# Simple Question about the direction of magnetic induction

## Homework Statement

There is a magnetic field B=0.02 Tesla and its direction is toward the plane (perpendicular to plane, shown by X X X X X - I'm not sure how to say this direction in English,sorry). we have a copper wire (uniform bar) perpendicular to magnetic field from south to north. we move the wire with speed 8 m/s from west to east. We want to find the induced emf and the place where the positive charge is induced (north or south)

Length of wire=0.25 m

2. The attempt at a solution
I can calculate the first part easily with V = BLV and we get 0.04 volt. But I don't know how to determine the place where the positive charge is induced. How I should solve this?

Thanks in advance and sorry for my English.

Last edited:

rude man
Homework Helper
Gold Member

## Homework Statement

There is a magnetic field B=0.02 Tesla and its direction is toward the plane (perpendicular to plane, shown by X X X X X - I'm not sure how to say this direction in English,sorry). we have a copper wire (uniform bar) perpendicular to magnetic field from south to west. we move the wire with speed 8 m/s from west to east. We want to find the induced emf and the place where the positive charge is induced (north or south)

2. The attempt at a solution
I can calculate the first part easily with V = BLV and we get 0.04 volt. But I don't know how to determine the place where the positive charge is induced. How I should solve this?

Thanks in advance and sorry for my English.
You didn't give L so we can't confirm your answer of 0.04V.

To get the charge locations, use the Lorentz law.

• titansarus
You didn't give L so we can't confirm your answer of 0.04V.

To get the charge locations, use the Lorentz law.
Sorry, The L is 0.25 meter. Can you give more information about how to use lorentz law (If you mean F = q v cross b). It gives the force, how can I get the position of positive charge with that?

rude man
Homework Helper
Gold Member
Sorry, The L is 0.25 meter. Can you give more information about how to use lorentz law (If you mean F = q v cross b). It gives the force, how can I get the position of positive charge with that?
Charge goes in the direction of force does it not?
Your answer is incorrect. Think about tthe direction of the bar in relation to the direction of the velocity.

• titansarus
Charge goes in the direction of force does it not?
Your answer is incorrect. Think about tthe direction of the bar in relation to the direction of the velocity.
The direction of force for q>0 is to north. so it means that positive charge goes to north,yes?

But I think v=0.04 volt is correct, Are you sure it is incorrect?

rude man
Homework Helper
Gold Member
The direction of force for q>0 is to north. so it means that positive charge goes to north,yes?
You must decide for yourself.
But I think v=0.04 volt is correct, Are you sure it is incorrect?
If the direction of the bar were east-west, i.e. along v, what would the emf be then?

• titansarus
You must decide for yourself.
If the direction of the bar were east-west, i.e. along v, what would the emf be then?
Oh, the main question was in another language and I mistranslated it. It was meant to be south-north. for south-west, it is sqrt(2) * 0.02, and for east-west, 0, yes?

rude man
Homework Helper
Gold Member
Oh, the main question was in another language and I mistranslated it. It was meant to be south-north. for south-west, it is sqrt(2) * 0.02, and for east-west, 0, yes?
Carry out the vector expression emf = B⋅(L x v). (This also answers your first question).