A LevelPhysicsOCRExam Questions4. Electrons, Waves & PhotonsPotential Dividers

Potential Dividers (OCR A Level Physics): Structured Questions

Exam code: H556

34 mins12 questions

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Potential Dividers

1a
3 marks

A light-emitting diode (LED) emits red light when it is positively biased and has a potential difference (p.d.) greater than about 1.8 V.

The energy of a photon of red light is about 1.8 eV.

Calculate the wavelength λ of this red light.

λ = .......................................... m

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1b
2 marks

The LED is connected into a circuit, as shown below.

q17b-paper-2-nov-2020-ocr-a-level-physics

The battery has electromotive force (e.m.f.) 4.5 V and negligible internal resistance.

The resistor R has resistance 150 Ω.

Assume the p.d. across the LED is 1.8 V.

Calculate the ratio power dissipated by LEDpower dissipated by resistor

ratio = .......................................

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1c
3 marks

The diagram below shows a circuit designed by a student.

q17c-paper-2-nov-2020-ocr-a-level-physics

The LED is very close to, and facing the light dependent resistor (LDR).

The circuit is taken into a dark room.

i) The student thought that the LED would switch on.

Instead, the LED was found to repeatedly switch on and off.

Explain this behaviour of the LED in this potential divider circuit.

[2]

ii) Suggest a possible refinement so that the LED switches on permanently when taken into the dark room.

[1]

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2
6 marks

A metal circular plate is rotated at a constant frequency by an electric motor.

The plate has a small hole close to its rim.

Fig. 17.1 shows an arrangement used by a student to determine the frequency of the rotating plate.

Diagram of the apparatus. Light from a lamp shines downwards through a small hole near the rim of a flat circular plate, which is turned by a motor above it. Below the hole is a light-dependent resistor. The light-dependent resistor is connected in series with a 1.2 kΩ resistor and a 4.5 V battery, and the two ends of the 1.2 kΩ resistor are connected to a data-logger.

Fig. 17.1

A light-dependent resistor (LDR) and a fixed resistor of resistance 1.2 kΩ are connected in series to a battery. The battery has e.m.f. 4.5 V and has negligible internal resistance. The potential difference V across the resistor is monitored using a data-logger.

Fig. 17.2 shows the variation of V with time t.

Graph of V in volts against t in seconds from 0 to about 1.2 s. V is 2.0 V for most of the time. It rises to 4.0 V in short, narrow rectangular pulses that begin at 0.2 s, 0.6 s and 1.0 s, then drops straight back to 2.0 V.

Fig. 17.2

Use your knowledge and understanding of potential divider circuits to explain the shape of the graph shown in Fig. 17.2. Include in your answer the maximum and minimum values of the resistance of the LDR.

Describe how the student can determine the frequency of the rotating plate.

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3a
2 marks

A student investigates a temperature-sensing circuit used to control a heater.

The circuit consists of a 6.0 V d.c. power supply, a fixed resistor of resistance 15 Ω and a negative temperature coefficient (NTC) thermistor, as shown in Fig. 17.1.

Circuit diagram. A 6.0 V battery is connected across a 15 Ω resistor in series with an NTC thermistor, with the resistor above the thermistor. Two leads, one from each end of the thermistor, end in arrows labelled To heater switch.

Fig. 17.1

The variation of the resistance of the thermistor with temperature is shown in the graph in Fig. 17.2.

Graph of the resistance of the thermistor in Ω (0 to 7) on the y-axis against temperature in °C (−20 to 140) on the x-axis, on a grid where each small square is 2 °C by 0.1 Ω. The curve falls steeply and then levels off, passing through about (−20 °C, 4.8 Ω), (0 °C, 2.7 Ω), (10 °C, 2.0 Ω), (20 °C, 1.5 Ω), (40 °C, 0.9 Ω), (60 °C, 0.5 Ω) and (140 °C, 0.07 Ω).

Fig. 17.2

Explain, in terms of charge carriers, why the resistance of the thermistor changes with temperature.

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3b
3 marks

A voltmeter is connected across the fixed resistor.

Calculate the reading on the voltmeter when the temperature of the thermistor is 40 °C.

voltmeter reading = .................................................. V

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3c
6 marks

The circuit is designed to operate a heater switch. The heater switches on when the potential difference across the thermistor is greater than 0.70 V.

Deduce whether this circuit will switch on the heater when the temperature falls below 10 °C.

Your answer should include data from Fig. 17.2 and suitable calculations.

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