# Homework Help: Current density

1. May 13, 2014

### cuongbui1702

that is my solution:
I=J.A=J.∏r^2
Since r =a => J=0 => I=0 ≠ Io
I think this is wrong but i cant answer why this solution was wrong. Help me Please

2. May 13, 2014

### Staff: Mentor

You need to write and solve the integral equation in order to show that the total current = Io...

3. May 13, 2014

### cuongbui1702

I read solution, and they also used the integral equation. I do other exercise, i only took I=J.A, and i had a right result. But in this problem, i did not know use the integral equation, why i need to use that?

4. May 13, 2014

### Staff: Mentor

Because the current density varies with radius. When something varies like that, to calculate the total accurately, you need to use integration. Does that make sense?

5. May 13, 2014

### tms

You need to use the integral because that is what you need to do to solve the problem. You are given the current density as a function of radius, and need to find the total current. When you have a problem like that, you integrate. That's what integration is, a kind of sum, and you here have to sum up a bunch of little bits of current into the total current.