6.1.3 Charging and Discharging Capacitors

• Time constant of a capacitor resistor circuit = C*R
• Once the power supply is disconnected:

  Electrons repel one and another and so flow around the circuit dissipating electric heat, once charges have equilibrated there is no p.d and the current = 0
• lower resistance in a discharging circuit means higher current - ohms law if the current is higher the charge will fall to zero in a faster time
• Capacitance is a measure of charged stored per rate that the charge flows
• For discharging graphs of V/t Q/t and I/t

  We can use the equation

  X = X0 e-t/RC
• Time constant for a discharging graph is the time taken for a discharging capacitor to fall 37% of its original value
• Equations

  I = Q/t = I = Q/RC

  I = V/R = Q/RC
• As the switch is closed there is a maximum current in the capacitor as it starts to gain charge from zero as it gains charge
• Equation for Charging Capacitors:

  I = I0 * e-t/RC

  Equation X = X0(1-e-t/RC)

  Can be applied to pd and charge
• Pd across all components adds to V0

  V0 = VR + Vc
• Time constant for a discharging capacitor is the time taken for the charge to fall 37% of its original value
• Time constant for a charging capacitor is the time taken for the charge to rise 63% of the original value