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Edexcel | Physics WPH0 | W 19 4 1 | 1 hour 30 minutes
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17
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The circuit shown is designed to test reaction time.

                   

The following instructions will test the reaction time of a student.

● Hide switch S1 from the student.

● Move S1 to position 1 and wait until the voltmeter reads 6.0 V.

● Close switch S2.

● At a random time, move S1 to position 2 so the buzzer sounds.

● As soon as the buzzer sounds, the student should open S2.

● Use the new voltmeter reading to calculate the student’s reaction time.

17
(a)
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3 Mark s

Explain how the circuit works to test reaction time.

(QWC – Work must be clear and organised in a logical manner using technical

wording where appropriate)

Capacitor charges when S1 moves to position 1

Capacitor starts to discharge when S1 moves to position 2

17
(b)
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When the circuit is used with a particular student, the final voltmeter reading is 0.70 V.

17
(b) (I)
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2 Mark s

Show that the student’s reaction time is about 0.2 s.

capacitance of capacitor = 270 μF

resistance of resistor = 332 Ω

resistance of buzzer = 38 Ω

17
(b) (II)
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2 Mark s

 

 

Non-zero intercept on y axis and p.d. decreasing

Exponential curve i.e. decreasing gradient, but not reaching x-axis

Example of graph

                    

17
(b) (III)
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4 Mark s

Explain how to make the circuit more sensitive for a reaction time of 0.2 s.

You should refer to the shape of your graph in (ii).

More sensitive means a bigger change in p.d. for a given change in time

Requires the gradient (at that time) to be greater

This requires the time constant to be close(r) to the approximate reaction time

Or If time constant too small it will be nearly discharged

Or If time constant too large the p.d. will be changing too slowly

Increase the resistance/capacitance

Or

More sensitive means a bigger change in p.d. for a given change in time

Requires the gradient (at that time) to be greater

Because the gradient at a given time is proportional to the initial p.d.

Increase the initial p.d.

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