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1. Capacitance time constant help
I need help with capacitance, or more specifically the equation for time constants and discharge,

A specific question is asking me to find out how long a capacitor will take to discharge from 6 volts to 2,

R = 100 Kilo ohms
C = 50 x 10^-6
V = 2 VOLTS
V (Initial) = 6 volts,

if V = V(Initial) x e^-(t / RC)

the equation will need to be shuffled to make t the subject, unfortunately i dont know how to do this, but i think that if t was the subject, the question could be answered,

2. Re: Capacitance time constant help
You need to take logs of both sides.
This gives
Ln V = Ln Vi - (t/RC)

Ln is log to the base e

Can you do the rest?
Last edited by Stonebridge; 15-09-2012 at 21:39.
3. Re: Capacitance time constant help
Thanks for the help but Im sorry i cant, i never did A level maths so i didn't even know what logs or exponentials were until reading the topic and im not familiar with the phrase "log to the base" or using logs or exponentials.

but if (ln V) Gives a whole number there will still be no visible intermediate steps to make t the subject, which i dont understand.
4. Re: Capacitance time constant help
(Original post by mar junior)
Thanks for the help but Im sorry i cant, i never did A level maths so i didn't even know what logs or exponentials were until reading the topic and im not familiar with the phrase "log to the base" or using logs or exponentials.

but if (ln V) Gives a whole number there will still be no visible intermediate steps to make t the subject, which i dont understand.
Hasn't your physics teacher gone through this bit of maths needed to solve questions like this? You will meet the same thing again when you do radioactive decay.
This isn't really the place to teach you maths. You need to know what an exponential is, and what a log is, to do A Level physics.

You can find the log (Ln) of a number using the ln function on your calculator.
So it's easy to get the values for Ln V and Ln Vi

In the equation I gave you, you then just need to rearrage to get the value of t, as you have been given R and C

I'm sure you are able to rearrange a formula such as this if you have done GCSE maths.

Can you do it now?