# RC and RL circuits

1. Mar 30, 2009

### driedupfish

Hi everyone. I am having trouble understanding RC AND RL circuits. Specifically the charging and discharging of circuits. The current and charge that goes through the inductor or capacitor at various time(At time=0 or some time after the switch is closed or a long time after the switch is closed).
Thanks

2. Mar 30, 2009

### tiny-tim

Hi there!

I'm sorry to hear you're driedup … you need to get a nice little bowl like mine
Inductors are difficult. Capacitors are easier.

Concentrate on understanding the process with just a capacitor first.

See the PF Library or wikipedia …

what part is worrying you?

3. Mar 30, 2009

### Staff: Mentor

4. Mar 30, 2009

### Staff: Mentor

I usually just remember the following rule of thumb:

At DC a capacitor is an open circuit and an inductor is a short circuit.

The corolary is that for very high frequencies a capacitor is a short and an inductor is an open circuit.

So when you first throw the switch you apply the high-frequency corolary. The capacitor is a short so all of the voltage is across the resistor and the current is given by Ohm's law. After a while it settles down and you apply the DC rule. The capacitor is an open circuit so all the voltage is across it and there is no current.