# The Naked Scientists Forum

### Author Topic: Current elements in electric fields  (Read 4473 times)

#### nndaia

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• Posts: 7
##### Current elements in electric fields
« on: 01/06/2006 11:23:30 »
I need help and my physics teacher can't give it to me.

I know that when a current-carrying conductor is placed in an electric field it is not parallel to, the perpendicular (to the field) component of the current will cause a force on the wire in a direction perpendicular to both the field and the perpendicular component of the current element.

What I don't know is... why?

I can picture in my mind a world without war, a world without hate.
And I can picture us attacking this world, because they'd never expect it.

#### another_someone

• Guest
##### Re: Current elements in electric fields
« Reply #1 on: 01/06/2006 12:15:19 »
The reason you have a perpendicular force is because every electric current creates a magnetic field perpendicular to it.

George

#### nndaia

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• Posts: 7
##### Re: Current elements in electric fields
« Reply #2 on: 01/06/2006 12:31:30 »
Yes, but why does this create a force on the conductor? The field generated by the current is circular in this case, around the conductor, and it has no net influence on the conductor.

I can picture in my mind a world without war, a world without hate.
And I can picture us attacking this world, because they'd never expect it.

#### another_someone

• Guest
##### Re: Current elements in electric fields
« Reply #3 on: 01/06/2006 19:19:08 »
Assuming the conductor is not ferromagnetic (i.e. it is not iron, nickel, or similar), then a constant magnetic field will have no effect upon it.

On the other hand, any change in the magnetic field will induce a current in the conductor (only as the field changes), and that current will have its own magnetic field that will interact with the existing magnetic field.

When you switch on a current in the wire, it will cause a rise in the magnetic field around the wire, and that rising magnetic filed will induce a current in the second conductor, which will then develop its own magnetic field that interact with the magnetic field created by the original current.

If you leave the current switched on for a long time, then the magnetic field will stabilise, and will have no effect on the second conductor.

But, even if the current is held constant for a time, if you move the conductors closer together, or further apart, it will alter the strength of the magnetic field that the second conductor will experience, and this will then induce a current in that conductor (that is how a dynamo works).

George

#### nndaia

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• Posts: 7
##### Re: Current elements in electric fields
« Reply #4 on: 02/06/2006 08:03:17 »
What second conductor?

If you have one current-carrying conductor in a uniform magnetic field that it is not parallel to, it (according to my textbook) experiences the force, but why?

I can picture in my mind a world without war, a world without hate.
And I can picture us attacking this world, because they'd never expect it.

#### another_someone

• Guest
##### Re: Current elements in electric fields
« Reply #5 on: 02/06/2006 10:58:21 »
quote:
Originally posted by nndaia

What second conductor?

If you have one current-carrying conductor in a uniform magnetic field that it is not parallel to, it (according to my textbook) experiences the force, but why?

I can picture in my mind a world without war, a world without hate.
And I can picture us attacking this world, because they'd never expect it.

You now ask about a current carrying conductor in a 'magnetic field', whereas your original question was about a current carrying conductor in an 'electric field'.

If you are talking about a static electric field (as distinct from a field around an electric current, which would be a magnetic field), then I am not sure what the answer is.

The present question, as to why a current carrying conductor should interact with a magnetic field is answered by what I said above, it is because the current carrying conductor itself creates a magnetic field that is perpendicular to the current, and so the two magnetic fields will create a force between them.

George

#### nndaia

• First timers
• Posts: 7
##### Re: Current elements in electric fields
« Reply #6 on: 02/06/2006 15:06:03 »
quote:
Originally posted by another_someone

You now ask about a current carrying conductor in a 'magnetic field', whereas your original question was about a current carrying conductor in an 'electric field'.

George

So I did. Sorry about that; I didn't notice. I meant magnetic field.

The magnetic fields (that of the conductor and the uniform background field) would interact and reinforce/negate each other (superposition), but why should this create a force?

I can picture in my mind a world without war, a world without hate.
And I can picture us attacking this world, because they'd never expect it.

#### another_someone

• Guest
##### Re: Current elements in electric fields
« Reply #7 on: 02/06/2006 20:24:18 »
quote:
Originally posted by nndaia
The magnetic fields (that of the conductor and the uniform background field) would interact and reinforce/negate each other (superposition), but why should this create a force?

They would interact in the way that two magnets placed near each other would interact (i.e. they would either attract or repel each other).  The force would be the force of attraction or repulsion.

It is a little more complex because the field is perpendicular to the conductor, and so rather than a direct force of attraction or repulsion, it would be a circular force around the conductor (but this is no different to the way that the Earth's magnetic field behaves near the equator, where the force is parallel to the surface of the Earth).

George

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##### Re: Current elements in electric fields
« Reply #7 on: 02/06/2006 20:24:18 »