Physics 201- Introduction to Electricity & Magnetism
October 25- Charging & Discharging Capacitors

Qualitative Charging and Discharging

Below you see a circuit composed of a battery, resistor, capacitor and switch.  Initially the capacitor is uncharged and the switch is open (not connected to 1 or 2).

After the switch is moved to position 1, current (or electrons) begins to flow out of the battery and toward one plate of the capacitor.  Current (or electrons) also flow from the other plate to the other side fo the battery.  This ensures that the capacitor has a net charge of zero.

Just after the switch is closed in position 1 (when the capacitor is nearly uncharged),
a) the voltage across C equals e.
b) the voltage across R equals e.
c) the current in the circuit is zero.
d) both a) and c) are correct.

After the switch has been closed (still position 1) for an extended period of time,
a) the current in the circuit is zero.
b) the current in the circuit is equal to e / R.
c) the voltage across R equals e.
d) both b) and c) are correct.

Charges continue to move around the circuit until, the battery is no longer able to push them and the system reaches its equilibrium state.  Here the capacitor is fully charged and the current goes to zero.

After this extended period of time, the switch is moved to position 2.  Now the current (or electrons) move from one side of the capacitor to the other by traveling through the resistor.  This will cause each side of the capacitor to become neutral.

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 Jeff Phillips phillips@lmu.edu Loyola Marymount University Fall 2002