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Finding kilowatt hours from parallel capacitors

by CIERAcyanide
Tags: capacitors, cost, electricity, kilowatts, parallel
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CIERAcyanide
#1
Mar24-10, 03:30 PM
P: 9
1. The problem statement, all variables and given/known data

A parallel-connected bank of 5.00 F capacitors is used to store electric energy. What does it cost to charge the 2900 capacitors of the bank to 45,000 V, assuming a unit cost of $0.03 per kWh?


2. Relevant equations

CV = Q
1 Joule = 2.77E-7 kWh
1 Volt = 1 Joule per Coulomb

3. The attempt at a solution
I'm not sure how to convert from Volts to Joules and then to kilowatt hours. I tried CV=Q to find the Coulombs, from where I wanted to find the number of Joules. If my thinking is correct, the 45,000 Volts means there are 45,000 Joules per Coulomb. When i multiplied 5E-6 times 45,000 to find Q my result was that the charge was only .225 C for one capacitor, which seems low. I'm not sure where to go from here, and I'm very unsure of how to convert from Volts to Joules to Kilowatt hours. I know at the end I need to remember to multiply my final result by 2900 for the number of capacitors. I also don't know how the fact they're connected by parallel plays into this.

Please help!
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ideasrule
#2
Mar24-10, 03:40 PM
HW Helper
ideasrule's Avatar
P: 2,322
Are you familiar with the formula E=1/2CV^2 or E=Q^2/2C? Those are the formulas for the energy stored in a capacitor.
CIERAcyanide
#3
Mar24-10, 03:55 PM
P: 9
Okay, i'll try that and see if i can work out the right answer

CIERAcyanide
#4
Mar24-10, 04:18 PM
P: 9
Finding kilowatt hours from parallel capacitors

The units for E is in joules correct?
ideasrule
#5
Mar24-10, 05:23 PM
HW Helper
ideasrule's Avatar
P: 2,322
That depends on what you use for C, V, and Q. If you use standard SI units, E will come out in joules.


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