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International A LevelPhysicsEdexcelExam Questions4. Further Mechanics, Fields & ParticlesCapacitance

Capacitance (Edexcel International A Level Physics)

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Capacitance

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Capacitance

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1a
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A student investigated the properties of capacitors.

The student connected a capacitor in series with a resistor. The capacitor was charged through the resistor by applying a potential difference (p.d.) using a battery of e.m.f. 6.0 V and negligible internal resistance. The capacitor was then discharged through the resistor. The p.d. across the capacitor was measured during the charging and discharging processes.

Add to the diagram to show a suitable circuit for charging and discharging the capacitor while measuring the p.d. across it.

q17a-wph14-04-june-2021-edexcel-int-as-and-a-level-phy

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1b
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The graph shows how the p.d. across the capacitor varies with time as it is being charged by the 6.0 V battery.

q17b-wph14-04-june-2021-edexcel-int-as-and-a-level-phy

i) Sketch, on the same axes, a graph to show how the p.d. across the resistor varies with time.

(2)

ii) Show that the p.d V across the capacitor when it is charging is given by  

V space equals space V subscript 0 space minus space V subscript 0 e to the power of negative fraction numerator t over denominator R C end fraction end exponent  

where V0 is the e.m.f. of the battery

t is the time

R is the resistance of the resistor

and C is the capacitance of the capacitor.

(3)

iii) During the charging process, the student recorded the p.d. across the capacitor and the resistance of the resistor.

The capacitors available had the following values:

10 μF 

15 μF 

47 μF 

100 μF 

150 μF

Deduce the value of capacitance for the capacitor that was used.

R = 330 kΩ

(4)

iv) Calculate the charge on the fully charged capacitor.

Charge = .........................................(2)

v) Calculate the energy stored by the fully charged capacitor.

Energy stored = .....................................(2)

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1a
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A student built the circuit shown. The capacitor was initially uncharged.

q14-wph14-04-jan-2022-edexcel-int-as-a-level-phy

She closed the switch and plotted a graph of current on ammeter A1 against time.

q14-1-wph14-04-jan-2022-edexcel-int-as-a-level-phy

Determine whether the initial value of current on the graph is consistent with the values stated on the circuit diagram.

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1b
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Explain how the current on ammeter A2 would vary over the same time interval.

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1c
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Determine the capacitance of the capacitor.

Capacitance = ...............................

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1d
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Determine the charge stored on the capacitor after 30 s.

Charge stored on capacitor after 30 s = ................................

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1e
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Determine the maximum energy stored by the capacitor.

Maximum energy = ...............................

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2a
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The train on a model railway is powered by contact with the rails. The train sometimes loses contact with the rails. The diagram shows a circuit that can maintain power for an electronic controller on the train even when the power is disconnected for a short time.

The switch represents the contact between the train and the rails.

q14-wph14-04-oct-2021-edexcel-int-as-and-a-level-phy

Explain how this circuit can maintain power to the electronic controller if the switch is opened for a short time.

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2b
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The switch is opened.

Calculate the time taken for the potential difference across the capacitor to decrease to 4.0 V.

Assume the resistance of the electronic controller is infinite.    

capacitance of capacitor = 47 mF    

resistance of resistor = 470 Ω

Time taken = ...........................................

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2c
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The switch is closed at time t = 0 and then opened at t = 20 s.

Sketch a graph on the axes below to show how current I through the resistor varies with t.

q14c-wph14-04-oct-2021-edexcel-int-as-and-a-level-phy

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