# Homework Help: The third law of Newton is always fulfilled?

1. Oct 5, 2006

### 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!

Last edited: Oct 5, 2006
2. Oct 5, 2006

### Noesis

Why do you think it wouldn't hold?

3. Oct 5, 2006

### wxrocks

of course it is

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.

4. Oct 5, 2006

### 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.

5. Oct 5, 2006

6. Oct 5, 2006

### Hootenanny

Staff Emeritus
Newton's third law certianly applies here.

7. Oct 6, 2006

### chuy

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

Greetings!

8. Oct 6, 2006

### 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.

9. Oct 6, 2006