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Edexcel | Physics WPH0 | W 15 4 1 | 1 hour 30 minutes
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15
Add To Classified

A student was investigating the charge and discharge of a capacitor.

He set up the following circuit.

15
(a)
Add To Classified
2 Mark s

Calculate the time constant for the circuit.

Time constant =

Use of T = RC

T = 0.3 s

15
(b)
Add To Classified
2 Mark s

The student wanted to plot a current-time graph as the capacitor charged, but found

that the current changed too rapidly for him to take readings.

Instead, he modelled the experiment using a spreadsheet. The switch was closed at

time t = 0 s. V is the potential difference across the capacitor.

Explain how the value in cell B5 is calculated.

B5 = (6V - E4)/1.5 (k Ω)

Or I = (E−V)/R

Or I = (6.0 – 3.33)/1.5

Correct units or comment about mA and kΩ

(allow 1 mark for B5 =C5/(A5-A4) and 1 mark for use of exponential

equation)

15
(c)
Add To Classified

Some of the data from the spreadsheet has been plotted on a graph of current I against

time t.

15
(c) (I)
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2 Mark s

Plot the missing points and draw a line of best fit.

3 points plotted accurately ± ½ small square

Line of best fit drawn (smooth by eye)

15
(c) (II)
Add To Classified
2 Mark s

Use the graph to determine a second value for the time constant

Time constant =

Initial tangent drawn

t = 0.19 s −0.26 s

Or 37% of initial current found

t = 0.24 s −0.26 s

Or half life determined and use of half life = 0.693RC

t = 0.23 s −0.26 s

Or uses a pair of points off the graph in exponential equation

t = 0.24 s −0.26 s

15
(c) (III)
Add To Classified
1 Mark

Suggest how the student might change his spreadsheet to give a more accurate

model of the charging of the capacitor.

Reduce the time interval

15
(d)
Add To Classified
3 Mark s

An alternative method of determining the time constant is to use a straight line graph.

State and explain the variables that the student should plot and how he should

determine the time constant from this graph.

Or states that there is exponential relationship between I and t

Plot ln I against t Or ln (I/I0) against t

The time constant = –1/gradient

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