# Homework Help: Magnetic Field

1. Dec 9, 2004

### MightyMeanie

I am getting confused. electrons, protons, which finger goes where in the left hand rule... agh!!! lol, im muddling myself all up. i understand the rule (well i hope i do by now as that means the rest of my homework is wrong) but the last question of my h/w is almost killing me.
heres the question (well the last part as i manages the beginning) any help will be wonderful, thanks

Yasmin
xxx

A beam of protons moving at constant speed is directed into a uniform magnetic field in the same direction as the field.

Describe and explain how the path of the beam in the field would have differed if the beam had been directed into the field at a slight angle to the field lines.

2. Dec 9, 2004

### dextercioby

HINT:According to Mr.Henrik Antoon Lorentz,the magnetic force with which a field is acting on a charged particle is proportional to the cross product between the field and the particle's velocity vector.
Pick the direction of the field along one axis of coordinates and write Newton's second law in terms of the particle's momentum derivative wrt to time and analyze that cross product.Solve the equations of movement and find the trajectory.

ANSWER:You should be getting a helicoid.

3. Dec 9, 2004

### cepheid

Staff Emeritus
Did you mean...in the same direction as the beam?

Well, the force on a charge moving in a magnetic field is given by:

$$\vec{F} = q\vec{v} \times \vec{B}$$

Do you understand how to compute the cross product of two vectors? What does it say about two vectors that are completely parallel (neither of them has any component in a direction perpendicular to the other one)? Compare that to two vectors with a non-zero angle between them.

Also, these charges are positive. Stick to the right hand rule.
I hope this helps get you started. If you're still stuck, let me know.