Understanding Magnetic Forces: Direction, Calculation, and Applications

[scorpa]

Hello,

I was doing some bonus homework questions and hit a few snags along the way. Any help would be appreciated!

1) What is the direction of the force on a current carrying wire in a magnetic field if the current is toward the left on a page and the magnetic field is down the page?

For this one I was thinking that the force was down out of the palm of your hand but I wasn't sure.

2) What is the force on a 3.5m long wire that is carrying a 12A current if the wire is perpendicular to Earth's magnetic field?

The thing that is screwing me up in this question is the lack of information given. I have the length and the current given, but to find the force magnetic I also need the magnetic field strength (B). I'm not quite sure how to figure it out without be given the three variables of magnetic force.

A galvanometer has a full-scale deflection when the current is 50.0 uA. If the galvanometer has a resistance of 1.0 kiloohms, what should the resistance of the multiplier resistor be to make a voltmeter with a full-scale deflection of 30.0V?

I thought that I was doing this one right but my answer didn't coincide with the one in the book. This is what I did:

V/I = R 30V / 50x10^-6 A = 6.0 x 10^5 ohms total

(1/Rtotal) - (1/R1) = (1/R2)

(1/600 Kohms) - (1/1 kohms) = 600/599 = (599/600ohms) which is wrong.

 

[Pyrrhus]

Problem 1)

Remember the magnetic force is defined as:

$$\vec{F}_{B} = q \vec{v} \times \vec{B}$$

Thus, by using the right hand rule, if the current is flowing on the left of the page then you must put your fingers pointing left and then close then towards the magnetic field, and then your thumb will point out of the page or on the Z positive axis, imagining the page is the xy plane.

 

[scorpa]

Ok thanks, I'll give that one a try. Anyone got any ideas for the others?