Physics Help Forum Can someone explain the concept of "change in electric or magnetic flux"?

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum

 Mar 3rd 2011, 10:03 PM #1 Junior Member   Join Date: Mar 2011 Posts: 2 Can someone explain the concept of "change in electric or magnetic flux"? What i mean to say is that, i understand the concept of "change of distance", "change of time" and other changes and other time rates.... but the "time rate of change of electric flux" really confuses me. Does change of flux mean increase or decrease in the number of electric lines passing through some body? or does it mean the "movement" electric lines through it?.... i mean, if a conductor, say a rod moves thru a UNIFORM magnetic field, there would be transient polarization of the rod. (one end would get slightly positive, the other negative)...So its obvious that there was some instantaneous induced emf in the rod, if so, then the concept of changing magnetic flux as the increase or decrease in the number of lines of magnetic induction (which my teacher provided) fails here...Because faraday's law states emf= -N (change of flux)/ (time) so my teacher's concept would say that emf should be zero( because there is no increase or decrease in the number of lines of magnetic induction ,since the magnetic field is uniform)...but still we have said that instantaneous emf existed. so whats the point here?....if my teacher's concept about the change of magnetic/electric flux is wrong...which is the right concept? so if u apply the i
Mar 4th 2011, 03:14 PM   #2

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 Originally Posted by khamaar What i mean to say is that, i understand the concept of "change of distance", "change of time" and other changes and other time rates.... but the "time rate of change of electric flux" really confuses me. Does change of flux mean increase or decrease in the number of electric lines passing through some body? or does it mean the "movement" electric lines through it?.... i mean, if a conductor, say a rod moves thru a UNIFORM magnetic field, there would be transient polarization of the rod. (one end would get slightly positive, the other negative)...So its obvious that there was some instantaneous induced emf in the rod, if so, then the concept of changing magnetic flux as the increase or decrease in the number of lines of magnetic induction (which my teacher provided) fails here...Because faraday's law states emf= -N (change of flux)/ (time) so my teacher's concept would say that emf should be zero( because there is no increase or decrease in the number of lines of magnetic induction ,since the magnetic field is uniform)...but still we have said that instantaneous emf existed. so whats the point here?....if my teacher's concept about the change of magnetic/electric flux is wrong...which is the right concept? so if u apply the i
There are a couple of ways to look at the electric field lines when the flux increases. The first is to say that there are extra field lines, the other is that to say that the "strength" of the field lines was increasing. Either way the energy density of the electric field is increasing over a given area.

The polarization of the rod is not due to a changing flux. The "conduction electrons" in a metal are basically free to move as they please. As you pass the rod over a magnetic field the Lorentz force causes these electrons to move to one side of the rod or the other. Hence you have a polarization.

The same counts for a ring, however here we have an extra effect. As the ring is entering or leaving the area where you have a uniform magnetic field the magnetic flux in the area of the ring changes. This causes a temporary electric field ala Faraday's Law.

Did that cover your questions or do you need to go into more detail?

-Dan
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