# Homework Help: Determining the capacitor value for an RC circuit

1. Oct 3, 2016

### matthew tivey

• OP warned about not using the homework template
hi, i have a question which i was hoping someone could help with,

a series circuit featuring a capacitor (C) charging via a 1Mohm (R) resistor and a 12volt dc supply(VS).

i know the equation to describe VC is VC=VS(1-e -t/RC)

the question is: assuming VC is 2V after a time of 4 seconds, determine approximate value of the capacitor.

i have rearranged the equation to C= -t / Rloge(1-VC/VS).

i have calculated it as follows: 2/12= 0.166, then 1 - 0.166= 0.834.

loge0.834= 2.267

1000000 x 2.267 = 2267047

4/2267047 = 0.0000017644uf

please could some one help with the calculation.

regards

matt.

2. Oct 3, 2016

### Staff: Mentor

Your value for the natural log of 0.834 doesn't look right. Check: the log of a value less than one should be negative.

3. Oct 3, 2016

### phyzguy

I think you've done the calculation correctly, but how did you deal with the units? You have a time in seconds divided by a resistance in Ohms, so what are the units of your answer?

Oops, I missed gneill's point. You need to correct that as well.

4. Oct 3, 2016

### Staff: Mentor

I've changed your thread title to better describe the problem. Too vague or general titles are frowned upon

5. Oct 3, 2016

### kuruman

This is not correct. I suggest that you solve this problem algebraically and substitute at the very end. It will be much easier to troubleshoot your work if you do that.