Capacitance

Capacitance

1. Capacitance

   • metal plates separated by insulating material called the dielectric
   • Store electric charge
   • Charging a capacitor
     • connect in series with a battery- electrons flow off one plate and on the other until the potential difference across the capacitor is equal to that of the battery
   • =charge stored by a capacitor per volt of pd applied across it measured in farads

2. Energy Stored by a capacitor

   • derivation of energy stored equation
     • E=1/2 QV
   • also shown by the area under the graph of p.d against charge for the capacitor when charging

3. Capacitor Discharge

   • Charging
     • can also plot charge on capacitor against time, exponential increase with the steady level being CV
     • initial charge=emf/R
     • as charge accumulates in capacitor the p.d across it increases
     • this p.d opposes that of battery and it becomes harder to push charge onto the capacitor
     • eventually the p.d equals that of the battery and the charging current drops to zero
     • rate at which current drops depends on R and C
       • Time constant = RC
       • time taken for current to drop to 1/e of it's initial value