MadSci Network: Physics

Re: Does Lenz' Law apply to long conductors?

Date: Mon Jul 14 15:06:49 2008
Posted By: Jim Guinn, Staff, Science, Georgia Perimeter College
Area of science: Physics
ID: 1206127815.Ph

Dear John,

This is a great question that deals with a number of interesting points.  
First, we need to remember that Lenz’s Law is one aspect of a more general 
result that is Faraday’s Law.  Faraday’s Law tells us that when we have a 
changing magnetic flux through a loop, there will be an induced 
electromotive force (EMF) around that loop.  The part of Faraday’s Law 
that is Lenz’s Law, basically the minus sign, tells us the orientation of 
that EMF.  The direction of the EMF is such that if a current was induced, 
the magnetic field created from the induced current would point in the 
opposite direction to the change in the flux, that is, the induced 
field “tries” to reduce the external field’s change.  I say that “if” a 
current is induced, because the electromotive force is produced whether a 
current is created or not.  The EMF is essentially a line integral of the 
electric field and does not require the presence of any material in which 
the current may flow.

So what is happening with your steel conductor?  The iron in the steel is 
ferromagnetic, that is, iron responds very strongly to external magnetic 
fields.  When you place iron in an external field, the iron will tend to 
align its own internal magnetic moments to the external field, thus 
increasing the magnitude of the field.  The dominant effect will then be 
an increase of the field due to the iron’s internal magnetic field, as 
opposed to a decrease due to Lenz’s Law.  Iron cores are often used within 
coils for just this purpose, to increase the total magnetic field produced.

Well, I hope I have answered your question, John.  If you would like some 
more information, please let us know.


Jim Guinn

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