# Magnetic Field due to current

## Homework Statement

..... the ones with arrow are my answer and the ones with circles are book's answers!!!

<MCQ 2>
i am not sure by same it means parallel or equal .... <MCQ 3>
how is a wrong and c correct
and if there is current there is electric field in conductor ... howz (c) correct ???
I am sure (b) is correct and i kind of think (a) is also correct

Help Plz ## Answers and Replies

For the second prob the only thing that I can think is that they mean they are both in the phi direction. ??? I'm kind of with you on this one.

As far as the third problem I'm assuming that there is no electric field in the vicinity because there are no static electrons so there is no electric field outside the conductor. There is likely microscopic electric fields inside but on average they are zero. I don't understand why (a) is not a correct answer either.

As far as the third problem I'm assuming that there is no electric field in the vicinity because there are no static electrons so there is no electric field outside the conductor. There is likely microscopic electric fields inside but on average they are zero. I don't understand why (a) is not a correct answer either.

But there will be electric field becuase of the emf source connected otherwise how will there be any current in the wire ... the emf source will create field in sire which will force electrons to move in opposite direction

That depends on if the current is steady or not. If the current is not changing then there is no change in magnetic flux then the emf is zero.

so according to you we can have currents without applying emf ... are you sure about this?

Yes. Steady currents. This is for electrostatics and magnetostatics. By definition
$$\epsilon = -\frac{d\Phi}{dt}$$

So if you have a constant magnetic field then you don't have an emf. My electrodynamics professor told us to think of it like the inertia of electromagnetism.

I believe that is induced emf, it means that if the net flux through a wire is constant ... there will be no emf ... and i.e no current in wire!!!

and what you said violates energy conservation principle!!!

I don't think that's true. The current has to start flowing to begin with. I think the problem that you were asking about is only looking at the steady current flow and not the starting and stopping points where and emf would be induced. That is magnetic flux not electric flux by the way. Sorry for not pointing that out in the equation.

What is violating conservation of energy?

Yes. Steady currents. This is for electrostatics and magnetostatics. By definition
$$\epsilon = -\frac{d\Phi}{dt}$$

So if you have a constant magnetic field then you don't have an emf. My electrodynamics professor told us to think of it like the inertia of electromagnetism.

if EMF is 0
then current is 0
and this is not the case in this question

And how are you relating induced emf to original current in wire?