RP09 - Charging + Discharging a Capacitor

• 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 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 it 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 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
  • This depends on how quickly the capacitor discharges
• What graph can be plotted to confirm the exponential decay of a capacitor's potential difference as it discharges?

  • A graph of In(V/V0) against t can be plotted.
  • This should form a straight line graph
• Gradient Derivation:

  V = V_0e^(-t/RC)

  Ln(V/V_0) = -t/RC

  Ln V - Ln V_0 = -t/RC

  Ln V = -t/RC + Ln V_0

  Where:

  Ln V = y

  -1/RC = m

  t = x

  Ln (V_0) = c