# Find current in a wire?

1. Mar 5, 2013

### Colts

1. The problem statement, all variables and given/known data
(a) The current through a wire is a steady 2.5 amps. How much current passes through it between t = 0 seconds and t = 4 × 10^-4 seconds?

For this one I thought it would be 2.5( 4 × 10^-4), but it says how much current and I thought current was a rate. So what does the question mean?

(b) The current through a wire is given by I(t) = I0e -at, where I0 = 2.5 amps and a = 6 × 103 s-1. How much electric charge passes through the wire between t = 0 seconds and t = 4 × 10-4 seconds?

This is the same way. Do I just plug t in?

2. Relevant equations

3. The attempt at a solution

2. Mar 5, 2013

### rude man

You're quite right. It should have asked for the charge, not the current. The current is 2.5A, no matter when.

(b) The current through a wire is given by I(t) = I0e -at, where I0 = 2.5 amps and a = 6 × 103 s-1. How much electric charge passes through the wire between t = 0 seconds and t = 4 × 10-4 seconds?

This is the same way. Do I just plug t in?
[/quote]

No. Charge is the integral of current. You need to integrate the current to get the charge.

2. Relevant equations

3. The attempt at a solution[/QUOTE]

3. Mar 5, 2013

### Staff: Mentor

You are correct: Current is a rate. I would just treat it as a sloppily worded problem and assume they meant how much electric charge passes through it. (Just like in the second question.)

The current isn't constant, so you need to be careful. Hint: Use a bit of calculus.