# Magnetic Field Problem! HELP NEEDED

[SOLVED] Magnetic Field Problem! HELP NEEDED!!!!!!!

1. Homework Statement

A proton of mass mp and charge e is in a box that contains an electric field E, and the box is located in Earth's magnetic field B. The proton moves with an initial velocity vertically upward from the surface of Earth. Assume gravity is negligible.

(a) On the diagram above, indicate the direction of the electric field inside the box so that there is no change in the trajectory of the proton while it moves upward in the box. Explain your reasoning.

(b) Determine the speed of the proton while in the box if it continues to move vertically upward. Express your answer in terms of the fields and the given quantities.
The proton now exits the box through the opening at the top.

(c) On the figure above, sketch the path of the proton after it leaves the box.

(d) Determine the magnitude of the acceleration a of the proton just after it leaves the box, in terms of the given quantities and fundamental constants.

2. Homework Equations

F=IL*B = qv*B

B= uI / 2piR u=4pi*10-7

I don't have the diagram but i will try to find a picture of it on the internet.

Last edited:

Related Introductory Physics Homework Help News on Phys.org
Kurdt
Staff Emeritus
Gold Member
well that is becasue i am very confused.
i don't even know where to start, that is why i haven't posted anything, usually i do, but i just am lost!

help? :(

Kurdt
Staff Emeritus
Gold Member
Well what have you been doing? I'm sure you have some sort of notes or a book that can give you a slight clue as to what the answer to part a) should be.

i should use the curved hand rule right?

Last edited:
If the magnetic field is also in the box, you need to balance the force by the magnetic field with the force of the electric field, so the trajectory within the box is straight.To do this, you need to think about which way the force from the magnetic points initially, and how to place an electric field so the force on the charge is in the opposite direction. Once you have figured this out, you can start working with some formulas. Also, In such a problem its reasonable to neglect gravity i would think.

thank you engageengage!

okay so for part a) the electircal field is in the postive z direction (out of the page) right?

and for part b) do i use the formula Fm=qv*B ? Where v is the velocity or in other words, the speed? (by the way, Fm=Force magnetic)

PLEASE help! thanks to all who have helped me till now. buti still need to understand how to do the rest of it.
thanks!

Doc Al
Mentor
okay so for part a) the electircal field is in the postive z direction (out of the page) right?
First figure out the direction of the magnetic force on the proton. (You'll need to use the right hand rule.)

First figure out the direction of the magnetic force on the proton. (You'll need to use the right hand rule.)
well if i use the right hand rule, the magnetic force is in the -x direction (to the left).

is that correct?

Doc Al
Mentor
well if i use the right hand rule, the magnetic force is in the -x direction (to the left).

is that correct?
Good! So which way must the electric force act in order to cancel out the magnetic force? So which way must the electric field point?

in order to cancel the magnetic force, the electric field must act in the positive x direction.

so the elctric field should point in the positive z direction?

Hootenanny
Staff Emeritus
Gold Member
so the elctric field should point in the positive z direction?
Now why would you say that? How does the electric field relate to the force experienced by a charged particle?

do they attract each other?

Doc Al
Mentor
do they attract each other?

You need to understand how to find the force on a charge due to both electric and magnetic fields. Look it up in your textbook.

we don't have a textbook.

can someone explain to me "how to find the force on a charge due to both electric and magnetic fields. " ????? Please?

Doc Al
Mentor
we don't have a textbook.
Say what? What do you have?

Hootenanny
Staff Emeritus
Gold Member
can someone explain to me "how to find the force on a charge due to both electric and magnetic fields. " ????? Please?

Edit: Sorry Doc, didn't see you there.

Please don't be rude to me.

Our teacher said that physics is better learned w/out a textbook. he justs gives us problems to work.

Does this have anything to do with induction?

Currently, i am studying my class notes. i have been, i just cna't seem to find anything about electic fields.
in my class notes it says taht E= Fe/q

Last edited:
Doc Al
Mentor
Last edited by a moderator:
Kurdt
Staff Emeritus
Gold Member
Please don't be rude to me.

Our teacher said that physics is better learned w/out a textbook. he justs gives us problems to work.

Does this have anything to do with induction?
I'd say your teacher was very wrong.

Doc Al
Mentor
Currently, i am studying my class notes. i have been, i just cna't seem to find anything about electic fields.
in my class notes it says taht E= Fe/q
That's all you need.

Seriously: You are taking a class with an instructor and there's no textbook?? You are just relying on notes that you take in class? Or notes given out by the instructor?

i know my teacher is wrong. but i can't do anything about it.
i thought that homework helpers would help me understand the problem like you were by asking me the magnetic force and field and stuff, but i don't understand electric fields so why can't somone explain it to to me?

That's all you need.

Seriously: You are taking a class with an instructor and there's no textbook?? You are just relying on notes that you take in class? Or notes given out by the instructor?

yes i am serious. out teacher gives us notes. he talks we write. period. no textbook or anything.

Hootenanny
Staff Emeritus
Gold Member
Our teacher said that physics is better learned w/out a textbook. he justs gives us problems to work.
No he doesn't just give you worked problems, he clearly gives you class notes as well,
Currently, i am studying my class notes.
In my class notes it says taht E= Fe/q
So what does that tell you about the direction of the electric force in relation to the electric field?
i can't right now. it is a Sunday. no store is open. this problem is due today.
You have homework due in for a Sunday?

Edit: I'm going to take a bow out of this thread, too many cooks spoil the broth.

Last edited: