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ASPhysicsOCRExam Questions4. Electrons, Waves & Photons4.5 Potential Dividers

Potential Dividers (OCR AS Physics)

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4.5 Potential Dividers

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4.5 Potential Dividers

1a3 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.

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

Calculate the wavelength λ of this red light.

λ = .......................................... m [3]

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1b2 marks
b)
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 fraction numerator power space dissipated space by space LED over denominator power space dissipated space by space resistor end fraction




ratio = ....................................... [2]

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1c3 marks
c)
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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26 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.

q17-paper-2-june-2018-ocr-a-level-physics

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.

q17-2-paper-2-june-2018-ocr-a-level-physics

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.

[6]

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