# Magnetic Field Problem! HELP NEEDED

Doc Al
Mentor
Please don't be rude to me.
No one's being rude to you. We are here to help, but we're not here to replace your instructor or textbook.
Our teacher said that physics is better learned w/out a textbook. he justs gives us problems to work.
That's kind of crazy.

Lucky for you there are several excellent physics texts available for free on the web. Here's one source: http://lightandmatter.com/

Does this have anything to do with induction?
No.

well the Electric Force divided by the charge equals the electric field.

so the electric force is in the postive direction and the charge is positive.

but i still don't understand.

Doc Al
Mentor
well the Electric Force divided by the charge equals the electric field.

so the electric force is in the postive direction and the charge is positive.
Good.

Regarding not having a text: You should complain about this policy. This might fly for graduate seminars (I've certainly had enough classes where the "textbook" was just a pile of original papers and the lectures merely consisted of stories about the good old days. Lots of luck!) but at the introductory level this is nonsense. There is way too much material to rely on just notes that you personally take in class. If anything, it's usually the opposite: The lectures are meant to augment, not replace, the text.

I can guarantee you that your instructor is using a text, even if he won't admit it.

EXACTLY doc al! but our teacher says it is good without a text. so yah, i can't help it now.
i just need to be able to finish this problem today.

so the electric field is in a positive directoion?

but which direction, that is the question, haha...

Astronuc
Staff Emeritus
The problem is one of a velocity selector, in which the force imposed by the electric field is equal and opposite the force imposed by the magnetic field on the moving proton.

Doc Al gave a good link to the Lorentz force on hyperphysics.

Both the electric and magnetic fields are vector fields.

No the proton moves up in a magnetic field which goes into the paper/screen. So what direction is the resultant force based on F = q (v x B)? Bold used to indicate vector.

Then to balance that force, the electric force must be in the opposite direction. In addition to opposite directions, the balancing forces must have equal magnitudes in order to add to zero.

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Doc Al
Mentor
so the electric field is in a positive directoion?
$$\vec{F}_e = \vec{E} q$$

For a positive charge, the electric force is in the same direction as the electric field.

I urge you to study the two sites I provided. Ben Crowell has kindly seen fit to make his textbooks available for free. Take advantage of this!

wait astronuc, what do you mean.

doc al and the other person said that the magnetic force is in the -x direction and the electric force is in the +x direction.

and you said that:
"the electric field is equal and opposite the force imposed by the magnetic field on the moving proton."

so since the magnetic field is into the page (-z) then the opposite would be +z ??

this is still for part a) btw.

okay so i am trying to understand this electric field stufff by the websites. and this is what i get "The electric field from a positive charge points away from the charge; the electric field from a negative charge points toward the charge. " and since we are talking about a proton, it is a positive charge. so the electric filed will point away from the charge.

so is the electirc filed in the positive x direction? since its pointing away?

Astronuc
Staff Emeritus
so since the magnetic field is into the page (-z) then the opposite would be +z ??
Careful. Do not confuse magnetic field with magnetic force.

The magnetic force on a moving charge is perpendicular to the plane formed by the velocity and magnetic field vectors by virtue of v x B.

wait, how am i confusing magnetic force and field?

and i really don't get the perpendicular part.

is doc al back yet?

Doc Al
Mentor
wait astronuc, what do you mean.

doc al and the other person said that the magnetic force is in the -x direction and the electric force is in the +x direction.

and you said that:
"the electric field is equal and opposite the force imposed by the magnetic field on the moving proton."

so since the magnetic field is into the page (-z) then the opposite would be +z ??
Read carefully. Equal and opposite the magnetic force, not the magnetic field. (Astro and I are saying the same thing, of course.)
okay so i am trying to understand this electric field stufff by the websites. and this is what i get "The electric field from a positive charge points away from the charge; the electric field from a negative charge points toward the charge. " and since we are talking about a proton, it is a positive charge. so the electric filed will point away from the charge.

so is the electirc filed in the positive x direction? since its pointing away?
Those sites are talking about the field of a point charge--but that's not related to this problem. You want to understand the force on a charge due to an external field. See my previous post (#32).

Astronuc
Staff Emeritus
The electric field (a vector field) imposes a force (a vector) which is parallel with the field.

A magnetic field (a vector field) imposes a force which is perpendicular to the magnetic field. The magnetic force is proportional cross (vector) product of the velocity vector and magnetic field vector.

One is trying to find the magnitude and direction of the electric field such that the force on the proton is equal in magnitude, but opposite in direction, to the force imposed by the magnetic field ($$\vec{B}$$).

i am not trying to find the magnitutde of the elctric field.

and now i understand you doc al.

but where do i find ben crowell's txt books.

i dont have that much time on my hands. its true.

so if the electric field is not in the +z or +x direction, it must be in the +y direction right?

Astronuc
Staff Emeritus
so if the electric field is not in the +z or +x direction, it must be in the +y direction right?
Go back to post #9, where you are correct on the direction of the magnetic force. The electric force must be in the opposite direction in order to be able to cancel the magnetic force.

i know that Astronuc, i know that the electric Force is in the +x but i am looking for the electic field.

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wiat astronuc said that electirc filed is parallel to magnetic field?
so is it also in the -z diretion??

Doc Al
Mentor
i am not trying to find the magnitutde of the elctric field.
But you do need the direction of the electric field.

but where do i find ben crowell's txt books.
I gave his link in post #26.

so if the electric field is not in the +z or +x direction, it must be in the +y direction right?
Who said that it's not in one of those directions? Answer each of these questions:

Which way does the magnetic force point?

Which way must the electric force point to balance the magnetic force?

So which way must the electric field point to produce such a force? (See post #32.)

Who said that it's not in one of those directions? Answer each of these questions:

Which way does the magnetic force point?

Which way must the electric force point to balance the magnetic force?

So which way must the electric field point to produce such a force? (See post #32.)

1) -x
2) +x
3) +x

3) ?

Doc Al
Mentor
1) -x
2) +x
3) +x
Exactly!

Astronuc
Staff Emeritus
1) -x
2) +x
3) +x
Correct.

If the velocity vector direction is +y, and the magnetic field is +z (into the page), then the cross (vector) product must be -x, by the right hand convention. Therefore the electric field vector must point in the opposite (+x) direction, in order to impose a force on a 'positive' charge in the +x direction.

THANKS!

now for part b) ;)