# Help on Electromagnetic Induction concept

1. Aug 28, 2007

### darkwhite87

This is the material i read from textbook, and it doesnt make any sense to me!! can somebody plz help me out if they can understand the concept:

To understand electromagnetic induction, we need to reconsider the concepts of electric
and magnetic fields.

A dc current I flowing through a stationary contour C in a coordinate system
(x, y, z) produces a magnetic flux density field B. Let us look at a charged particle Q
moving at a velocity v with respect to contour C. We add a second coordinate system
(x', y', z') that moves together with the charge Q, that is, with respect to which Q is
stationary.

In our thought experiment we have two observers (electrical engineers or physicists,
of course), one stationary in (x, y, z), and the other in (xf, yf, z'). They are interested
in measuring the electric and magnetic forces acting on the charged particle

Let Jack be in the first coordinate system. His instruments record a force acting
on a moving particle. He concludes that the charge is experiencing a magnetic force
F = Qv x B, since it is moving in a time-invariant magnetic field. If the charge stops,
there is no force. Therefore, Jack's conclusion is that in his system there is no electric
field.

Jill, in the second coordinate system, comes to a different conclusion. She also
measures a force, proportional to Q, acting on the charge. However, for her the charge
is not moving. Therefore, she concludes that the force she measured is an electric one,
F = QE.
She notices, of course, that this force is time-varying. She also notices that in
her system there is a time-varying magnetic field (since the source I of the magnetic
field is moving with respect to her coordinate system). Thus, Jill's conclusion is that in
her coordinate system both a time-varying electric field and a time-varying magnetic
field exist.

Let us rephrase the important conclusion we reached: a time-varying magnetic
field is accompanied by a time-varying electric field. We found this to be true in the
case of motion of the observer with respect to the source of a time-invariant magnetic
field. We shall now argue that a time-varying magnetic field is always accompanied
by a time-varying electric field, no matter what the cause of the variation of the field
is.

In red is the part which i dont get, if the charge is not moving with respect to her, how can she conclude there is a electric force??

2. Aug 28, 2007

### mgb_phys

3. Aug 29, 2007

### Paulanddiw

That's how she concludes. Since it's not moving (to her) and there IS a force, and since only motion causes forces on charges in magnetic fields, it MUST be an electric field.

The next line says: " she notices, of course, that this force is time-varying. She also notices that in her system there is a time-varying magnetic field (since the source I of the magnetic field is moving with ..." So, the moving source of I is the cause of the electric field that Jill concludes must be there.

They sure make you guys jump through hoops in school these days.

4. Aug 29, 2007

### mgb_phys

That a magnetic field is just a relativistic view of an electric field is a fairly advanced concept.

5. Sep 25, 2007

### TVP45

Wow!

The measurement of either electricity or magnetism depending on relative velocity is, of course, well known. But, I have to point out that the person who wrote this textbook has done physics a great disservice. May I suggest either Jackson or Lorrain and Corson? Or, of course, the good old book, Feynman Lecures II.

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