# Magnetic fields

1. Mar 23, 2004

### athenaroa

A Fe+ ion is traveling at 200 m/sec in a direction 20 deg south of west at a point. The magnetic field at that point is 0.4 T directed at 10 deg west of south. What is the instantaneous magnetic force that acts on the Fe is how many newtons?

2. Mar 23, 2004

### Chen

The magnetic force on a charge that travels through a magnetic field is:
$$\vec F_m = q\vec V \times \vec B = q|V||B|\sin \alpha$$
([itex]\alpha[/tex] being the angle between the field vector and the velocity vector of course.)

Last edited: Mar 24, 2004
3. Mar 24, 2004

### ZapperZ

Staff Emeritus
I normally would not be picky on such things. However, it should be corrected that the general form of Lorentz force for the magnetic field term is proportional to q(v x B), not q(B x v).

Since the original question involves the exact vectorial direction of the force, and since cross products are not commutative, this can produce an error in the direction.

Zz.

4. Mar 24, 2004

### Chen

Of course, I've corrected my post. Thanks for pointing my mistake out. Usually I just use the right-hand rule to find the direction of the magnetic force.