Networks, Time and Fields
Capacitors & RC Time
13 minStudent plan
What a capacitor stores, why it blocks DC but passes AC, and the time constant that sets how fast a circuit responds.
After this lesson you can
- Say what a capacitor actually stores
- Work out an RC time constant and what it means in practice
- Explain why a capacitor blocks DC and passes AC
Storing charge
A capacitor is two conducting plates separated by an insulator. Charge cannot cross the gap, but it can pile up on one plate and be pushed off the other, and that separation of charge stores energy in the electric field between them.
Capacitance C, in farads, measures how much charge is stored per volt applied: Q = CV. A farad is enormous, so real parts are in microfarads (10⁻⁶), nanofarads (10⁻⁹) and picofarads (10⁻¹²).
Because no charge crosses the insulator, a capacitor blocks steady DC completely once charged. But while the voltage is changing, charge is moving on and off the plates, so current does flow. That is the whole behaviour in one sentence: a capacitor passes changes and blocks the steady state.
In the full lesson
- Storing charge
- The time constant
- What capacitors are actually for
This lesson comes with the Student plan. The first module of Physics for Engineers is free.