A-LevelPhysicsD.C. circuits9702/42 · May/June 2017 · Q810 Marks

A student designs a circuit incorporating an operational amplifier (op-amp) as shown in Fig. 8.1. [Figure 8.1]

(a)(i)On Fig. 8.1, draw a circle around the output device.[1]
(a)(ii)State the purpose of this circuit.[2]
(b)(i)The resistors X and Y each have resistance R. When conducting, the LED labelled B emits blue light and the LED labelled G emits green light. State whether blue light or green light is emitted when the resistance of component C is greater than the resistance Rᵥ of the variable resistor. Explain your answer.[3]
(b)(ii)State and explain what is observed as the resistance of component C is reduced.[3]
(c)Suggest the function of the variable resistor.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[B1] circle around both diodes
[M1] indicates (whether) temperature
[A1] (is) above or below a set value
[M1] (when resistance of C > R_v,) V⁻ > V⁺ or V⁺ <3V or p.d. across R_v < p.d. across R/Y/3V or p.d. across C > p.d. across R/X/3V
[M1] op-amp output is negative
[A1] (only) green
[B1] resistance of C becomes less than R_v or V⁻ < V⁺
[A1] green (LED) goes out
[A1] blue (LED) comes on
[B1] changes/determines temperature at which LEDs switch

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