# Confused on the power through a Capacitor formula help!

1. Apr 4, 2013

### nchin

I know that the energy stored in a capcitor is U = 1/2 CV2. but the power through it? I have in my notes that the power is the derivative of U.

so for example the energy is 26 Joules so the derivative of 26 J? doesnt make sense cause 26 is a constant.

i attached a picture of the problem. i first find the C equivalent.

oops! theres actually a switch on the top of the circuit! which closes at t = 0

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Last edited: Apr 4, 2013
2. Apr 4, 2013

### Staff: Mentor

Can you find an expression for the potential across the equivalent capacitance with respect to time? How about the stored energy? This expression should be differentiable w.r.t. time...

3. Apr 4, 2013

### nchin

potential across is C = QV --> V(t) = C/Q(t). is that it?

4. Apr 4, 2013

### Staff: Mentor

No. C is capacitance, not potential. C = QV is not a correct formula. And Q(t) does not provide any information about how the charge Q changes over time.

What is the expression for the voltage across the capacitance with respect to time for the given circuit? Hint: it involves an exponential function with a time constant.

Last edited: Apr 4, 2013