# Ampere's circuital law on finite length wire

1. Apr 17, 2007

### s.gautam

When we apply ampere's circuital law to finite length wire,we get the wrong answer.Why is that? The symmetry rule is being followed,so thats not the problem.
Is it because a finite length wire means that charge is piling up somewhere which means that the current is not steady?

2. Apr 17, 2007

### Hootenanny

Staff Emeritus
3. Apr 17, 2007

### s.gautam

Thanks a lot,that was really helpful.

4. Apr 17, 2007

### Hootenanny

Staff Emeritus
Pleasure Welcome to the forums!

5. Apr 21, 2007

### s.gautam

Hey another thought occured to me,how can we apply biot-savart's law to determine the magnetic field of a finite length wire? Biot-savart's law is also valid for only steady currents.

6. Apr 21, 2007

### s.gautam

I guess I should post it in a new thread.

7. Apr 21, 2007

### arunma

A wire of finite length can have a steady current, same as a wire of infinite length. Indeed this is precisely the case in any DC circuit.

8. Apr 22, 2007

### s.gautam

Yes,it can,but not until its in a closed loop.With biot-savart law,we find out the field of a finite segment of wire which is not a closed loop,and this is whats troubling me.

9. Apr 22, 2007

### NeoDevin

You're right, it does need to be in a closed loop, but what we can do when we calculate it, is calculate the contribution to the magnetic field of the finite straight wire. It is still left to calculate the field of the rest of the loop. If you're working out of griffith's 2nd ed. See for example Problem 5.37, where you calculate the contribution from one side of the square then multiply it by four.