# Homework Help: Magnetic Fields

1. Oct 21, 2004

### kristi.lynn

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...