# Homework Help: Magnetism help - direction of charges

1. Mar 17, 2006

### Da-Force

I've already taken the test but this is lingering in my head.

We have two electrons in an area of 0 magnetic field.

One electron is at rest while the other is moving westward. We then apply a magnetic field B moving eastward. What direction are the electrons moving?

Note: No direction is not an answer... All 5 possible choices stated that both electrons moved in a direction. The question is what DIRECTION are they moving.

Image for you, animated by me just in case you are better with an image

http://xs72.xs.to/pics/06116/electronmotion.gif [Broken]

Last edited by a moderator: May 2, 2017
2. Mar 17, 2006

### Staff: Mentor

Start by answering this question: What force does the magnetic field exert on each electron?

(I assume that you can ignore the forces the electrons exert on each other.)

3. Mar 17, 2006

### Da-Force

Right Hand Rule doesn't work LOL (Left Hand in this case)

It's kinda tricky.. I've been able to answer most of all other kinds of problems, but this one is tough.

Moving Electron has no force while I ain't sure about stationary.. :-X

4. Mar 17, 2006

### Staff: Mentor

Why does the moving electron experience no force? (You're correct, but why?)

Does a stationary charge experience a force in a magnetic field?

What's the rule for the magnetic force on a charge?

5. Mar 17, 2006

### Da-Force

|F|=qVBsin@ where @ = 180 degrees.

A stationary charge couldn't experience a force if v=0 based on the equation...

Doc, I know all this.. What's wrong with this problem?

6. Mar 17, 2006

### Staff: Mentor

Right. So you agree that the force on each charge is zero. So, what does Newton's 2nd law tell you?

Nothing. You just have to get your mind around it. Hint: If the force is zero, how will the velocities change due to the presence of the magnetic field?

7. Mar 17, 2006

### Da-Force

They don't change, but all the possible choices stated both particles moved.

That's what I can't get my mind over.

8. Mar 17, 2006

### Staff: Mentor

Can you recall the exact choices?

(The only way that both charges will move is if you are meant to include the force that they exert on each other: They repel each other.)

9. Mar 17, 2006

### Da-Force

Umm..

Something like:

Stationary e-:
North
South
East
Up
Down

Moving e-:
East
West
Southward towards Earth
North
Down

10. Mar 17, 2006

### Staff: Mentor

I'd have to say that the given answers sound bogus. (What was the relative position of the two charges?)

11. Mar 17, 2006

### Da-Force

They don't give you relative positions.

They explained that they put these two charges at a point in space where there is no magnetic field. Then they add a magnetic field X_X