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Edexcel | Physics WPH0 | W 17 4 1 | 1 hour 30 minutes
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16
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A timer circuit includes a capacitor and a variable resistor in series.

 

16
(a)
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The graph shows how the current in the timer circuit varies with time when the

capacitor discharges through the variable resistor. The resistance of the variable

resistor is 

16
(a) (I)
Add To Classified
3 Mark s

Show that the capacitance of the capacitor is about 0.02 F.

Use of I = I0 /e (I0 from 2.35 mA to 2.4 mA) to find time constant

Or intercept with t axis using initial tangent to find time constant (range 125 s

to 135 s)

Use of time constant = RC

C = 0.015 (F) to 0.017 (F)

Or

Attempts a pair of readings of I and t from graph

C = 0.015 (F) to 0.017 (F)

Or

Attempts to obtain ‘half-life’ from graph

Use of t ½ = RC ln2

C = 0.015 (F) to 0.017 (F)

16
(a) (II)
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3 Mark s

Calculate the initial charge on the capacitor.

 

 

                                                           intial charge=.......................

Use of V = IR for initial p.d. using initial current

Use of C = Q/V ecf from (i)

Q = 0.32 C

Example of calculation

V = 0.0024 A × 8200 Ω = 19.7 V

16
(a) (III)
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2 Mark s

Calculate the energy initially stored in the capacitor. 

 

 

                                                        Energy=................................

16
(b)
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3 Mark s

In another timer circuit the capacitance of the capacitor is 470 μF. The circuit

switches off when the potential difference falls to 15% of its initial value. The

variable resistor is adjusted so that the timer circuit switches off after 3.5 minutes.

Calculate the resistance of the variable resistor.

 

 

                                                        Resistance=......................

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