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Question about electromagnetism

by scientist91
Tags: electromagnetism
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scientist91
#1
May31-07, 12:30 PM
P: 133
I have one question. It is about current in conductor. It is about electromagnet induction. So I move the magnet among conductor in closed circular loop, like on this picture.
So is the way of moving of the electrons depends from the way of the moving magnet?
Let's say I move the magnet in direction of the arrow (like on the pic), so the electrons will move in the direction of the arrow (like on the picture), right? Thank you.
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mcstar
#2
May31-07, 03:48 PM
P: 11
hi!

there's the third maxwell relation: curl E == -d/dt(B).
You than use stokes theorem to get its integral form:
int[E*dl](contour)==int[-d/dt(B)](surface, enclosed by contour).
So, you have your permanent magnet, it's like a manetic dipole. Its magnetic field attenuates like 1/r^3. So you pick a turn in your coil, and move your dipole into it. Than you're integrating the magnetic flux density(B) on the surface, enclosed by the wire(the one turn in the coil), and you get an electromive force, that drives electrons through the wire, like a voltage applied on some other wire, according to Ohm's law.

So as you can see, bringing the dipole closer to the loop, B is rising, than as you left the center of the loop, the B is getting weaker. So first you get a current flowing one direction, and than in the opposite direction. Than you can generalize, and take the whole coil into consideration.
scientist91
#3
May31-07, 04:16 PM
P: 133
Quote Quote by mcstar View Post
hi!

there's the third maxwell relation: curl E == -d/dt(B).
You than use stokes theorem to get its integral form:
int[E*dl](contour)==int[-d/dt(B)](surface, enclosed by contour).
So, you have your permanent magnet, it's like a manetic dipole. Its magnetic field attenuates like 1/r^3. So you pick a turn in your coil, and move your dipole into it. Than you're integrating the magnetic flux density(B) on the surface, enclosed by the wire(the one turn in the coil), and you get an electromive force, that drives electrons through the wire, like a voltage applied on some other wire, according to Ohm's law.

So as you can see, bringing the dipole closer to the loop, B is rising, than as you left the center of the loop, the B is getting weaker. So first you get a current flowing one direction, and than in the opposite direction. Than you can generalize, and take the whole coil into consideration.
So when I move the magnet it is possible that the electrons will move on any direction right?

mcstar
#4
May31-07, 04:33 PM
P: 11
Question about electromagnetism

no, not any direction. either direction is correct.
you grab the coil with you right hand, and in the direction, your fingers curl, will the current flow, when you pull out the dipole, so the loop experiences weaker and weaker B-field. when you mive the dipole in, the opposite is true.
scientist91
#5
Jun1-07, 01:01 AM
P: 133
Quote Quote by mcstar View Post
no, not any direction. either direction is correct.
you grab the coil with you right hand, and in the direction, your fingers curl, will the current flow, when you pull out the dipole, so the loop experiences weaker and weaker B-field. when you mive the dipole in, the opposite is true.
do the electrons move from the stronger B field to the weaker B field?
mcstar
#6
Jun1-07, 05:04 AM
P: 11
you dont seem to understand the whole idea.
the electrons doesnt move because of the B field. they move, because a time-dependent magnetic field is present in the loop, and it induces an electric field, which is not curl-free. this field drives the electrons.
magnetic fields exert a force on charges, its the Lorentz-force:
F=q*v x B, where x is the cross product of the velocity of the electron and the magnetic flux density vector.

however i strongly recommend you to grab some physics textbooks, and read them carefully, these things must be in them, and probably they can teach you better than me.
scientist91
#7
Jun1-07, 01:31 PM
P: 133
Quote Quote by mcstar View Post
you dont seem to understand the whole idea.
the electrons doesnt move because of the B field. they move, because a time-dependent magnetic field is present in the loop, and it induces an electric field, which is not curl-free. this field drives the electrons.
magnetic fields exert a force on charges, its the Lorentz-force:
F=q*v x B, where x is the cross product of the velocity of the electron and the magnetic flux density vector.

however i strongly recommend you to grab some physics textbooks, and read them carefully, these things must be in them, and probably they can teach you better than me.
I know all of that things. But my question is that the electrons go from anode to cathode. So there are two direction that can electrons move. From the right and the left, can they move (the electrons) in opposite directions?
mcstar
#8
Jun1-07, 04:17 PM
P: 11
Quote Quote by mcstar View Post
no, not any direction. either direction is correct.
you grab the coil with you right hand, and in the direction, your fingers curl, will the current flow, when you pull out the dipole, so the loop experiences weaker and weaker B-field. when you mive the dipole in, the opposite is true.
i already wrote the answer.
scientist91
#9
Jun1-07, 05:25 PM
P: 133
Quote Quote by mcstar View Post
i already wrote the answer.
can you draw some picture please (because of my bad english)? Thank you very much.
Mentz114
#10
Jun1-07, 05:27 PM
PF Gold
P: 4,087
I know all of that things. But my question is that the electrons go from anode to cathode.
I don't think you know anything. Did you understand posts #2 and #6 ?

Electrons go from cathode to anode. There's no cathode or anode in your setup. As has been told to you in all your threads - the answer is in Maxwell's equations. Go away and study them.
scientist91
#11
Jun2-07, 02:33 AM
P: 133
Quote Quote by Mentz114 View Post
I don't think you know anything. Did you understand posts #2 and #6 ?

Electrons go from cathode to anode. There's no cathode or anode in your setup. As has been told to you in all your threads - the answer is in Maxwell's equations. Go away and study them.
Is the magnetic field different, of the both of the cases?
mcstar
#12
Jun2-07, 12:01 PM
P: 11
hello!

i worked out the problem, its available here:
http://users.hszk.bme.hu/~pf650/em_problem.pdf


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