# RC circuit discharge help

1. Dec 11, 2011

### charlies1902

I attached the problem. Is the answer they have for part b wrong? shouldn't the current be 0?

#### Attached Files:

File size:
55.7 KB
Views:
70
• ###### d.png
File size:
6.1 KB
Views:
63
2. Dec 11, 2011

### sandy.bridge

I'd say it is right. The capacitor chargers to a value equal to the voltage of the source, E. Upon moving to position B, the capacitor discharges but you can assume that at t=0- the voltage across the capacitor is indeed E.This is the instantaneous current at t=0-, which degrades as t increases.

3. Dec 11, 2011

### Simon Bridge

Yes and no.
At the level the problem is set (in terms of ideal components): no (the initial current should not be zero).
Initially in the RC circuit, the capacitor does not resist the flow of current so the current in the circuit is the same as through the resistor alone.

Perhaps you are thinking that since the switch is open at the start, then there must be zero current at the instant the switch is closed?

4. Dec 11, 2011

### Staff: Mentor

Nope. The current won't be zero. The capacitor has a charge on it, hence a potential difference. When the switch is closed on position b, that potential difference "sees" a path through the resistor. So current flows immediately.

5. Dec 11, 2011

### charlies1902

ah I see I got my concept wrong. I thought that when a capacitor is fully charged, the current would be 0. I just did kirkoffs loop rule and got their answer. thanks.