# Capacitor help needed (voltage needed and time constant question)

1. Feb 14, 2013

### LTME

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

#1. If you needed to store 1 coulomb of charge on a 1000uF capacitor what voltage would be required and what electric field would exist within the capacitor if the plate spacing is .1mm.

#2. Using Vc=Vo*e^(-t/RC) show that Vc will be equal to 37% of Vo when (RC) seconds of time have passed.

2. Relevant equations

#1. I'm not sure which equations are relevant and that is part of the problem.

#2. given in the question.

3. The attempt at a solution

#1. I (think) I get the first part of the question. q=C*V so 1 coulomb = 1000uF/v than v=1mV. However I do not know where to start on the second half of the question.

#2. I suppose my math skills are just lacking for this one because I don't even know where to start.

Thanks for any help.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 14, 2013

### dydxforsn

For part one you're simply using Q = CV where 'Q' is the charge of the capacitor, 'C' is the capacitance, and 'V' is the voltage required to do so. To get the electric field from the spacing and the voltage you just need to use the equation E = V/d (E is the rate at which V changes with distance).

For part two you need to plug in t = RC which will make the argument in the exponent go to -1 (the RC's cancel.) Then you will see that your answer is just V0 times some constant value ($e^{-1}$).

3. Feb 14, 2013

### LTME

Thanks for the help.

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