0 V,
ii) from 1.0 V to 8.0 V.
b) At normal brightness, the p.d. across the lamp is 8.0 V.
Calculate, for normal brightness,
(i) the resistance of the lamp
resistance =
(ii) the power of the lamp.
power =
(c) Five of these lamps, operating at normal brightness, are connected in parallel to a power
supply.
Determine
(i) the electromotive force (e.m.f.) of the power supply,
e.m.f. =
(ii) the current from the power supply.
current =
Answer/Explanation
Answer
(a)(i) Resistance constant
(ii) Resistance increases
(b)(i) I = V/R in any form OR (R=) V/I
8.0/0.72
11 Ω
(ii) (P = ) IV OR 0.72 × 8.0
5.8 W
OR \(I^2\)R OR \(0.72^2\)candidate’s (b)(i)
OR \(V^2\)
/R OR \(8^2\) / candidate’s (b)(i)
5.7 W or 5.8 W (dependent on exact data used)
(c)(i) 8.0V
(ii) (5 × 0.72 =) 3.6 A
Question
(a) The resistance of a circuit component varies with the brightness of the light falling on its surface.
i. State the name of the component.
ii. Draw the circuit symbol for this component.
(b) Fig. 9.1 shows a 6.0 V battery connected in series with a 1.2 k Ω resistor and a thermistor.
(i) At a certain temperature, the resistance of the thermistor is 2.4 kΩ.
Calculate the reading on the voltmeter.
voltmeter reading =
(ii) The battery connected to the circuit in Fig. 9.1 is not changed.
Suggest a change that would cause the reading of the voltmeter to decrease.
Answer/Explanation
Answer:
(a)(i) LDR OR light-dependent resistor
(a) (ii) 
(b)(i) I = V/R
(total resistance =) 1.2 + 2.4 OR 3.6 seen
I = 6.0 /(1.2 + 2.4) OR 1.67 or 1.7 (mA)
(V =) 4.0 V
OR
\((V^1 )\)= \([R_1/(R_1 +R_2)]\) V
(total resistance =) 1.2 + 2.4 OR 3.6 seen
(\(V_1\) ) = (2.4 /3.6) 6.0
= 4.0 V
(b) (ii) Replace the 1.2 kΩ resistor with one of higher value
OR Increase the temperature (of the thermistor or the room)