# Magnetic fields

1. Nov 20, 2005

### lonelygirl

hey physics experts could you help me out with these two problems please?

1/ A positively charged particle of mass 5 x 10^-5 kg is traveling due east with a speed of 80 m/s and enters 0.3 T uniform magnetic field. The particle moves through one quarter of a circle in a time of 2 x 10^-2s, at which time it leaves the field heading due south. All during the motion the particle moves perpendicular to the magnetic field.
a) What's the magnitude of the magnetic force acting on the particle?
b) Determine the magnitude of its charge.

2/ The wire in the figure carries a current of 10A. Suppose that a second long, straight wire is placed right next to this wire. The current in the secon dwire is 25A. use Ampere's law to find the magnitude of the magnetic field at a distance of r = 0.7 m from the wires when teh currents are
a) in the same direction and
b) in the opposite direction

appreciate a lot for your help ;-)

2. Nov 20, 2005

### Physics Monkey

Hi lonelygirl, could you please tell us what work you've done so far. We can't help you until you post some of your own thoughts on the problem.

3. Nov 20, 2005

### lonelygirl

i have no idea how to start it could please give some hints :-(

thanks

4. Nov 20, 2005

### Physics Monkey

Ok, question one asks about the magnetic force, so do you know how to find the magnetic force on a charged particle? Hint: look for the Lorentz force. Also, what do you know about the way particles move when they are in a magnetic field?

For question 2, you are told to use Ampere's law, so what is Ampere's Law? Have you used Ampere's Law to find the magnetic field of a long straight wire before? How is the situation modified if you have another wire? Hint: think about the superposition principle.

Hope this helps.

5. Nov 20, 2005

### lonelygirl

Lorentz force = Electric force + Magnetic force right?

Can you do me the first step please? i know its concept but still didn't get it :(

6. Nov 20, 2005

### Physics Monkey

I can't do the step for you, and it wouldn't help you anyway. I will try to guid you to the answer. Since the electric field is zero, there is only a magnetic force. Can you tell me what you need to know to figure out what the magnetic force is? Hint: there are three ingredients you need to find the magnetic force.