# Confused about the cause of Magnetism

My physics book says that the magnetic field is caused by moving electrical charges, acts only on charges with a motion vector perpendicular to itself, and the force it exerts on a charge is proportional to the velocity of the charge. How does the moving charge cause this force, which does not act directly towards or away from the line of motion? Thanks.

Nabeshin
My physics book says that the magnetic field is caused by moving electrical charges, acts only on charges with a motion vector perpendicular to itself, and the force it exerts on a charge is proportional to the velocity of the charge. How does the moving charge cause this force, which does not act directly towards or away from the line of motion? Thanks.

Why does mass create gravity? Why does gravity attract and not repel?

This is about the caliber of question you are asking. The simple answer is it just does, and it's a fundamental property of our universe. We can describe the interaction very well with maxwell's equations etc., but to explain it is likely beyond the realm of science.

I thought that it was a product somehow of the electric field, not its own, separate force, this is what confused me.

Born2bwire
Gold Member
I thought that it was a product somehow of the electric field, not its own, separate force, this is what confused me.

Electromagnetic force is one of the four fundamental forces. The electric force and mangetic force are put in the same category but they are distinct. I can have a magnetic force without any associated electric force. Electric and magnetic fields are coupled, but they become decoupled in the static case (and can be estimated to be somewhat decoupled in the quasi-static approximation).

This is the kind of thing people write PHd thesis' on...

...and fail.

hah no offense, but it as stated above...its exactly like gravity - we can predict it, analyse it, use it, and tell everything there is to know about it, but we still have no idea how it works on the level you are asking about.

Is magnetism based on the relative velocities of the charges? Would two charges moving in the same direction at the same speed have a magnetic force between them?

I thought that it was a product somehow of the electric field, not its own, separate force, this is what confused me.

Actually, you are not really too far off base here. An electric field in a static reference frame looks like a magnetic field in a moving reference frame. Similarly if a moving charge generates a magnetic field, this will appear to be an electric field in a frame of reference moving with the same velocity as the charge. Things get a little more confusing when many charges are moving with different velocity since no reference frame has only electric fields or only magnetic fields. What is really interesting is when the charges are accellerating in which case electromagnetic radiation is generated. Take a look at the following video in which Prof. Lewin at MIT gives a very nice intuitive explaination of electromagnetic radiation.

http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoAndCaptions/detail/embed28.htm [Broken]

You can try to understand the situation from the point of view that the laws of physics should look the same in any inertial reference frame (special relativity). Maxwell's equations were known to obey the Lorentz transformation even before Einstein developed his theory. Of course none of this can explain why there are electromagnetic forces, but the magnetic field seems a little less mysterious if you think along these lines.

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The force F on a charged particle with charge q and velocity v by a magnetic field B is given by the empirical Lorentz Force equation:

F = q[E + (v x B)]

where the x denotes a vector cross product.

Because the force is perpendicular to the velocity v, no work is done. I believe that this equation (F = I x B) is used to define the magnetic field amplitude in Tesla units.