# Magnetism questions

1. Apr 22, 2010

### rvlotstein

Ok I need a bit of help with my Physics 212 homework.I know I am supposed to list my guesses and all available formulas,but my my problem is I don't actually know where to start.I didn't find any example of these in the book so Im just kinda stumped.I suppose my big problem is that I am currently taking both math 144 and 170 (trigonometry and calculus one respectively) while I am taking the physics course so when people talk about intergration or what not it just kinda goes over my head.With that being said if you could just tell how to do the problem or formulas that I can "plug and chug" that would be great.Thanks in advance.If anything I already answered 7/10 problems on the homework,and at this point in time a few more zeros can't hurt that much...not.

First one:

The uniform 29.0 mT magnetic field in the figure points in the positive z-direction. An electron enters the region of magnetic field with a speed of 4.70 x10^6 m/s and at an angle of 30* above the xy-plane.
PART 1:
Find the radius r of the electron's spiral trajectory in mm.

PART 2:
Find the pitch p of the electron's spiral trajectory in mm.

the book isn't much of help,that and we didn't even talk about this sorta stuff in class.So Im kinda stumped.

Next One:

two springs are holding a wire in a uniform magnetic field of .5T(Coming out of the page, 20 cm square area ).The springs each have a spring constant of 8.00 N/m. The springs are stretched .9 cm when current is ran through the wire(current points straight up).

How Big is the current?

The book goes over to parallel wires and the force they exert on each other but not the force a magnetic field exerts on a current carrying wire.Im guessing that the force is whatever force it takes to stretch the two springs that distance.But after that I don't know how to get Amps from the force.

last one:

A bar magnet experiences a torque of magnitude .075 N/m when it is perpendicular to a 0.50 T external magnetic field.

What is the strength of the bar magnet's on-axis magnetic field at a point 20 cm from the center of the magnet?
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2. Apr 23, 2010

### jdc15

For the first one, start with your formulas for force on a moving charge (F=qvB) and circular motion (F=mv^2/r). Use the right hand rule to find the direction of the force and thus its circular motion bound to one axis. As for pitch, first determine the period of rotation, and then using the constant velocity in one direction, find the distance it travels each time it circles around.

For the second one, use your formulas for force on a spring (F=-kx) and for on a wire in a magnetic field (F=IlB).

Not sure about the last one. A little rusty on my magnetic fields and calculus atm.