Capacitance: Start Here
Capacitance: Start Here
- A capacitor has two conductors with an insulator between them. It separates equal amounts of positive and negative charge.
- A charged capacitor stores electric potential energy. It can transfer that energy when it discharges.
Follow the charge
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- A power supply pushes electrons onto one plate and removes electrons from the other plate.
- The plate charges are equal in magnitude and opposite in sign. Q means the magnitude on one plate.
- Charge moves through the outer circuit. It does not cross the insulating dielectric.
A capacitor does not create charge. It separates charge.
Capacitance measures charge per volt
Definition
2 marksWhat is meant by capacitance?
Model answer: The ratio of charge to potential. For an isolated conductor, Q is its charge and V is its potential. For a parallel-plate capacitor, Q is the charge on one plate and V is the potential difference between the plates.
The exact meanings of charge and potential earn the second mark in a recent definition question (9702/42/O/N/24 Q7(a)).
Symbols
- = capacitance (F)
- = charge on one plate (C)
- = p.d. between the plates (V)
Worked example
Charge stored by a capacitor
A 220 μF capacitor is connected across 15 V. Find the charge on one plate.
Answer
Your turnquick check
A 47 μF capacitor stores 0.94 mC. Find its p.d.
Show worked answer
Answer