Capacitors - energy stored vs work done

AI Thread Summary
A 180 microfarad capacitor charged to 100 volts stores 0.9J of energy, calculated using the formula 0.5CV². However, the work done by the battery in charging the capacitor is 1.8J, leading to confusion about why the work done differs from the energy stored. The discussion highlights that some energy is lost during charging, even without a resistor, and calculations indicate that approximately half of the energy used is dissipated. The discrepancy arises because the work done includes energy lost as heat or other forms during the charging process. Understanding this concept is crucial for exam preparation.
RK1992
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this isn't homework, I am 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
 
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RK1992 said:
this isn't homework, I am doing revision for an exam :)

So work you're doing at home, but not homework? :wink:
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?
 
jarednjames said:
So work you're doing at home, but not homework? :wink:
yes, I am a geek :p

jarednjames said:
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 isn't charging through a resistor so surely 99% of the energy used charging will be stored?
 
RK1992 said:
yes, I am a geek :p

I meant, work you do at home = homework.
well no some will dissipate right.. but this isn't 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
 
jarednjames said:
I meant, work you do at home = homework.
hm i don't think so, its revision :S

jarednjames said:
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 I am not sure i understand that thread particularly well :/
 

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