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What is Capacitance?

Definition of Capacitance

  • Capacitance is defined as the amount of charge stored per volt
  • C=Q/V
  • Capacitance is measured in Farads(F)
  • Farads are a big unit- therefore usually in micro/nano/pico farads
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Practicals involving Capacitors

You can investigate the Charge stored by a Capacitor Experimentally

  • Set up a test circuit to measure current and potential difference
  • Constantly adjust the variable resistor to keep charging current constant for as long as possible
  • NB-it is impossible to keep charging current constant when capicitor is nearly fully charged
  • At regualr intervals record p.d until it equals the battery p.d
  • It is then possible to plot graphs of Current(I)/Amps vs Time(t)/seconds and Charge(Q)/Coulombs vs Potential Difference(V)/Volts
  • Gradient of Q/p.d is Capacitance
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Capacitance and Energy

Capacitors store Energy

  • When charge builds up on plates of the capacitor, electrical energy is stored
  • Area under graph of p.d vs charge stored=energy stored in the capacitor
  • p.d across the capacitor is proportional to charge stored on it
  • therefore the graph will be a straight line through the origin
  • Area under graph(triangle) -E=1/2xQV


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Charging a Capacitor

A capacitor can be charged by connecting it to a battery

  • when connected to a battery, a current flows in circuit until  capacitor is fully charged, then stops
  • Electrons flow onto negative terminal plate- (-ve) charge builds
  • Build up of (-ve) charge repels electrons off plate connected to positive terminal of battery, making that plate positive
  • These electrons are attracted to the positive terminal of the battery
  • An equal and opposite charge builds plates. Causes Potential Difference between them (NB-no charge can flow between plates-Insulator)
  • Current starts high. As more charge builds up electrostatic repulsion makes it more difficult for electrons to be deposited
  • When p.d across capacitor=p.d across battery, current is 0
  • Capacitor is now fully charged
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Discharging a Capacitor

To Discharge a Capacitor, Take out the Battery and Reconnect the Current

  • When a charged capacitor is connected across a resistor, the p.d drives a current through the circuit
  • This current flows in opposite direction from charging current
  • If a voltage sensor attached to a datalogger is connected across the capacitor , a discharge curve can be plotted
  • Capacitor is fully discharged when p.d across plates and current in circuit are both zero.
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Charging and Discharging

The Time Taken to charge or discharge depends on:

  • Capacitance of Capacitor (C). This affects the amount of charge that can be transferred at a given voltage.
  • Resistance of circuit (R). This affects the current in the circuit.



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Charging and Discharging

The Charge on a Capacitor Decreases Exponentially

  • When the capacitor discharges, the charge left on the plates falls exponentially with time
  • Therefore it always takes the same length of time for the charge to halve, no matter how much charge you start with
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Time Constant = RC

  • Time constant, is time taken for charge on a discharging capacitor to fall to 37% of Qo
  • Therefore its also time taken for charge of a charging capacitor to rise to 63% of Qo
  • The larger the resistance in series with the capacitor the longer it takes to charge/discharge
  • In practice, time taken for capacitor to charge/discharge fully is about 5RC
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Mrs Jones

Great set of revision cards with very clear explanations of how to investigate charging and discharging of capacitors experimentally.  The resulting graphs that can be drawn from these experiments are outlined and linked to equations.  Brilliant

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