# Homework Help: Parallel RC circuits

1. Mar 22, 2016

### erisedk

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

A parallel combination of 0.1M Ω resistor and a 10 μF capacitor is connected across a 1.5 V source of negligible resistance. The time (in seconds) required for the capacitor to get charged upto 0.75 V is approximately

(A) ∞
(B) ln 2
(C) log 2
(D) Zero
2. Relevant equations

$V = V_0(1-e^{\frac{-t}{RC}})$

3. The attempt at a solution

So, V0 should be 1.5 V, and C is 10 μF. However, should't the voltage across the capacitor be 1.5 V at all times because it's in parallel with the resistance? Even if I use Thevenin's theorem, I get the resistance across the capacitor as 0 ⇒ time constant is 0. Please help?

2. Mar 22, 2016

### Staff: Mentor

If the 1.5 V source has negligible resistance then your Thevenin idea is correct and the circuit's time constant will be correspondingly negligible (effectively zero seconds). So how long will it take to fully charge the capacitor from 0 V to 1.5 V?

3. Mar 22, 2016

### erisedk

Zero, so that should be the answer.

4. Mar 22, 2016

### Staff: Mentor

Yup.

5. Mar 22, 2016

Thanks!!