# Homework Help: Capacitor: charge and energy calculations

1. Dec 4, 2014

### hibphys

1. The problem statement, all variables and given/known data
A capacitor of capacitance 12*10^6 is fully charged from a 20V d.c. supply.
i Calculate the charge stored by the capacitor.
ii calculate the energy delivered by the dc supply
iii calculate the energy stored in the capacitor.
iv account for the difference between your answers for ii and iii.
2. Relevant equations
i charge is Q=CV
ii I think this is W=QV
iii I think this is W=1/2 CV^2

3. The attempt at a solution
iv I am not so sure it may be the fact that the dc current is equal to the voltage across the circuit or potential difference or something. I don't really get that.
i 2.4e-4
ii4.8e-3
iii2.4e-3

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2. Dec 4, 2014

### jbriggs444

Apparently this capacitor is actually $10^{-6}$ farads, not $10^6$ as written. With that understanding, your answers for i, ii and iii are correct.

For part iv, consider what is happening as the capacitor is halfway charged. The dc supply (ideally) is putting current out across a 20 V potential difference between its two leads. The capacitor (ideally) has a 10 V potential difference between its two leads. How can one account for the other 10 V of potential difference?

3. Dec 4, 2014

### hibphys

LOST TO RESISTANCE ?

4. Dec 4, 2014

### jbriggs444

Yes. You could elaborate on that answer by offering possibilities for where the resistance could exist. Thinking about conservation of energy, you could think about where the energy that is "lost to resistance" goes.