A-LevelPhysicsD.C. circuits9702/42 · May/June 2019 · Q96 Marks

Part of a circuit incorporating an operational amplifier (op-amp) is shown in Fig. 9.1.

(a)(i)A relay is connected to the output of the op-amp circuit so that a lamp may be switched on or off. Complete Fig. 9.1 to show the relay connected into the circuit.[2]
(a)(ii)State and explain whether the output of the op-amp is positive or negative for the lamp to be switched on.[2]
(b)(i)State the device in Fig. 9.1 that could be used so that the circuit indicates a change in: the bending of a rod[1]
(b)(ii)the level of daylight to switch on a street light.[1]

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Mark scheme

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

9(a)(i) relay coil shown connected between diode and earth B1 switch shown connected across lamp B1
9(a)(ii) Any one from: M1 • (for diode to conduct) current flow is into output of op-amp • when earth is at higher potential diode is forward biased • diode blocks current when output positive • diode must conduct SO VOUT is negative A1
9(b)(i) strain gauge B1
9(b)(ii) light-dependent resistor B1

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