# E and magnetic field

1. Oct 8, 2011

### charlies1902

A beam of protons is moving in the +x direction through a region where the e field is perpendicular to the magnetic field. The beam is not deflected.

For the part "the beam is not deflected," how do you know if it'll be deflected or not? Or will that usually be given? Also why does the force of the e field and the force of the magnetic field add up to = 0?

2. Oct 8, 2011

3. Oct 8, 2011

### SammyS

Staff Emeritus
Please don't "bump" your thread before waiting 24 hours. That's one of the rules for these Forums.

How much do you know about the force that a magnetic field can exert on a charged particle?

How much do you know about the force that an electric field can exert on a charged particle?

4. Oct 8, 2011

### charlies1902

I'm not sure if I understand what you're asking. can you clarify?

5. Oct 8, 2011

### SammyS

Staff Emeritus
Sure, what part of my post wasn't clear?

1) Bumping?

2) what you know regarding mag. field?

3) what you know regarding E field?

6. Oct 8, 2011

### charlies1902

The last 2 questions. I know the equations and how to use them, if that's what you're asking.

7. Oct 8, 2011

### Staff: Mentor

If they tell you that the electrons are not deflected, then the net force must be zero (Newton #1). That tells you something about the orientation and relative magnitudes of the fields. Otherwise, if they give you the field details it's up to you to determine the net result!

8. Oct 9, 2011

### charlies1902

Does deflection basically mean the direction the proton is going is changed when it moves thru the field? So if it stays in the same motion that means sum of forces=0.

Also, the next question asks if the protons were replaced with electrons, would they be deflected? I don't know how to explain this. I just said when you sum up forces charge ends up cancelling out so the sign of the charge will not matter. Is that a legit justification?

9. Oct 9, 2011

### Staff: Mentor

Correct.
I think what you mean is that, since the forces involved are both proportional to the magnitude of the charge, if they cancel for one sign of charge they must cancel for the other.

10. Oct 9, 2011

### charlies1902

after I stated that I second guessed myself. Since the question asks if the e- will be deflected we can't assume that the force of the e field and the force of the magnetic field is equal to 0 right? If so, then I can't say the charges canceled out.

11. Oct 9, 2011

### Staff: Mentor

In order for the force of the individual fields to be zero there would have to be no fields.

In order for the sum of the forces due to the fields to be zero they must have a particular ratio of strengths and particular orientations with respect to the particle motion.

The charges do not cancel out if you are summing the forces. But they do affect the magnitude of the forces to the same degree. Write the equations for each force. Write their sum. Can you factor the charge out of the sum?

12. Oct 9, 2011

### charlies1902

Uh, if I factor it out then wouldn't that means the charge does come into play? The only way I see them as not having an effect is if the 2 forces are set to 0, thus the charge is canceled.

13. Oct 9, 2011

### Staff: Mentor

Of course it comes into play! It's setting the magnitudes of both forces. But if both forces scale in the same way with the charge, then they will have the same ratio no matter what the charge, and will continue to cancel.

Write the equations for each. Write the expression for the sum of them. If you can factor q out of that expression so that it stands alone, i.e., F = q*(stuff - otherstuff), and if F=0, then either q is zero (and it's not!) or (stuff - otherstuff) is zero and will be zero for ANY charge q.