# understanding magnetic field generated by a straight wire at atomic scale

Discussion in 'Homework Help' started by PG1995, Aug 24, 2012.

1. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Hi

It is said motion of electrons give rise to magnetism. Please see the diagram below.

When a straight wire is connected to a battery circular magnetic field is generated around the wire.

Once wire is connected to a battery electrons start moving inside the wire at a very low speed. But how come collective movement of electrons generate such a perfect circuit field when individually electrons do not generate such field? Please help me with it. Thank you.

Regards
PG

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2. ### steveb Senior Member

Jul 3, 2008
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When learning the basics of classical electromagnetism, it is best not to worry about the magnetic field and spin properties of individual electrons because this brings in quantum mechanical effects that will just confuse you and slow you down.

Personally I would suggest that you reword your question and instead ask about the Biot Savart law which says that a short current element has a dipole like magnetic field pattern. This law can then be used to integrate over the length of a long wire to find the net magnetic field of any arbitrary shaped wire. Hence, when you integrate over an infinitely long wire, the complicated dipole pattern reduces to the simpler pattern of a current carrying wire.

Last edited: Aug 24, 2012
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3. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Hi Steve

I value your advice here that I should think in terms of classical magnetism which doesn't concern things at atomic scale. Thank you.

Regards
PG

4. ### steveb Senior Member

Jul 3, 2008
2,433
469
Yes, at first this will help you because usually the electrons are randomly oriented and the magnetic effects of individual electrons cancel out, leaving only the effect of charge as significant.

However, in classical magnetism, we still deal with permanent magnets and ferromagnetic materials. Here, the quantum effects at the atomic scale are manifesting themselves at the macroscopic scale. The case of ferromagnetism is handled classically by modifying the magnetic permeability, and we can forget about the microscopic scale. However, to really understand permanent magnets, in the future, you'll need to consider the atomic level in more detail.

In other words, I think my advice will help you in the short term, but eventually, you will want to break my advice. My main point in this thread is that the magnetic moment of the electron is not relevant for understanding the magnetic field in a wire. However, your question is similar to a very relevant question in classical theory. That is, the short wire has a dipole-like magnetic field pattern, yet a long wire has a much simpler pattern; and, it is natural to wonder why this is so.

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5. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Hi

There is some problem and I can't start a new thread so I have to use older threads to post related problems.

Regards
PG

• ###### mm drop.jpg
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6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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It wasn't called 'drop' when I was taught but the term can be applied because it takes more mmf to create a given magnetic field when the magnetic resistance rises due to a smaller cross section of steel, or a longer distance in the circuit. In this respect it is identical to resistance and voltage 'drop'.

7. ### PG1995 Thread Starter Well-Known Member

Apr 15, 2011
813
6
Thank you, Kermit.

So, in your opinion, even the definition of Ampere's law is quite right? Please let me know. Thank you.

Best wishes
PG