# Magnetic Forces on Moving Charges True or False

1. Apr 10, 2013

### devilaudy

1. The problem statement, all variables and given/known data

Consider a charged particle moving with velocity v through a magnetic field B. Answer True or False.

1) The magnetic field B is always perpendicular to the acceleration a of the particle.
2) The magnetic force F is always perpendicular to the acceleration a of the particle.
3) The acceleration a of the particle is always perpendicular to the velocity v.
4) The force F exerted by the magnetic field is always perpendicular to the velocity v.

2. Relevant equations

F = qv x B (cross product, magnetic force on a moving charged particle).

3. The attempt at a solution

I tried 1) false 2) true 3) false 4) true and FFFT, but it was wrong. I was sure about my first answer since the magnetic force is always perpendicular to B and V.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 10, 2013

### tiny-tim

hi devilaudy!

what does the magnetic force equation mean?

3. Apr 10, 2013

### devilaudy

Thank you for answering!! I was not really thinking about the acceleration, I thought it would have the same direction as the velocity of the particle! But would it be perpendicular to its motion? Then the acceleration would be parallel to the force exerted by the magnetic field, so the answers would be TFTT. I am not sure if I am getting it right...

4. Apr 10, 2013

### Ampere

^ That looks right to me.

It's easier to think about this with a concrete example. Suppose the particle is positively charged, moving to the right, and the magnetic field points into the page/screen. Use the right hand rule to figure out the direction of the force. What kind of motion does the particle undergo?

Last edited: Apr 10, 2013
5. Apr 11, 2013

### tiny-tim

hi devilaudy!

(just got up :zzz:)
that's right … good ol' newton's second law … acceleration is always parallel to force!

(so in a constant magnetic field, everything goes in a circle …*the faster it goes, the larger the circle … see if you can prove that)
i think you're confusing field with force