The third law of Newton is always fulfilled?

[chuy]

Is certain that this law not fulfilled in the case of two charged particles moving in perpendicular directions (the force exerted in the particle A by the magnetic field of B is not equal to the exerted one in B by the magnetic field of A) ? Why?

Bye!

[Noesis]

Why do you think it wouldn't hold?

[wxrocks]

How would the third law not apply? Sure, the particles would have motion different than uncharged particles, but if you consider the magnetic and electrical forces, everything works.

[P3X-018]

Indeed Newtons 3. law fails this situation. If you have 2 charged particles moving perpendicular like in the following config.

A
*->

* B
| (x) \mathbf{B}
v

You can look at it as currents in those directions. Then A will create a magnetic field given by the righthand rule. The direction of the B-field from A at B is as shown into the page (x). If particle B (with a charge q) have the velocity \mathbf{u}, then the force on B from A will be

\mathbf{F}=q(\mathbf{u}\times\mathbf{B})

But there is no magnetic field from the charge B at exactly that spot of A, so there wont be a force from B acting on A there, but there is a force acting on B from A. And here is the failure of Newtons 3. law.

[wxrocks]

What about the electric field?

[Hootenanny]

What about the electric field?
Newton's third law certianly applies here.

[chuy]

Then, if we also consider electric field, Isn't violated the law? How?

Greetings!

[P3X-018]

There will be an electric field not matter what, otherwise it wouldn't be called an electric charge. But there will also be a magnetic field if they are moving, with an electric field, obeying the 3rd law of Newton, but it's the force due to the magnetic field that fails the 3rd law.

[Chi Meson]

Here's an earlier thread on the same topic. All sides are considered nicely.
http://www.physicsforums.com/showthread.php?t=94627