Question

When is Emf= vBl negative like (Emf=- vBl ) and when is it positive?

Answer 1

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

The emf would be negative if the vector \(\vec v\times\vec B\) is negative.

It depends on your coordinate system, how you allocate the signs.

But you can always look at the velocity and magnetic field vectors.

In this drawing, the current will be out of the screen.
Swap the vectors, and it will be into the screen.

Check out the right hand rule! :)

Answer 2

thanks!

Answer 3

You're welcome! When I answered this, I was in a rush. Let me know if you have any questions!