- #1

- 19

- 0

## Homework Statement

1. Referring to the picture, switch S1 is closed at t=0.

a) What is the current in the circuit loop at t=0 and t=∞?

b) What is the voltage on C at t=0 and t=∞? (4M)

(Picture's Link: http://s29.postimg.org/bfsbw2hc5/physexam1q7.png)

## Homework Equations

When charging,

1.) ε - iR - q/c = 0

2.) max. current = -ε/R

3.) max. charge = C*ε

4.) q = Q

_{final}(1 - e^(-t/(RC))

5.) i = (Q

_{final}/RC)*e^(-t/(RC))

## The Attempt at a Solution

a) In fact I don't understand what t = 0 means. So when t = 0, is there any current yet?

For t = ∞, I tried to sub t = ∞ to equation 5) and use equation 3) to find the Q

_{final}. But it doesn't seem correct. Should I do it this way?

b) When t = 0, voltage on C = ε - iR? I assume the current has already thrown through the circuit when t = 0?

For t = ∞, should I sub t = ∞ into equation 5)? But it seems i = ∞ then? doesn't seem correct.

Very confused.:( Please help.

(Plus, how do we calculate the resistance of a capacitor? When I google it, I found someone said it's assumed to be zero. So in calculation, I assumed its resistance to be zero?)

Last edited: