# Homework Help: Positively charged side with induced emf

1. Jun 11, 2009

### VenaCava

1. The problem statement, all variables and given/known data
A solid conductor travels at 150 m/s across a uniform .045T magnetic field. Which side is positively charged and what is the emf across this block?

2. Relevant equations

I actually solved for the emf no problem. I am having difficulty determining which side is positively charged. There is a diagram which accompanies the question. It can be found in this pdf file (first question): http://pittmath.com/Physics12/Ultimate%20Induction%20Review%20Assignment.pdf [Broken]

if you can't open it, the diagram simply shows a rectangular block with a velocity to the left going through a magnetic field going out of the page. The question asks if the positive side of the rectangle is the top or the bottom.

3. The attempt at a solution

I was told to use the right hand rule, but I'm not clear which one or even what I'm looking for.

If you could help that would be greatly appreciated.

Last edited by a moderator: May 4, 2017
2. Jun 11, 2009

### diazona

Do you know the Lorentz force law? (force in terms of charge, velocity, and magnetic field) I'd use that to determine the direction of force on the electrons in the metal. Remember that the direction of the cross product of two vectors is given by the right hand rule.

3. Jun 11, 2009

### VenaCava

I think you lost me a little bit.
I don't understand what makes a side "positive." That's my main problem.

4. Jun 11, 2009

### diazona

Well, think about what happens to the electrons in the metal. They are charged particles, moving through a magnetic field (not parallel to the field), so they will feel a force. That force will tend to push them toward one side of the metal bar. Now, since electrons are negatively charged, the side that the electrons are pushed toward will have more electrons than protons, so it will have a slight excess of negative charge. That's the negative side. The opposite side of the bar will be missing some electrons, so it will have a slight excess of positive charge. That's the positive side.

Does that make sense?

5. Jun 11, 2009

### VenaCava

thank you that helped a lot.