Retrieval Practice
Capacitance & Charge/Discharge — AQA A-Level Physics
Q1. Define capacitance and state its unit.
- The charge stored per unit potential difference.
- Unit: farad (F).
- C = Q/V.
Q2. What is a dielectric and how does it affect capacitance?
- An insulating material between the plates.
- Its polar molecules align with the applied field, creating an opposing field that reduces the p.d. and increases capacitance.
Q3. State the equation for the capacitance of a parallel plate capacitor.
C = A epsilon_0 epsilon_r / d, where A is the area of one plate, d is the separation, and epsilon_r is the relative permittivity.
Q4. State all three equations for the energy stored in a capacitor.
- E = 1/2 QV, E = 1/2 CV^2, E = \(Q^{2}\) / (2C).
- All three are on the data sheet.
Q5. Describe how the current varies with time when a capacitor charges through a resistor.
The current starts at a maximum value I0 and decreases exponentially to zero as the capacitor fully charges.
Q6. Describe how the charge on the plates varies with time when a capacitor discharges.
- The charge starts at Q0 and decreases exponentially to zero.
- The rate of decrease is proportional to the charge remaining.
Q7. Define the time constant for a discharging capacitor.
The time taken for the charge, current, or voltage to decrease to 37% (1/e) of its original value. tau = RC.
Q8. Define the time constant for a charging capacitor.
The time taken for the charge or voltage to rise to 63% (1 - 1/e) of its maximum value. tau = RC.
Q9. State the half-life equation for a discharging capacitor.
- t_1/2 = 0.69RC = 0.69 tau.
- It is the time for Q, I, or V to halve.
Q10. Write the discharge equation for charge on a capacitor.
- Q = Q0 e^(-t/RC).
- The same form applies to current (I = I0 e^(-t/RC)) and voltage (V = V0 e^(-t/RC)).
Q11. Write the charging equation for charge on a capacitor.
- Q = Q0(1 - e^(-t/RC)).
- The same form applies to voltage.
- Current during charging still decays: I = I0 e^(-t/RC).
Q12. In the required practical, what do you plot and how do you find C?
- Plot ln(V) against t.
- The gradient = -1/RC.
- Rearrange: C = -1/(R x gradient).
Q13. What does the gradient of a charge-time graph represent?
The current at that instant: I = DeltaQ / Deltat.
Q14. What does the area under a current-time graph represent?
The total charge stored (or released) in that time interval: Q = I x t.