Charging and Discharging
Charging a Capacitor
- When a capacitor is connected to a d.c. power supply (e.g. a battery), a current flows in the circuit until the capacitor is fully charged, then stops.
- The electrons flow from the negative terminal of the supply onto the plate connected to it, so a negative charge builds up on that plate.
- At the same time, electrons flow from the other plate to the positive terminal of the supply, making that plate positive. These electrons are repelled by the negative charge on the negative plate and attracted to the positive terminal of the supply.
- The same number of electrons are repelled from the positive plate as are built up on the negative plate. This means an equal but opposite charge builds up on each plate, causing the potential difference between the plates.
- Remember that no charge can flow directly between the plates because they're separated by an insulator (dielectric).
Equations
- Charge on a charging Capacitor: \(\(Q=Q_{0}\left( 1-e^{-\frac{t}{RC}} \right)\)\)
- Charge on a discharging capacitor: \(\(Q=Q_{0} e^{-\frac{t}{RC}}\)\)
- Voltage on a discharging capacitor: \(\(V=V_{0} e^{-\frac{t}{RC}}\)\)
- Current on a discharging capacitor: \(\(I=I_{0} e^{-\frac{t}{RC}}\)\)