Find charge given current and time?

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SUMMARY

The discussion centers on calculating electric charge using the formula q = It, where I is the current (3.5 A) and t is the time in seconds (1500 seconds). The correct calculation yields a charge of 5250 C, contrasting with the incorrect calculation of 10,500 C derived from an erroneous application of the work-energy principle. The confusion arises from the use of different equations for work done on charges, specifically W = qV and W = (1/2)qV, which apply in different contexts, such as charging a capacitor versus moving individual charges.

PREREQUISITES
  • Understanding of electric current (I = q/t)
  • Familiarity with the concept of charge (q) in physics
  • Knowledge of work-energy principles in electrical contexts
  • Basic understanding of capacitors and their charging equations
NEXT STEPS
  • Study the relationship between current, charge, and time in electrical circuits
  • Learn about the work-energy principle in the context of electric fields
  • Explore the differences between W = qV and W = (1/2)qV in capacitor charging
  • Investigate practical applications of electric charge calculations in circuit design
USEFUL FOR

Students studying physics, electrical engineers, and anyone interested in understanding electric charge calculations and their applications in circuits.

ObviousManiac
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Homework Statement


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

Homework Equations


P=W/t
W=1/2qV
P=VI

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?
 
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ObviousManiac said:

Homework Statement


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

Homework Equations


P=W/t
W=1/2qV
P=VI

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

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

I=q/t

q=It

q=(3.5)(1500)

q=5250 C

......

Okay so I get that, but now I'm struggling with...
SammyS said:
For what situation is your equation, W = (1/2)qV, valid?
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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