Energy for magnetism

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Main Question or Discussion Point

If,
-There is an iron rod.(It does not have magnetic potential energy at this time)
-I made an electromagnet in a far away place.
-When I bring the magnet closer to the iron rod,the iron rod suddenly gets kinetic energy.Why?

As energy cannot be created,from where does the kinetic energy come from?
 

Answers and Replies

  • #2
russ_watters
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The potential energy is created when you switch on the magnet.
 
  • #3
jtbell
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More precisely, the energy stored in the electromagnet's field comes from the current source in the circuit that powers the electromagnet. When you switch the current source on, it has to do work against the "back emf" produced by the electromagnet as the current increases. This work ends up as energy stored in the magnetic field.
 
  • #4
CWatters
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There is a common follow up question which goes something like...

There is a heck of a lot of iron in the universe. It can't all have gained PE w.r.t the magnet? Wouldn't that be a lot more energy than stored in the magnetic field :-)

Similarly...if you dig a small hole in the ground then rather a lot of matter (eg that building next to the hole) suddenly gained PE w.r.t the bottom of your hole :-)
 
  • #5
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The potential energy is created when you switch on the magnet.
How can you say that energy is created?Do you mean energy in the magnetic field is converted to potential energy?
 
  • #6
russ_watters
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How can you say that energy is created?Do you mean energy in the magnetic field is converted to potential energy?
It is created from the electrical energy that is fed to it. Created/converted - either word works. Yes, energy is conserved.
 
  • #7
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What about this?
There is a common follow up question which goes something like...

There is a heck of a lot of iron in the universe. It can't all have gained PE w.r.t the magnet? Wouldn't that be a lot more energy than stored in the magnetic field :-)

Similarly...if you dig a small hole in the ground then rather a lot of matter (eg that building next to the hole) suddenly gained PE w.r.t the bottom of your hole :-)
 
  • #8
russ_watters
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Yes, it can. Just like there's a heck of a lot of GPE associated with most of the rest of the matter in the universe being far away from Earth.
 
  • #9
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Yes, it can. Just like there's a heck of a lot of GPE associated with most of the rest of the matter in the universe being far away from Earth.
That's kinda confusing.Having GPE is understandable because things that created earth always had mass.But this magnet is created just now.
 
  • #10
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A magnetic field has a certain energy density. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/engfie.html

If an iron bar is used to extract energy from the field then it does so by reducing the field and therefore reducing the remaining energy. The iron in the universe does not gain more PE than the energy in the field.
 
  • #11
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I see.This is all about magnetic field's energy.Now I understand.I think I'll have to learn more about electromagnetism.Thanks
 
  • #12
Magnetism is not an energy in itself. Magnets have a force, and if that force is used to move an object over a certain distance, the energy is called work.

1 way switch
 
  • #13
adjacent
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What is the speed of the magnetic field?
What is we use a permanent magnet?From where does this magnet's field gets energy?(electromagnet's gains its energy from electric current source).But as we see,permanent magnet's is able to lift without getting "exhausted".
 
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  • #14
CWatters
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What is the speed of the magnetic field?
Light has both electric and magnetic components.

What is we use a permanent magnet?
I don't understand that question.

From where does this magnet's field gets energy?(electromagnet's gains its energy from electric current source).But as we see,permanent magnet's is able to lift without getting "exhausted".
Read about work and energy. For example..

work (aka Energy) = force * displacement

A fridge magnet does not move so the displacement is zero. This means the energy required is zero. So a fridge magnet does not use any energy when stuck to a fridge.

It's not 100% clear what you mean by "permanent magnet's is able to lift without getting "exhausted". Please give an example so we can explain.
 
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  • #15
adjacent
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I don't understand that question.
It was a spelling mistake,it should be what if.
Read about work and energy. For example..

work (aka Energy) = force * displacement

A fridge magnet does not move so the displacement is zero. This means the energy required is zero. So a fridge magnet does not use any energy when stuck to a fridge.

It's not 100% clear what you mean by "permanent magnet's is able to lift without getting "exhausted". Please give an example so we can explain.
What I meant was this;
If an object is in a distance 'd' form the magnet,it has potential energy which is gained from the energy in the magnet's field.Then the potential energy of the object changes to kinetic energy and then to heat etc..As DaleSpam said,the remaining energy is reduced.Then why doesn't all the remaining energy get's reduced?(i.e. the magnet is able to move infinite iron pieces(if brought close to it)).
 
  • #16
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But as we see,permanent magnet's is able to lift without getting "exhausted".
The magnetic field around a permanent magnet follows the same rules as the magnetic field around an electromagnet. It has an energy density determined by the same formula I linked to earlier. The energy in the field limits how much work can be done, so a permanent magnet cannot pick up an unlimited amount of material. Each item that it lifts reduces the total amount of energy in the field. Work must be added to the system to increase the field back to normal.
 

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