Investigating charge stored on a capacitor
- set up test circuit with battery, variable resistor, ammetre, voltmetre, switch and capacitor.
- adjust variable resistor in order to keep charging current constant.
- record pd at regular intervals until it equals battery pd.
- plot a current-time graph. the area under this is the charge stored on the plates.
- plot a charge time graph ( Q = IT). the gradient of this equals capacitance.
Charging a capacitor
- capacitor is connected to a battery
- current flows in circuit until capacitor is fully charged, then stops.
- electrons flow to plate connected to negative terminal
- this repels charge from other plate (connected to + terminal)
- initialy, current through circuit is high
- charge builds up
- electrostatic repulsion makes it harder for charge to be deposited
- pd across capacitor = pd across battery, current falls to 0
Discharging a capacitor
- Either take out battery or reconnect circuit
- pd drives current through circuit
- current flows in opposite direction to charging current
- pd across plates and current = 0
- capacitor = fully discharged
charge decreases exponentially with time (it always takes the same amount of time for charge to halve)
graphs of q, i or v against t for charging and discharging are exponential.
Time taken to charge or discharge depends on:
Time constant, t = time taken for charge on discharging capacitor to fall to 37% of its original value.
it's also the time for a charging capacitor to rise to 63% of Qo
the larger the resistance in series wuth the capacitor, the longer it takes to charge.
5RC is time for capacitor to fully discharge.
Energy stored by a capacitor
- work done when charge,q moves through pd, v, = w=vq
- pd varies as capacitor charges
- v and q are proportional
- average pd = half of final pd
- v against q graph - area equals the work done in charging the capacitor
- the capacitor stores the transferred energy as electric potential energy.