# Electromagnetic Field

DB
Why is it that as a particle propagates through an electromagnetic field, the electric and magnetic forces are perpendicular to each other? (through the straight line of the particles motion) What effect does this have on the particle's motion, wavelenght and frequency?

Thanks

The force on the particle is Lorentz's Force : F = qE + q(VxB). Are you asking for a demonstration of this?

If you want a specific trajectory for the particle, you need to describe the EM field more specifically, as an EM field can be shaped to cause any trajectory you want.

In classical physics, particles do not have a wavelength or frequency. (unless they have an oscillatory trajectory, of course.

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DB said:
Why is it that as a particle propagates through an electromagnetic field, the electric and magnetic forces are perpendicular to each other? (through the straight line of the particles motion) What effect does this have on the particle's motion, wavelenght and frequency?

Thanks

Let's discuss the very simple common case when an electrically charged particle is moving in the electromagnetic field created by other sources (not by the particle itself).Then the nonrelativistic force is the famous Lorentz force...The fields E and B are solutions of the Maxwell equations and it can be shown that they are perpendicular one to another...That's all.

Things could become really complicated if the particle is relativistic...Or if the field is quantum or if the particle is quantum...

Daniel.

DB
So their is no specific reason why an E and B field are perpidicular to each other?

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I just told you that there is:if both E & B are solutions to the field equations,then in the EM field they are perpendicular one to another and moreover,it can be shown to be BOTH perpendicular on the direction the field (par éxample a EM wave) is propagating...

Daniel.

pmb_phy
DB said:
Why is it that as a particle propagates through an electromagnetic field, the electric and magnetic forces are perpendicular to each other? (through the straight line of the particles motion) What effect does this have on the particle's motion, wavelenght and frequency?

Thanks
The simple and most truthful answer is - nobody knows. That's why there is a law regarding it. Its taken as a postulate in the theory or electrodynamics and relativity. The law states that any charged particle moving in an EM field is subjected to the force

F = q(E + vxB)

This is called the Lorentz Force Law. It seems like I did something more with this in the past year but I don't recall where I put it. I'll get back to you if I think of it.

Pete

DB
thanks pete

Staff Emeritus
DB said:
So their is no specific reason why an E and B field are perpidicular to each other?

One can set the direction of the E field at a particular point to be any desired direction with a pair of parallel charged plates. One can indpendently set the direction of the B field with a magnet or electromagnet. Therefore E and B do not have to be perpendicular.

marlon
pervect said:
One can set the direction of the E field at a particular point to be any desired direction with a pair of parallel charged plates. One can indpendently set the direction of the B field with a magnet or electromagnet. Therefore E and B do not have to be perpendicular.

Are you sure. I don't see how this can be done indepently of each other. Please elaborate because i really would like to know.

After introdicing the B-field, it WILL have influence on the already present E-field and the other way around. This is basic electrostatics and magnetostatics...

marlon

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Marlon,please "elaborate" on the "magnetostatics,electrostatics" and the "influnce" of B over E...

Daniel...

P.S.U got me all confused,man...Are u trying to rewrite electrodynamics...??

marlon
dextercioby said:
Marlon,please "elaborate" on the "magnetostatics,electrostatics" and the "influnce" of B over E...

Daniel...

P.S.U got me all confused,man...Are u trying to rewrite electrodynamics...??

What do you mean ?

Are you saying you don't know what electrostatics and magnetostatics are ?

marlon :uhh:

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:rofl: :rofl: Me??Maybe,u...U didn't answer (intentionally avoided) my question...

What have electrostatics,magnetostatics and "field influence" in common??

Daniel.

marlon
dextercioby said:
:rofl: :rofl: Me??Maybe,u...U didn't answer (intentionally avoided) my question...

What have electrostatics,magnetostatics and "field influence" in common??

Daniel.

Well, i am very sorry but if you claim to know these subjects then why are you asking about the connection between the E-field and the B-field?

Really, what are you trying to prove here ?

marlon

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That u made an erroneous remark... :uhh:

Daniel.

marlon
dextercioby said:
That u made an erroneous remark... :uhh:

Daniel.

Hmmm

then what, according to you is wrong about my remark ?

marlon

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marlon said:
After introdicing the B-field, it WILL have influence on the already present E-field and the other way around. This is basic electrostatics and magnetostatics...

marlon

To me this sounds very conspicuous... :yuck: Electrostatics and magnetostatics have nothing in common with B->E and viceversa...
Please take another look at the equations of electrostatics & magnetostatics and tell me I'm right... :tongue2:

Daniel.

marlon
dextercioby said:
To me this sounds very conspicuous... :yuck: Electrostatics and magnetostatics have nothing in common with B->E and viceversa...
Please take another look at the equations of electrostatics & magnetostatics and tell me I'm right... :tongue2:

Daniel.

This is not a reason. Again i ask you : what is wrong with my remark?

marlon

Are you saying that the E field and the B field have no mutual connection...We are talking about the combination of both these subjects...

Homework Helper
Marlon,i ain't going to quote you on each reply...It's not magntostatics and neither electrostaics involved when discussing the "bond" between the electric field & the magnetic field...
Your mistake was insterting the 2 long words in the paragraph they should have never been in the first place.Plus the rather dubious:
marlon said:
This is basic electrostatics and magnetostatics...
What is basic electrstatics and magnetostatics...??

Daniel.

P.S.Damn,i quoted u again... :tongue2:

marlon
dextercioby said:
Marlon,i ain't going to quote you on each reply...It's not magntostatics and neither electrostaics involved when discussing the "bond" between the electric field & the magnetic field...
Your mistake was insterting the 2 long words in the paragraph they should have never been in the first place.Plus the rather dubious:

What is basic electrstatics and magnetostatics...??

Daniel.

P.S.Damn,i quoted u again... :tongue2:

Sighs,...please, stop starting discussions based upon personal views...

marlon

ps : basic means elementary

Homework Helper
Is that a rather elegant way to end discussions in which u're wrong...??

Daniel.

P.S.Maybe i'll teach QM one day and i'll surely as hell won't use the word "orbital" as a substantive... :tongue2:

marlon
dextercioby said:
Is that a rather elegant way to end discussions in which u're wrong...??

Daniel.

P.S.Maybe i'll teach QM one day and i'll surely as hell won't use the word "orbital" as a substantive... :tongue2:

For the last time, i politely ask you what was my mistake ?

ps : you already know what orbital means ?

marlon

Homework Helper
Well,Marlon,no mean to offend,but you've had almost an hour to figure out that the your remark with "basic electrostatics and megnetostatics" was totally inappropriate in the context your previous phrase had created...

Daniel.

Staff Emeritus
If you put a charge up a parallel plate capacitor to a voltage V, the charge on each plate will be q = C*V, and the electric field will be V/d, where d is the distance between the plates. The direction of the field will be normal to the plates.

The magnetic field inside a charged capacitor will be zero.

If you place a magnet near a charged capacitor, the arrangement of charge on the capacitor will not be disturbed. The Lorentz force law shows that a magnetic field will affect only moving charges. The charges on the capacitor plates will be nonmoving when the capacitor is in equilibrium.

You may get some small disturbance of the electric field if the magnet is conductive, but this can be minimized. There's no intrinsic reason that the presence of a static magnetic field will affect a static electric field.

The presence of the electric field will not disturb the magnet either.

In short, there are six independent quantities, Ex, Ey, Ez, Bx, By, Bz that describe the electromagnetic field at any point in space. Each of them can be set independently of the other. If you set Ex and Bx to be nonzero, and the other four components to be zero, you have set up a situation where the electric and magnetic fields are not perpendicular.

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Okay,and to put it a form so that Marlon could finally understand what u were talking about..."That is basic electrostatics and magnetostatics"... :tongue2: The field equations are decoupled.U can do whatever u want with the fields,since they are independent of each other...

Daniel.

Gza
i guess dexter wins that round!

DB said:
Why is it that as a particle propagates through an electromagnetic field, the electric and magnetic forces are perpendicular to each other?
Electric and magnetic forces need not be perpendicular at all. For example, magnetic and electric fields can be perpendicular to each other in such a way that the electric and magnetic forces are parallel. One can make them in opposite directions so that they cancel. Such a device is called a spin rotator, or Wien filter. It is in common use; I've designed some myself.

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pervect said:
If you place a magnet near a charged capacitor, the arrangement of charge on the capacitor will not be disturbed. The Lorentz force law shows that a magnetic field will affect only moving charges. The charges on the capacitor plates will be nonmoving when the capacitor is in equilibrium.

Now, if i place the magnet near the capicitor and then move with a constant velocity v, then the charges on the capicator will have a velocity relative to me. Therefore they should experience a force and are expected to accelerate and rearrange.
But there will also be Electric field in the new frame. My question is whether the existing electric field due to the charges in the capactior will change so as to produce no net force, or whether there will be an electric field due to the magnet also in the new frame.

Staff Emeritus
siddharth said:
Now, if i place the magnet near the capicitor and then move with a constant velocity v, then the charges on the capicator will have a velocity relative to me. Therefore they should experience a force and are expected to accelerate and rearrange.
But there will also be Electric field in the new frame. My question is whether the existing electric field due to the charges in the capactior will change so as to produce no net force, or whether there will be an electric field due to the magnet also in the new frame.

I'm not quite sure who is moving in your example.

Assuming we have a magnet moving relative to a capacitor:

In the magnet frame, the charges experience the Lorentz force because they are moving through a magnetic field, and there is no electric field.

In the capacitor frame, the charges experience an electrostatic force due to the fact that a moving magnetic field transforms to an electric field.

However, because the charges are stationary in the capacitor frame, the Lorentz force due to the magnetic field is zero - force = v cross B, and v=0, so the force due to the magnetic field is zero.

So both observers agree that there is a force, one however attributes the force as being due to a magnetic field, and the other oberver interprets the force as being due to an electric field.