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Capacitor Questions

Q6. A capacitor, initially charged to a pd of 6.0V, was discharged through a 100 kΩ resistor.

A datalogger was used to record the pd across the capacitor at frequent intervals. The graph shows how the pd varied with time during the first 40 s of discharge.

(a)

(i) Calculate the initial discharge current.

(1 mark)

(ii) Use the graph to determine the time constant of the circuit, giving an appropriate unit.

(4 marks)

(iii) Hence calculate the capacitance of the capacitor in microfarads.

(1 mark)

(iv) Show that the capacitor lost 90% of the energy it stored originally after about 25 s.

(3 marks)

(b) In order to produce a time delay, an intruder alarm contains a capacitor identical to the capacitor used in the experiment in part (a). This capacitor is charged from a 12V supply and then discharges through a 100 kΩ resistor, similar to the one used in the experiment.

(i) State and explain the effect of this higher initial pd on the energy stored by this capacitor initially.

(2 marks)

(ii) State and explain the effect of this higher initial pd on the time taken for this capacitor to lose 90% of its original energy.

(1 mark)

(Total 12 marks)