Charging and discharging capacitorsEdexcel International A Level Physics: Flashcards

What these 12 flashcards ask

State the equation for the charge on a discharging capacitor.
State the equations for I and V in a discharging capacitor circuit.
What is the time constant of an RC circuit?
What is the significance of the time constant in discharging?
What fraction of the final p.d. is reached after one time constant of charging?
State the log form of the discharge equation for V.
What does a graph of ln V against t look like for a discharging capacitor?
Why does the charging current fall as a capacitor charges?
What is the initial current when charging through R from a supply of p.d. V?
What effect does a larger resistance have on the discharge?
Rearrange V = V0e^(−t/RC) to find t.
Name two instruments used in Core Practical 11 to record the capacitor p.d.

Exam questions on Charging and discharging capacitors

A 2200 μF capacitor is charged to a p.d. of 9.0 V and is then discharged through a 4.7 kΩ resistor.
Calculate the p.d. across the capacitor 20 s after the discharge begins.2 marks
A 100 μF capacitor, initially uncharged, is charged through a 47 kΩ resistor using a 6.0 V supply that has negligible internal resistance.
Explain why the current in the circuit decreases as the capacitor charges.2 marks
In Core Practical 11 a student records the p.d. V across a capacitor as it discharges through a resistor, using a data logger. She plots ln V against the time t in seconds (V in volts). Her line of best fit is straight, with a gradient of −0.050 s⁻¹ and an intercept of 2.20 on the ln V axis.
Use the equation for exponential discharge to explain the shape of her graph, and determine the time constant of the circuit and the initial p.d. across the capacitor.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).