# Magnetic field question?

1. Jun 26, 2012

### Questions999

1. The problem statement, all variables and given/known data

The conductive bar CD with a length of 20 cm and resistance R, moves with speed v=5 m/s. The magnetic field is perpendicular with the plan and the induction is B=2 T.
The mass of the object in the figure is m=80 grams and g=10 m/ s^2
a) find the direction and the intensity of the induced current that moves in CD
b) The resistance of this bar

2. Relevant equations

Figure : http://s1171.photobucket.com/albums/r549/Elaia06/?action=view&current=hrtfg.jpg

3. The attempt at a solution
I tried using Emf=B*L*v and then I=emf/R but i dont know the R..
I JUST WANT TO KNOW HOW TO RELATE Tension and weight with induction//

2. Jun 26, 2012

### truesearch

If the bar is moving at a constant speed this tells you something about the resultant force on the bar?
There is a force due to the weight of the mass and there is a force due to any induced current flowing in the wire CD.

3. Jun 26, 2012

### Yukoel

Hello Elaia06,
You have to find the induced current first which as truesearch points out bases itself on the fact that the rod is moving at a constant velocity.Tension can be found out basing your derivations on the same fact.Direction of induced current comes out as a result of the same(analyse the forces on the rod and see which direction of current corresponds to equilibrium).Then for resistance you have adequate expressions in your attempt .
regards
Yukoel

4. Jun 27, 2012

### Questions999

Ah, the resultant force is zero.This means the only force acting on the bar is the magnetic force F=B*i*l.
Now we have the Tension= mg-ma but acceleration is zero so we write
Magnetic force = Force on the string
B*i*l=m*g
I know how to solve this now
The magnetic force will have the direction <---- based on the figure and the induced current the direction from left to right .

IS IT CORRECT NOW?

Last edited: Jun 27, 2012
5. Jun 27, 2012

### tiny-tim

you haven't used v or R

6. Jun 28, 2012

### Questions999

after i find I, then I use I=b*L*v/R and so I find R :)
Is it correct in total?

7. Jun 28, 2012

### tiny-tim

yes, that's correct

8. Jun 28, 2012

### Questions999

Thank you very much sweetheart! :D