Flashcards for 21.5 Charging Capacitors

Q: When the switch is open




A:
 The battery provides a constant e.m.f V0
 The capacitor has capacitance C and the resistance of the resistor is R
 The capacitor is initially uncharged and the switch is open




Q: When the switch is closed




A:
When the switch is closed

 There is a max current in the circuit and the capacitor starts to charge up
 The p.d across the capacitor starts to increase from zero as it gathers charge
 According to Kirchhoff's 2nd law:
o The p.d VR across the resistor and the p.d VC across the capacitor must
always add up to V0
o So VR must decrease as VC increases with time
 After a long time, depending on the time constant CR of the circuit, the capacitor will
be fully charged with a p.d of V 0 and VR will be zero
o When this happens, the current I in the circuit is zero




Q: Current I in the circuit decreases exponentially and is given by the equation:
A: