3.7.4.1
Capacitance is the charge stored per unit potential difference
Capacitance & Charge/Discharge — AQA A-Level Physics
Key Definition
Capacitance — The charge stored per unit potential difference between the plates. Measured in farads (F).
$$C = \frac{Q}{V}$$
- Where:
- $C$ = capacitance (F)
- $Q$ = charge stored (C)
- $V$ = potential difference (V)
- A capacitor is two conductive plates separated by an insulator (dielectricAn insulating material placed between the plates of a capacitor that increases its capacitance by reducing the electric field strength for a given chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..). It stores energyThe capacity to do work. Measured in joules (J). in the electric field between the plates.
- 1 F is a very large unit. Typical values are in microfarads (uF), nanofarads (nF), or picofarads (pF).
- The chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). Q is the magnitude of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). on each plate, not the total charge of the capacitor.
- Greater capacitanceThe charge stored per unit potential difference across a capacitor. Measured in farads (F). means more charge stored for the same p.d., and more energyThe capacity to do work. Measured in joules (J). stored.
Worked Example
A parallel plate capacitor has a capacitanceThe charge stored per unit potential difference across a capacitor. Measured in farads (F). of 1 nF and is connected to a 0.3 kV supply. Calculate the charge on the plates.
Show Solution
1
List known values
- $C = 1 \times 10^{-9}$ F
- $V = 0.3 \times 10^{3} = 300$ V
2
Rearrange C = Q/V for Q
$$Q = CV$$
3
Substitute and calculate
$$Q = (1 \times 10^{-9}) \times 300 = 3 \times 10^{-7} \text{ C} = 300 \text{ nC}$$
Answer
$Q = 300$ nC
Common Mistake
MEDIUM
Students often: Confusing C for capacitanceThe charge stored per unit potential difference across a capacitor. Measured in farads (F). with C for coulombs.
Instead: Context tells you which is meant. $C = Q/V$ uses C for capacitance (farads). Q is measured in C (coulombs).
Instead: Context tells you which is meant. $C = Q/V$ uses C for capacitance (farads). Q is measured in C (coulombs).