Where do magnets get their energy from?

  1. madmike159

    madmike159 371
    Gold Member

    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?
     
  2. jcsd
  3. 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.
     
  4. madmike159

    madmike159 371
    Gold Member

    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.
     
  5. We are still uncertain about this..
     
  6. 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.
     
  7. 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.
     
  8. 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.
     
    Last edited: Sep 30, 2008
  9. 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
     
    Last edited: Sep 30, 2008
  10. thats news to me. I'm quite sure you are completely wrong about that.
     
  11. Yes. I admit it.
     
  12. madmike159

    madmike159 371
    Gold Member

    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?
     
  13. force doest require energy. movement through a force requires (or gives up) energy.
     
  14. HallsofIvy

    HallsofIvy 40,809
    Staff Emeritus
    Science Advisor

    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.
     
  15. The scalar magnetic potential is "NI" or "amp-turns". Any e/m fields text will elaborate.
     
  16. 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.
     
  17. 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
     
  18. 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.
     
    Last edited: Oct 1, 2008
  19. 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
     
  20. http://www.nationmaster.com/encyclopedia/Magnetic-potential#Magnetic_scalar_potential

    The magnetic scalar potential is another useful tool in describing the magnetic field around a current source. It is only defined in regions of space in absence of (but could be near) currents.

    The magnetic scalar potential is defined by the equation:
    mathbf{H} = - nabla mathbf{psi}Applying Ampere's Law to the above definition we get:
    mathbf{J} = nabla times mathbf{H} = - nabla times nabla mathbf{psi} = 0Since in any continuous field, the curl of a gradient is zero, this would suggest that magnetic scalar potential fields cannot support any sources. In fact, sources can be supported by applying discontinuities to the potential field (thus the same point can have two values for points along the disconuity). These discontinuities are also known as "cuts". When solving magnetostatics problems using magnetic scalar potential, the source currents must be applied at the discontinuity.

    The magnetic scalar potential is suited to use around lines/loops of currents, but not a region of space with finite current density. The use of magnetic potential reduces the three components of the magnetic field mathbf{H} to one component mathbf{psi}, making computations and algebraic manipulations easier. It is often used in magnetostatics, but rarely used in other applications.

    I found another link but I cant post it because the website uses words like love and peace and oneness and I dont want to offend anybody.
     
    Last edited: Oct 1, 2008
  21. I'm sure he might be talking about how with electromagnets it magnetic field potential is determined by Amps x Turns(Coils of wire).

    or NI like he said.
     
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