# Magnetism/Magetic Fields

1. Oct 21, 2004

### kristi.lynn

Hey I tried posting this in another thread but no one was answering and I'm really quite stuck....

ok here's my problem(s).... I think the first one I'm missing something...

it says:

A particle initially moving south in a vertically downward magnetic field is deflected toward the east. What is the sign of the charge on the particle? Explain your answer with a diagram.

So the way I drew it I've got the B-field pointing down and the particle moving in the same direction as the B-field... so if I'm looking at something like the RHR then I figure there's like no force on the particle which makes it not be deflected! I've read the chapter and I figure I MUST be missing something but it's not helping at all... so if anyone can give me some guidance that would be great... ok second question...

A 150-g ball containing 4.00E8 excess electrons is dropped into a 125-m vertical shaft. At the bottom of the shaft, the ball suddenly enters a uniform horizontal magnetic field that has magnitude 0.250 T and direction from east to west. If air resistance is negligibly small, find the magnitude and direction of the force that this magnetic field exerts on the ball just as it enters the field.

ok now this one I assumed that q= -4.00E8 and my biggest problem is I can't figure out how to find the velocity of the ball. I tried F=mg and got the force due to gravity but that didn't tell me the velocity of the ball and I don't know if it CAN exactly... I was going to first use F=qv X B but then I figured maybe F=Il X B would be better bc they gave me the length too but I just don't know where to go! Please help me!

kristi.lynn

2. Oct 21, 2004

### kristi.lynn

ok I think I might have gotten the first one.... I just figured if it's gotta be deflected east then there must be some force to the east so by the equation F=qv X B for F to be in the +x direction or East, and with v being in the - y direction and B also in the -y direction, then + = q (-)(-) and so q must be positive... that's the best I could figure...

3. Oct 22, 2004

### kuenmao

In the second question, the first thing you need to know is the formula for electromagnetic force(which you already do: F=qv x B.

So, to find the force, you need to know the charge, the velocity and the strength of the B-field. The strength of the B-field is given: 0.250 T. Then you need to know the charge, which is also given: 4.00E8 multiplied by the charge of a electron. What is left is the velocity. You know the length of the shaft, the acceleration(what forces act on it while it is in free fall?) and the initial velocity(....electrons is dropped into a 125m.....). Can you find the velocity of the ball the moment it enters the magnetic field?

If you work all that out, you'll realize that the question is pretty straight forward.