Capacitors

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  • Created by: Sagaana
  • Created on: 11-11-18 13:46
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  • Capacitors
    • What is a capacitor?
      • It is an component made up of two conducting plates separated by a gap or a dielectric.
      • When  connected to a power source positive and negative charge builds up on the opposite plate creating a uniform electric Field
      • Capacitance= charge per unit potential difference
        • unit= Farad (this is a huge unit)
      • Dielectrics
        • Permittivity= how difficult it is to generate electrical field
          • Relative Permittivity= the ratio of Pmaterial and Pvacuumn
            • aka Dielectric Constant
        • when no charge applied
          • Polar Molecule that have a positive and negative end are in random directions
        • When charge is applied
          • polar molecules are aligned that negative ends are towards the positive plate and vice versa
            • Polar Molecules have their own Electric Field and this opposes the applied electric field
              • Larger permittivity the larger the opposing field
                • so capacitance increases
      • Capacitors Stores energy
        • as charge builds up on the plates it stores electrical energy
        • This can be seen in the area underneath the graph of charge against p.d
    • Charging Capacitors
      • Connect switch to uncharged capacitor to the power supply
        • This should charge up the capacitor. Ammeter is 0 this means the capacitor is fully charged
      • Current flows  ans electrons flow to the plate connected to the negative terminal so a negative charge builds up
        • Build up of negative charge causes electrons to repel and attract to the positive plate.
          • Equal and opposite charge builds up on each plate creating a potential difference
            • As charge builds electrostatic repulsion makes it harder for more electrons to be deposit causing the ammeter to fall to 0.
    • Discharging Capacitors
      • Remove the power supply to discharge the capacitor
        • when both the p.d and current is 0 the capacitor is fully discharged
    • Time Constant
      • time taken to charge depends on two things: capacitance and resistance
      • Time taken to charge the charge/p.d/current of a discharging capacitor to fall to 37% and also the time take for capacitor to rise to 63%
      • larger resistance the  longer it takes to charge and discharge.
      • You can find the time constant using the log linear graph
      • Half life =0.69RC

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