Practical Skills: RP 09 Charging and Discharging Capacitors: Section A - Paper 3: Physics A Level AQA

What must always be checked when using an electrolytic capacitor in a circuit?

The electrolytic capacitor is a polarised component and so must be connected with the correct polarity in the circuit. If connected incorrectly, it can overheat and become a safety hazard.

What equation is used to calculate the time constant of a resistor-capacitor circuit?

Time Constant = Resistance x Capacitance

𝛕 = RC

What information does the time constant tell us?

The time constant tells us how long it takes for the capacitor to charge to 63% of its full capacity, as well as how long it takes for it to discharge to 37% of its full capacity.

What device is used to measure the potential difference across a capacitor and how should be connected?

A voltmeter should be connected in parallel across the ends of the capacitor.

What is the advantage of taking logarithms before plotting an exponential relationship?

Taking logarithms allows the exponential relationship to be more easily confirmed when plotted as it allows the graph to be matched to the equation of a straight line. This also means that the desired variables can be more easily obtained.

What graph can be plotted to confirm the exponential decay of a capacitor’s potential difference as it discharges?

A graph of ln(V/V₀) against t can be plotted. This should form a straight line graph.

When plotting a discharge graph of ln(V/V₀) against t, how can the capacitor’s time constant be obtained?

The graph will have an equation of

ln(V) = ln(V₀) - t/RC, and so the time constant (RC) is given by -1/gradient.

What is the benefit of doing a trial discharge before carrying out this experiment in full?

A trial discharge with your chosen values of R and C allows you to choose a suitable time interval to take recordings at, depending on how quickly the capacitor discharges.