OCR A Level Physics

Topic Questions

A LevelPhysicsOCRTopic Questions6. Particles & Medical Physics6.2 Charging & Discharging Capacitors

6.2 Charging & Discharging Capacitors

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11 mark

A capacitor is charged through a resistor.

q13-paper-2-june-2019-ocr-a-level-physics

The cell has electromotive force (e.m.f.) 1.50 V and negligible internal resistance.
The time constant of the circuit is 10 s. The switch is closed at time t = 0. At time t, the potential difference across the resistor is 0.60 V.

Which expression is correct?

  • 0.60 = 1.50e–0.10t

  • 0.90 = 1.50(e–10t − 1)

  • 0.60 = 1.50e–10t

  • 0.60 = 1.50(1–e–0.10t)

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11 mark

The diagram below shows a circuit used to charge a capacitor.

q7-paper-2-nov-2020-ocr-a-level-physics

The power supply has electromotive force (e.m.f.) 10 V and negligible internal resistance.
The capacitor has capacitance C and the resistor has resistance R.
The switch is closed at time t = 0.
The table below shows potential difference V across the resistor at various values of time t.

V / V 10 6.3 5.0 3.7
t / s 0 2.8 4.2 6.0


 What is the product C × R for this circuit?

  • 0 s

  • 2.8 s

  • 4.2 s

  • 6.0 s

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21 mark

A capacitor discharges through a resistor.
At time t = 0 the potential difference V across the capacitor is V0.
At time t = 2.0 s, V = 0.90 V0.

Which statement is not correct?

  • At t = 4.0 s, V = 0.81 V0.

  • The capacitor is fully discharged after t = 10 s.

  • The potential difference across the resistor is the same as that for the capacitor.

  • The potential difference V decreases exponentially with time t.

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31 mark

A student is modelling the decay of charge for a capacitor discharging through a resistor using the equation fraction numerator increment Q over denominator increment t end fraction space equals space – 0.2 Q.

The student decides to use ∆t = 0.5 s.
The initial charge on the capacitor is 1000 μC.

Part of the modelling spreadsheet from the student is shown below.

t / s Charge Q left on capacitor at time t / μC Charge ∆Q decaying in the next 0.5 s / μC
0 1000 100
0.5 900  
1.0    
1.5    

What is the value of Q in μC at t = 1.5 s?

  • 700

  • 720

  • 729

  • 800

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41 mark

A capacitor discharges through a resistor. At time t = 0, the charge stored by the capacitor is 600 μC. The capacitor loses 5.0% of its charge every second.

What is the charge left on the capacitor at time t = 4.0 s?

  • 111 μC

  • 120 μC

  • 480 μC

  • 489 μC

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51 mark

A capacitor is discharged through a resistor.

q14-paper-2-june-2019-ocr-a-level-physics

The capacitor is fully charged at time t = 0. The time constant of the circuit is 10 s. The switch is closed at time t = 0. The current in the resistor is I.

Which row is correct?

  Current bold italic I at bold italic t = 0 Current bold italic I at bold italic t = 10 s
A maximum 0
B maximum 37% of the current at t = 0
C 0 63% of the current at t = ∞
D 0 37% of the current at t = ∞

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61 mark

A capacitor is charged through a resistor.

q9-paper-2-june-2018-ocr-a-level-physics

The cell has e.m.f. 1.50 V and negligible internal resistance.
The capacitor is initially uncharged. The time constant of the circuit is 100 s.
The switch is closed at time t = 0.

What is the potential difference across the capacitor at time t = 200 s?

  • 0.20 V

  • 0.55 V

  • 0.95 V

  • 1.30 V

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