# When a magnetic field changes and an emf is produced,

by jaredvert
Tags: field, magnetic, produced
 P: 62 Then what gives the energy to create this emf?
 P: 649 What makes the magnetic field change from top to bottom of the tube if we drop a magnet in a copper tube? http://www.youtube.com/watch?v=JstwmJjn34w
P: 62
 Quote by nsaspook What makes the magnetic field change from top to bottom of the tube if we drop a magnet in a copper tube? http://www.youtube.com/watch?v=JstwmJjn34w

It is the induced emf in the copper tube correct? It would make a counterclockwise current. But what I don't understand is where is the energy for this induced emf coming from?

Thanks
PF Gold
P: 1,908
When a magnetic field changes and an emf is produced,

 Quote by jaredvert It is the induced emf in the copper tube correct? It would make a counterclockwise current. But what I don't understand is where is the energy for this induced emf coming from?
See Lenz's Law: http://hyperphysics.phy-astr.gsu.edu...ic/farlaw.html
P: 62
 Quote by UltrafastPED See Lenz's Law: http://hyperphysics.phy-astr.gsu.edu...ic/farlaw.html

Yeah I mean I think I get lenz law but can u explain what I'm missing here? The magnet falling induced a counterclowise current in the copper. Where the energy for this emf is what I don't understand
P: 649
 Quote by jaredvert Yeah I mean I think I get lenz law but can u explain what I'm missing here? The magnet falling induced a counterclowise current in the copper. Where the energy for this emf is what I don't understand
What makes the magnet move?
P: 62
 Quote by nsaspook What makes the magnet move?

Well gravity but I imagine it's acceleration is slower due to the magnetic force exerted on it
P: 649
 Quote by jaredvert Well gravity but I imagine it's acceleration is slower due to the magnetic force exerted on it
Yes, the person uses energy to pick up the magnet to some height from it's rest position and then release it to fall in the tube due to gravity.

http://www.physicsclassroom.com/clas...tential-Energy
P: 62
 Quote by nsaspook Yes, the person uses energy to pick up the magnet to some height from it's rest position and then release it to fall in the tube due to gravity. http://www.physicsclassroom.com/clas...tential-Energy

Oh so I see because it has less velocity it's KE is less and so the energy induced to the emf is equal to mgh - 1/2mvf^2. Where vf is velocity in coil. Correct?
 P: 649 Judging by the slow speed of the magnet drop in the tube a good percentage of the KE is in the induced emf and it's magnetic field.
P: 62
 Quote by nsaspook Judging by the slow speed of the magnet drop in the tube a good percentage of the KE is in the induced emf and it's magnetic field.

How come when he spins it the magnet accelerates faster down? Is it because the area of the plates perpendicular to the sides are smaller and so smaller current induced?
P: 649
 Quote by jaredvert How come when he spins it the magnet accelerates faster down? Is it because the area of the plates perpendicular to the sides are smaller and so smaller current induced?
At what point in the video do you mean? His spin is mainly to stabilize the magnet during it's drop.

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