# Find charge given current and time?

1. Jan 21, 2012

### ObviousManiac

1. The problem statement, all variables and given/known data
If a current of 3.5 A flows through a wire for 25 minutes, how much charge passes through the wire?

I = 3.5
t = 25 min = 1500 seconds
2. Relevant equations
P=W/t
W=1/2qV
P=VI

3. The attempt at a solution

If P = W/t
and
W = 1/2qV
then
P = qV/2t

so (qV/2t) = VI

qV = 2tVI

q = 2tI

q = 2(1500)(3.5)

q = 10,500 C

.......................

the answer given by my teacher is 5250 C. So somewhere, I'm off by a factor of two. Can someone help me find my error? Or did my teacher make a mistake?

2. Jan 21, 2012

### SammyS

Staff Emeritus
For what situation is your equation, W = (1/2)qV, valid?

All you really need for this problem is the definition of electric current.

3. Jan 21, 2012

### ObviousManiac

I=q/t

q=It

q=(3.5)(1500)

q=5250 C

...........................

Okay so I get that, but now I'm struggling with...
I'm confused... i.e. the difference between using
W=(1/2)qV
and
W=qV

One is the charge of the capacitor the other is the charge on the electron/proton (usually these particles are the one's being talked about). This is the W=qV equation... i think...

I read online that "W = qV is more for moving just one or a set number of charges across a voltage difference V (ie a capacitor) while W = 1/2QV is the work required to fully charge, or move ALL of the charges, across a capacitor to fully charge it"

...is that right? I don't really understand...

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