• Created by: emma
  • Created on: 14-05-13 18:36

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.
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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
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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:

  • capacitance
  • resistance
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Time constant

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.

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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.
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