4.1 Capacitance

• Capacitor charging

  when a charge is connected, electrons flow from one plate (which becomes positive) to the other (which becomes negative)

  the rate of flow of charge falls exponentially over time as the capacitor become more charged
• Capacitance

  the capacitance of a capacitor is defined as the charge stored per unit potential difference change

  unit: Farad, F
• Dielectrics

  when the capacitor is charged the dielectric becomes polarised, which enables a larger charge to be held on the capacitor plates for a given pd
• energy stored in a capacitor, U=1/2*Q*V

  sub in Q=CV to get U=1/2*C*V^2

  sub in V=Q/c to get U=1/2(Q^2/C)
• Capacitors in parallel

  C(total)= C(1)+C(2)+C(3)
• Capacitors in series

  1/C(total)= 1/C(1) + 1/C(2) + 1/C(3)
• in a field between parallel plates, the charge is strongest in the middle
• Electric field strength

  force per unit test charge placed in the field

  units= NC^-1
• Capacitor discharge

  when a capacitor is discharged through a resistor, we get exponential decay equations for Q, V and I
• Time constant (RC)

  time taken for a capacitor to discharge 0.37 of its initial charge