# Homework Help: Voltage Across Capacitor

1. Oct 29, 2011

### roam

1. The problem statement, all variables and given/known data

[PLAIN]http://img189.imageshack.us/img189/4555/capacitorh.jpg [Broken]

2. Relevant equations

$I(t)=I_0 e^{-t/\tau}$ (current as a function of time for a capacitor being charged)

3. The attempt at a solution

The time constant of the circuit is τ=RC=7s. And the initial current in the circuit is

v/Req=300/1489=0.2A (I know this is correct)

Now pluging into the equation:

$I(5)=0.2 e^{-5/7}=0.0979 \ A$

$V=IR= 0.0979 \ A \times 1489 \Omega = 145.73 \ V$

But the correct solution must be 153V. I'm very confused, what's wrong with my working?

Last edited by a moderator: May 5, 2017
2. Oct 29, 2011

### ehild

Is V=IR the voltage across a capacitor???

ehild