# Magnetic field and conducting rails

1. Apr 8, 2014

### BOAS

Hello,

i'm really struggling with this problem and don't understand some of the terminology.

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

A bar of length 20cm and negligible mass can slide over two conducting rails connected to a dc generator producing an emf V0 = 6V, connected so as to produce a current as in the figure. The resistance of the bar is R = 0.08Ω, all other parts have negligible resistance. The bar is connected through a pully to a body of mass 1.2kg the system is immersed in a uniform magnetic field orthogonal to the rails, as in the figure, whose magnitude is 1T. The system is designed in such a way that, after a while, the body is pulled upward with a constant limiting speed.
Compute:

a) The current flowing in the circuit and the power provided by the dc generator when the limiting speed is reached.

b) The magnitude of the limiting speed.

c) The value of the resistance of the bar corresponding to which the body does not move at all.

2. Relevant equations

3. The attempt at a solution

I know that the deal here is that I must show some working, but I feel like i'm wandering around in the dark not knowing what the question means by 'limiting speed'.

Does 'limiting speed' mean the maximum speed at which the mass can be raised?

I have found that the magnetic force acting on the bar due to the current and magnetic field is $F = BIL = 75(0.2) = 15N$ Where $I = \frac{V}{R} = \frac{6}{0.08} = 75A$

$emf = VBL$ so $v = \frac{emf}{BL} = 30ms^{-1}$

If that's correct, then it should be the speed at which the bar would move if it wasn't connected to the mass.

I am very confused about how to proceed, and would really appreciate some guidance.

Thanks,

BOAS

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Last edited: Apr 8, 2014
2. Apr 8, 2014

### rude man

"Limiting speed" since the bar accelerates from zero but reaches a constant speed eventually.

Are there more than one source of emf here?

Compute the current i in the bar based on the force it has to exert. Then, compute the velocity such that the sum of potential drops around the loop = 0, or sum of emf's = iR.
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