Charging and discharging a capacitor from a d.c. source
Watch this video for a practical demonstration of charging and discharging capacitors.
The circuit shown is used to investigate the charge and discharge of a capacitor. The supply has negligible internal resistance.
The capacitor is initially uncharged. When the switch is moved to position \(1\), electrons move from the negative terminal of the supply to the lower plate of the capacitor. This movement of charge is opposed by the An electrical component that restricts the flow of electrical charge. Fixed-value resistors do not change their resistance, but with variable resistors it is possible to vary the resistance. \(R\), so the initial Moving electric charges, eg electrons moving through a metal wire. in the circuit is \(I= \frac{E}{R}\)
Charging
During the charging of a capacitor:
- the charging current decreases from an initial value of \(\frac {E}{R}\) to zero
- the potential difference across the capacitor plates increases from zero to a maximum value of \(E\), when the capacitor is fully charged
- at all times the sum of the potential difference across the capacitor and the potential difference across the resistor equals the Electromotive force is defined as energy per unit charge. It is measured in Volts. of the supply
- the potential difference across the resistor (given by \({V_R}= IR\)) decreases from an initial value of \(E\) to zero when the capacitor is fully charged
When the switch is moved to position \(2\), electrons move from the lower plate through the resistor to the upper plate of the capacitor.
The movement of electrons through the A device used to measure electric current. is in the opposite direction to that of charging.
Discharging
During the discharging of a capacitor:
- the discharging current decreases from an initial value of \(- \frac {E}{R}\) to zero
- the potential difference across the capacitor plates decreases from \(E\) to zero, when the capacitor is fully discharged
- the potential difference across the capacitor is always equal to the potential difference across the resistor
- the potential difference across the resistor (given by \({V_R}= IR\)) decreases from an initial value of \(E\) to zero when the capacitor is fully discharged