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Where do magnets get their energy from?

by madmike159
Tags: energy, magnets
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madmike159
#1
Sep29-08, 04:03 PM
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Since everything has to obey the law of conservation of energy, where do magnets get their energy from? They can lift something up through a gravitational field which requires energy. If the energy for a solid state magnet comes when you magnetize it (e.g. moving it with your hands with another magnet) would it lost some of its strength as it picks up objects etc?
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granpa
#2
Sep29-08, 04:31 PM
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maybe you should first ask about electromagnets. suppose a superconducting ring carries a current I. it then attracts a piece of metal to itself. what happens to the current. I'm sure it decreases but I'm not sure why.
madmike159
#3
Sep30-08, 06:48 AM
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That is also a good question. I know electromagnets get their energy from the current in the wire but not what happens to it. I guess it would be disturbed in some way.

Glenn Rudge
#4
Sep30-08, 11:44 AM
P: 9
Where do magnets get their energy from?

We are still uncertain about this..
MrEnergy
#5
Sep30-08, 12:09 PM
P: 14
Magnets, which have a stabilized magnetic field, does not lose or get energy from anywhere. For example lifting an object or pushing one; if you close the magnet to an object and if the magnet pushes it, then the energy required to push the object is given by your fingers. And you can not produce a "generator" with magnets because they do not produce energy, actually it is a field and the field does not change without any exterior changes made. To make magnet do something, you use another type of energy and magnet is only a phase for transmitting the energy(for cases like lifting, pushing, rotating etc.). And electromagnets, they surely get their energy from electricty. Hope I could explain.
ManDay
#6
Sep30-08, 12:57 PM
P: 154
Magnets do not gain any energy from anywhere. The question "where does something get its energy from" is wrongly asked by itsself.

To make it easy for you think of the magnetic force as the same as the gravitional force. Would you ask where "the earth gets its energy from"? I hope not.
granpa
#7
Sep30-08, 01:13 PM
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a gravitational field has a scalar potential. a magnetic field doesnt. if 2 charges attract one another then the resulting combination has a reduced field due to the superposition of the original 2 fields. if 2 magnets attract then naive superposition of fields results in an increased net field. one has to factor in induction to explain the actual reduced net field.

its a legitimate question. gravity and magnetism are very different.
ManDay
#8
Sep30-08, 01:28 PM
P: 154
Quote Quote by granpa View Post
a gravitational field has a scalar potential. a magnetic field doesnt. if 2 charges attract one another then the resulting combination has a reduced field due to the superposition of the original 2 fields. if 2 magnets attract then naive superposition of fields results in an increased net field. one has to factor in induction to explain the actual reduced net field.

its a legitimate question. gravity and magnetism are very different.
No. They are not as different at this level of interpretation. Both do have a scalar potential and both are, of course, superpositional. Both do find an equivalent in eachother.

Magnetism <=> Mass
Magnetic Force <=> Gravitational Force
granpa
#9
Sep30-08, 01:35 PM
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Quote Quote by ManDay View Post
Both do have a scalar potential
thats news to me. I'm quite sure you are completely wrong about that.
ManDay
#10
Sep30-08, 01:38 PM
P: 154
Yes. I admit it.
madmike159
#11
Sep30-08, 03:20 PM
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So if I stuck a magnet to a fridge, where does the energy come from. Gravity will be trying to pull it to earth, does the fridge creat a resistive force and where does the energy for that come from?
granpa
#12
Sep30-08, 06:41 PM
P: 2,258
force doest require energy. movement through a force requires (or gives up) energy.
HallsofIvy
#13
Sep30-08, 06:45 PM
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I think granpa meant that "force does NOT require energy". Which is, of course, true.

Yes, gravity tries to pull the magnet down and the force between the magnet and the refridgerator (which comes from the electrons in both) pulls it up. The two forces are equal and opposite. There is no "energy" involved.
cabraham
#14
Oct1-08, 12:02 PM
P: 1,035
Quote Quote by granpa View Post
thats news to me. I'm quite sure you are completely wrong about that.
The scalar magnetic potential is "NI" or "amp-turns". Any e/m fields text will elaborate.
granpa
#15
Oct1-08, 12:10 PM
P: 2,258
we are evidently talking about different things. the electric field is the gradient of a scalar potential field. the magnetic field does not and can not have such a scalar potential field.

not sure what 'amp-turns' is. I would have to look it up.
cabraham
#16
Oct1-08, 04:28 PM
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Nothing personal, but you're telling me I'm wrong, and yet you don't know what "amp-turns" means.

Every e/m fields text describes "scalar magnetic potential". Yet you insist that it doesn't exist!

As far as E = -grad V goes, that is only true under limited conditions, i.e. E fields due to charged particles. With time varying magnetic fields, E = -dA/dt, where A is the vector magnetic potential.

This stuff is well documented. Honestly, there is a scalar magnetic potential. Honestly, I wouldn't lie to you. BR.

Claude
granpa
#17
Oct1-08, 04:35 PM
P: 2,258
I know that potential increases as you move along a field line and magnetic field lines move in circles. therefore there cant be a magnetic scalar potential. (in that sense)

you are referring to something completely different.

you are right about some electric fields not being the derivative of a scalar potential.

the original question was about where magnets get their energy. when charged particles interact the energy can be said to come from the potential energy of their fields. this is simple to show. magnetic fields arent so simple. their fields dont simply superimpose. you have to take induction into account when they interact.
cabraham
#18
Oct1-08, 04:46 PM
P: 1,035
What I'm referring to is something the scientific community defined in the 19th century. It may be completely different from your concept, but it is valid in the eyes of science.

With electric fields, the scalar potential is related to the energy. Take a simple parallel plate capacitor. A uniform E field exists between the plates, and energy is stored in said field. The scalar potential is the voltage. The energy stored is proportional to the voltage squared, i.e. W = C*(V^2)/2.

Now, in an inductor, energy is stored in the magnetic field. The scalar magnetic potential, is the amp-turns, NI. The energy is proportional to the square of the amp-turns. For an inductor, the inductance is proportional to the turns squared. The energy is L*(I^2)/2. Since L varies with N^2, then energy varies with (N^2)*(I^2), which is (N*I)^2.

V = scalar electric potential.

NI = scalar magnetic potential.

A = vector magnetic potential.

Claude


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