How to Determine Proton Movement in Electric and Magnetic Fields?

AI Thread Summary
To determine proton movement in electric and magnetic fields, it's crucial to understand the forces acting on the proton. The electric field must be oriented to counteract the magnetic force, ensuring the proton's trajectory remains straight. The magnetic force direction can be identified using the right-hand rule, while the electric force direction aligns with the electric field. The discussion highlights confusion over the relationship between electric and magnetic forces, emphasizing the need for clarity on vector fields and their interactions. Understanding these principles is essential for solving the problem effectively.
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[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.
 
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The reason nobody is answering this thread is because you haven't tried to solve any of the problems yourself.

https://www.physicsforums.com/showthread.php?t=94379
 
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? :(
 
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?
physicstestprobelm.jpg
 
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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!
 
physicsbhelp said:
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.)
 
Doc Al said:
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?
 
  • #10
physicsbhelp said:
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?
 
  • #11
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?
 
  • #12
physicsbhelp said:
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?
 
  • #13
do they attract each other?
 
  • #14
physicsbhelp said:
do they attract each other?
:confused:

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.
 
  • #15
we don't have a textbook.
 
  • #16
can someone explain to me "how to find the force on a charge due to both electric and magnetic fields. " ? Please?
 
  • #17
physicsbhelp said:
we don't have a textbook.
Say what? What do you have?
 
  • #18
physicsbhelp said:
can someone explain to me "how to find the force on a charge due to both electric and magnetic fields. " ? Please?
If you haven't got a textbook, read your class notes.

Edit: Sorry Doc, didn't see you there.
 
  • #19
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
 
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  • #20
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  • #21
physicsbhelp said:
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.
 
  • #22
physicsbhelp said:
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?
 
  • #23
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?
 
  • #24
Doc Al said:
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.
 
  • #25
physicsbhelp said:
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,
physicsbhelp said:
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?
physicsbhelp said:
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.
 
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  • #26
physicsbhelp said:
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.
 
  • #27
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.
 
  • #28
but i still don't understand.
 
  • #29
physicsbhelp said:
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.
 
  • #30
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...
 
  • #31
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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  • #32
physicsbhelp said:
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!
 
  • #33
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 ??
 
  • #34
this is still for part a) btw.
 
  • #35
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?
 
  • #36
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.

Please follow Doc Al's guidance.
 
  • #37
wait, how am i confusing magnetic force and field?

and i really don't get the perpendicular part.

is doc al back yet?
 
  • #38
physicsbhelp said:
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.)
physicsbhelp said:
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).
 
  • #39
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}).
 
  • #40
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 don't have that much time on my hands. its true.
 
  • #41
so if the electric field is not in the +z or +x direction, it must be in the +y direction right?
 
  • #42
physicsbhelp said:
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.
 
  • #43
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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  • #44
wiat astronuc said that electirc filed is parallel to magnetic field?
so is it also in the -z diretion??
 
  • #45
physicsbhelp said:
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.

physicsbhelp said:
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.)
 
  • #46
Doc Al said:
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
 
  • #47
3) ?
 
  • #48
physicsbhelp said:
1) -x
2) +x
3) +x
Exactly!
 
  • #49
physicsbhelp said:
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.
 
  • #50
THANKS!


now for part b) ;)
 

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