# Finding the direction of current given a varying magnetic...

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1. Apr 18, 2017

### alexdr5398

1. The problem statement, all variables and given/known data

2. Relevant equations
μ0 In,e = ∫B⋅ds

3. The attempt at a solution
I really don't how to approach this question at all.

Do you have to integrate counterclockwise around the loop every time? If the field was decreasing as y decreased, would you integrate CW or is it still CCW?

Why does μ0 In,e = ∫B⋅ds being >0 mean that it must be out of the page.

2. Apr 19, 2017

### BvU

The direction of $\vec {ds}$ has a meaning. You go around in a mathematically positive direction (CCW).

Ampere's law is an integral form of one of the maxwelll equations.

If $d\vec l$ in the link (your $\vec {ds}$ ) goes around CCW then $\vec {d\bf {S}}$ is pointing towards you (the positive z direction).

If the field is decreasing you still go around CCW and you get a negative $\vec I$ -- i.e. pointing in the negative z direction.

3. Apr 19, 2017

### alexdr5398

Okay, I understand. Thank you.