What force acts against a magnet passing through a coil?

In summary, a moving magnet passing through a conducting coil will induce an EMF due to the change in magnetic flux. The energy comes from the movement of the magnet and the induced current in the coil will oppose the motion of the magnet, slowing it down. The presence of a current in the coil creates a magnetic field that opposes the magnet's field, causing it to lose kinetic energy. This is due to the magnetic forces exerted between the current-carrying coil and the magnet. The voltage induced in the coil will peak when the magnet and coil are aligned and then return to zero. Passing the magnet over the coil will also induce a voltage but the polarity will not instantly change, rather it will peak twice as the magnet and coil align
  • #1
Zorodius
184
0
A magnet will induce EMF when it passes through a conducting coil. Where does that energy come from? Does the coil do work on the magnet somehow? The B field induced in the coil should be parallel to the magnet's motion, so I would think it could not slow the magnet's progress. Does the magnet become de-magnetized to some extent, so that it loses some kind of internal energy to the coil?
 
Physics news on Phys.org
  • #2
the energy come from the movement of the magnet... no movement no emf.
 
  • #3
So the magnet loses kinetic energy - what force causes that to happen?
 
  • #4
Realize that something must be pushing the magnet through the coil--that something is the source of the energy.
 
  • #5
Doc Al said:
Realize that something must be pushing the magnet through the coil--that something is the source of the energy.
Er, I don't quite follow - if the magnet has some initial velocity that is aimed through the coil, it will move through the coil without anything pushing it.
 
  • #6
The moving magnet will induce a current in the coil that will oppose its motion and slow it down. If you want it to go through the coil without losing speed, you'll have to push it.
 
  • #7
Doc Al said:
The moving magnet will induce a current in the coil that will oppose its motion and slow it down. If you want it to go through the coil without losing speed, you'll have to push it.
this is what I'm confused about - why does the presence of a current in the coil slow the magnet? The magnet has no net charge, and so no Lorentz force acts on it.
 
  • #8
Look at it this way. The current-carrying coil is itself a magnet. Its magnetic pole opposes the magnetic pole of the moving magnet.
 
  • #9
Zorodius said:
The magnet has no net charge,

Neither does a current-carrying wire.

and so no Lorentz force acts on it.

But current-carrying wires do experience magnetic forces, and so do magnets.

In classical electrodynamics, we "explain" a magnet by invoking surface currents along the surface of the magnet, which produce a magnetic field and in turn have forces exerted on them by external magnetic fields. At the quantum level, we explain magnets via the intrinsic magnetic moment of electrons, which is related to their intrinsic angular momentum (also known as "spin").
 
  • #10
Doc Al said:
The moving magnet will induce a current in the coil that will oppose its motion and slow it down. If you want it to go through the coil without losing speed, you'll have to push it.

What if the magnet is passes OVER instead of through the coil?

To slow down the magnet the side of the coils nearest the magnet would have to have the same polarity as the magnet when approaches and then change polarity when the coil and magnet are aligned, so the magnet isn't pushed/accelerated when it moves off the coil. This swaping of polarity in the coil would mean the the voltage in the coil would need to change direction, ie. from + to - or - to +.

I always thought that passing a magnet over a coil makes a voltage that starts at zero, ramps up to a peak when they are aligned, and then ramps back down to zero. From what you guys have said the voltage would have to go from one peak and then instantly change direction to another peak of a diffent sign then ramp back down to zero.

Code:
Your way            What I thought
    /|                     /\
   / |                    /  \
 _/  |   _              _/    \_
     |  /
     | /
     |/
Can someone please explain which it really is.
 
Last edited by a moderator:
  • #11
Murtnowski said:
What if the magnet is passes OVER instead of through the coil?
I'm not clear as to what you mean. I think you are talking about moving a pole of a magnet from one side of the coil to the other as you pass over it (moving perpendicular to the plane of the coil). (If I'm mistaken, let me know.)
To slow down the magnet the side of the coils nearest the magnet would have to have the same polarity as the magnet when approaches and then change polarity when the coil and magnet are aligned, so the magnet isn't pushed/accelerated when it moves off the coil. This swaping of polarity in the coil would mean the the voltage in the coil would need to change direction, ie. from + to - or - to +.
What matters is the rate of change of the magnet's flux through the coil. The induced current will oppose that changing flux. When magnet and coil are aligned, the rate of change in flux will already have dropped to zero. Why would you think the polarity will swap? As the magnet approaches the coil, the near side must have the same polarity, and as it leaves the near side must have opposite polarity--but that means the voltage peaks have the same polarity since the "near side" changes as you move from one side to the other.
I always thought that passing a magnet over a coil makes a voltage that starts at zero, ramps up to a peak when they are aligned, and then ramps back down to zero. From what you guys have said the voltage would have to go from one peak and then instantly change direction to another peak of a diffent sign then ramp back down to zero.
As I already noted, the peak voltage would occur before the magnet and coil are aligned, return to zero as they reach alignment, then peak again. Nothing takes place "instantly".

Code:
Your way            What I thought
    /|                     /\
   / |                    /  \
 _/  |   _              _/    \_
     |  /
     | /
     |/
Can someone please explain which it really is.
I'd say neither is correct: The first because the polarity reverses (with infinite slope!); the second because it only has one peak.
 

1. What is the force that acts against a magnet passing through a coil?

The force that acts against a magnet passing through a coil is called electromagnetic induction.

2. How does electromagnetic induction act against a magnet passing through a coil?

Electromagnetic induction occurs when the magnetic field created by the moving magnet induces an electric current in the coil, which in turn creates a magnetic field that opposes the movement of the magnet.

3. Is the force of electromagnetic induction always in the opposite direction of the magnet's movement?

Yes, the force of electromagnetic induction always acts in the opposite direction of the magnet's movement.

4. Can the strength of the force of electromagnetic induction be controlled?

Yes, the strength of the force of electromagnetic induction can be controlled by changing the speed of the magnet's movement, the number of coils in the coil, and the strength of the magnetic field of the magnet.

5. What is the practical application of the force of electromagnetic induction?

The force of electromagnetic induction is used in various technologies, such as generators, transformers, and electric motors, to convert mechanical energy into electrical energy and vice versa.

Similar threads

Replies
5
Views
1K
  • Electromagnetism
Replies
10
Views
1K
  • Electromagnetism
Replies
16
Views
989
Replies
3
Views
5K
  • Electromagnetism
6
Replies
191
Views
14K
Replies
2
Views
787
Replies
8
Views
2K
Replies
10
Views
1K
  • Electromagnetism
Replies
7
Views
1K
Replies
22
Views
1K
Back
Top