# Capacitors - energy stored vs work done

RK1992
this isnt homework, im doing revision for an exam :)

a 180 microfarad capacitor is charged so that is has a PD of 100v across the plates. i calculate the energy stored as 0.5cv² = 0.9J

the question then asks me to calculate the work done by the battery in charging the capacitor. the mark scheme says that i should use W = QV = CV² and should therefore get 1.8J

why is the work done in charging the capacitor different to the energy stored on it? i have never heard my teacher mention this fact in lessons, so i seriously have no idea where this has come from.. where does this equation come from? thanks

## Answers and Replies

JaredJames
this isnt homework, im doing revision for an exam :)

So work you're doing at home, but not homework? a 180 microfarad capacitor is charged so that is has a PD of 100v across the plates. i calculate the energy stored as 0.5cv² = 0.9J

the question then asks me to calculate the work done by the battery in charging the capacitor. the mark scheme says that i should use W = QV = CV² and should therefore get 1.8J

why is the work done in charging the capacitor different to the energy stored on it? i have never heard my teacher mention this fact in lessons, so i seriously have no idea where this has come from.. where does this equation come from? thanks

Well firstly, do you believe the energy it takes to charge the capacitor should equal the energy stored on it?

RK1992
So work you're doing at home, but not homework? yes, im a geek :p

Well firstly, do you believe the energy it takes to charge the capacitor should equal the energy stored on it?

well no some will dissipate right.. but this isnt charging through a resistor so surely 99% of the energy used charging will be stored?

JaredJames
yes, im a geek :p

I meant, work you do at home = homework.
well no some will dissipate right.. but this isnt charging through a resistor so surely 99% of the energy used charging will be stored?

99% is a very specific number you picked there. Do you have the calcs to back it up?

If you work through the calcs you'll see that half the energy is lost when charging.

Here is a similar thread answering the question for you: https://www.physicsforums.com/showthread.php?t=292838

RK1992
I meant, work you do at home = homework.
hm i dont think so, its revision :S

99% is a very specific number you picked there. Do you have the calcs to back it up?

If you work through the calcs you'll see that half the energy is lost when charging.

Here is a similar thread answering the question for you: https://www.physicsforums.com/showthread.php?t=292838
i just meant most, really..

okay thanks, although im not sure i understand that thread particularly well :/