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Edexcel | Physics WPH0 | W 13 6 1 | 1 hour 30 minutes
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  • Q1
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2
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A student writes a plan for an experiment to measure the current as a capacitor

discharges through a resistor. His aim is to find a value for the time constant for the

exponential decay of the current. His outline plan, which includes a circuit diagram, is

shown below.

Suggest improvements to the plan that would allow the student to carry out the

experiment successfully.

Your improvements should include:

2
(a)
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2 Mark s

the initial current and the range he should set on the multimeter,

Current = 130 μA

Meter range plausible and greater than their value (e.g. 200 μA)

Example of calculation

I0 = 6.0 V/47 kΩ = 128 μA

2
(b)
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2 Mark s

the expected value for the time constant and for how long he should take readings of

the current,

Calculates RC as 47 kΩ x 470 μF = 22 s

[accept 22.1 s or 22.09 s]

Record for a time equivalent to 3 - 5 times their time constant

2
(c)
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1 Mark

a reason why this stopwatch is suitable,

Precision (0.01 s) < human reaction time (0.1 s)

2
(d)
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2 Mark s

a technique he should use to ensure his readings are as accurate as possible and one

safety precaution that might be necessary,

Technique

Meter and stopwatch close together Or both in eye line at same time

Or use of lap-timer

(not repeat and average)

Safety

Capacitor connected correct way round Or capacitor rated voltage at least

6 V

2
(e)
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2 Mark s

an explanation of why a graph of ln I against t would give a straight line and how he

should find a value for the time constant from the graph.

ln I = −t/RC + ln I0 is same form as y = mx + c and since RC is constant

(hence the line is straight)

(equation need not be seen if other valid statement made)

T = −1/gradient ( must have negative sign)

2
(f)
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A teacher suggests that with this circuit it would be necessary to wait for some time

before switching from position X to position Y.

2
(f) (I)
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1 Mark

Comment on why this wait is necessary.

Capacitor will require time to charge up (through the resistor)

Or capacitor will not be fully charged

2
(f) (II)
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1 Mark

Draw below a different arrangement of the circuit components so that there is no

time delay.

Suitable circuit drawn (values not needed)

eg

  • Q1
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  • Q3
  • Q4

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