O-LevelPhysicsElectricity5054/22 · Oct/Nov 2019 · Q67 Marks

An electric circuit contains a 700Ω resistor and a light-dependent resistor (LDR). [Figure 6.1] is the circuit diagram. The electromotive force (e.m.f.) of the battery is 12V. An oscilloscope is connected across the fixed resistor. [Figure 6.2] shows the oscilloscope, including the settings of the timebase and the Y-gain controls. Line Q shows the position of the trace on the oscilloscope when the switch S is open.

(a)(i)The switch S is closed and the trace on the oscilloscope moves to the position shown by line P in Fig. 6.2. Determine the potential difference (p.d.) across the 700Ω resistor.[1]
(a)(ii)Determine the resistance of the LDR.[3]
(b)The intensity of the light incident on the LDR gradually increases. State and explain how the trace on the oscilloscope screen moves.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[B1] 4.0 V c.a.o.
[C1] (V=) 12-4.0 or 8.0 or 4.0/700 or 5.7 x 10⁻³
[C1] (R =) V/I or 8.0 × 1 / (4.0/700) or 700 × (8.0/4.0) or 12/(4.0/700) or 2100 (Ω)
[A1] 1400 Ω
[B1] resistance of LDR decreases
[M1] current increases or p.d. across 700 Ω resistor/oscilloscope increases or p.d. across LDR decreases
[A1] trace moves up the screen

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