Practise Edexcel A-Level Physics capacitor discharge equations by applying exponential decay, ln plots and threshold timings.
Two identical capacitors were connected in series and charged. They were then discharged through a resistor and ammeter.
A student investigated how the current in the resistor varied as the capacitors discharged.
The student had a stopwatch.
Describe how the student should determine an accurate value for the total capacitance of the capacitors.
Award any six valid method points. Choose a resistor giving a reasonable discharge time;
measure its resistance;
charge the series capacitors to a known initial p.d.;
start timing when switching to discharge;
record current and time simultaneously as closely as possible;
take many readings over at least a half-life or time constant.
A graphical route may plot lnI against t, for which the gradient is -1/(RC);
or obtain t1/2 from an I–t graph and use t1/2=RCln2;
or obtain the time to reach 37% and use τ=RC.
A non-graphical route may repeat at least three times and average the measured half-life or time constant before using the same formulae.