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Potential Work by a Magnet

  1. Dec 7, 2012 #1
    I am studying Mechanical Engineering at college, so please forgive me for any assumptions I make.

    I know (been told) that magnets are used to convert mechanical energy into electrical energy. And have no energy of their own. I have been thinking why a magnetic generator powering it self will not work.

    I know that over time the alignment of the magnetic domains will become less aligned over time as it is heated and affected by other magnetic fields.

    So in order to re- align it will take energy to produce another magnetic field to align it. So am I right in thinking that magnets store energy. And if so how much energy is needed (not specific quantities) to create such magnetic fields. And how much potential energy will a spinning magnet have, in a generator (any, such as a wind turbine) before it becomes de-magnetized.

    I can understand if it is less than the amount is needed to magnetize it. However if this is so, how do wind turbines generate surplus amounts of energy from the wind, if the same amount of energy would be needed when the magnets in it have become demagnetized.

    Thanks for reading and answering.

    Louis Hughes
     
  2. jcsd
  3. Dec 7, 2012 #2

    russ_watters

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    You're barking up the wrong tree here. What you are describing is an unrecoverable loss unrelated to the production of electricity (we don't get electricity by de-magnetizing magnets). The reason a "self-powered" generator can't work is that you have to power the generator with a motor and the motor will (at best) use all of the energy produced by the generator.
     
  4. Dec 7, 2012 #3

    berkeman

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    The fact that magnets do not contain energy does not have to do with them being demagnetized over time. Magnets are used to convert mechanical energy into electrical energy (or visa-versa).

    A moving magnet can create a current in a coil of wire that it moves over. That's how the generators connected to wind turbines generate electrical power.

    And a current in a wire coil generates a magnetic field, which can attract ferrous metals (that's how relays work), and can attract or repel other magnets.

    Does that help?
     
  5. Dec 7, 2012 #4
    There are self initiating dynamos. In fact, self initiating dynamos don’t even need a permanent magnet. Ferromagnetism can interfere with the operation of a self initiating dynamo.

    Self initiating dynamos can be generated by electrically conducting fluids. However, the electric current fluctuates in a chaotic manner. This results in a dipole that flips in a nonperiodic way.

    Self initiating dynamos are actually very common in nature. Any a fluid conductor forced to flow in spiral patterns can generate electricity resulting in a magnetic field. If the fluid conductor is rotating and if there is a temperature gradient in the fluid conductor, convection currents cause the flow of electric fields.

    Most celestial bodies have magnetic fields that are generated by convection currents in conducting fluids. This includes the earth, the outer planets and most stars. Some celestial bodies have a magnetic field that was originally produced by a conducting fluid, but are maintained by the frozen solid which the fluid became.

    Artificial self initiating generators have been built on earth. However, they are not very efficient generators of electricity compared to the permanent magnet type of generator. Self initiating generators are sometimes used to simulate celestial bodies including the earth. Most artificial self initiating generators have been manufactured using liquid sodium.

    I am sorry they are not more practical. However, here are some links on dynamo theory.

    http://www.physics.nmt.edu/~dynamo/dynsExp131.pdf
    “Dynamo and Hydromagnetic Liquid Metal Experiments
    Magnetic dynamo experiments are being performed in many laboratories throughout the world. The most successful have been at Kalsruhe and Riga where liquid sodium has been forced in constrained flows and produced predicted positive dynamo gain. The unconstrained flows are being investigated at Wisconsin, Cadarache , and Maryland.

    Over the past few years it has been possible to demonstrate sustained self-excited dynamo action in the laboratory using liquid sodium (Gailitis et al. 2001, Stieglitz & Müller 2001, Monchaux et al. 2007). Although the magnetic Reynolds numbers are currently lower than what is possible with simulations, the fluid Reynolds numbers are much higher than what can be achieved in simulations because of the very small magnetic Prandtl number. In this respect we may expect a lot of new results to come from laboratory experiments.”


    http://www.scholarpedia.org/article/Hydromagnetic_Dynamo_Theory
    “ Dynamo theory is a vast field with almost a hundred years of history, starting with early ideas by Joseph Larmor in 1919. Dynamos are particularly important in connection with understanding magnetic fields in astrophysics. Much of the work is at the level of analytic theories and numerical simulations. During the last decade also various liquid metal experiments have been performed.
    Traditionally, dynamos are divided into kinematic dynamos, where the flow can be considered given, and nonlinear dynamos, where the flow is affected by the magnetic field through the Lorentz force.
    The latter are sometimes also referred to as hydromagnetic dynamos, which emphasizes the importance of hydromagnetic interactions.”



    http://en.wikipedia.org/wiki/Dynamo_theory
    “In geophysics, dynamo theory proposes a mechanism by which a celestial body such as the Earth or a star generates a magnetic field. The theory describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales.

    Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. The conductive fluid in the geodynamo is liquid iron in the outer core, and in the solar dynamo is ionized gas at the tachocline. Dynamo theory of astrophysical bodies uses magnetohydrodynamic equations to investigate how the fluid can continuously regenerate the magnetic field.”





    I also read that there are self initiating dynamos that don't require a conducting fluid. They use some type of spinning coil of wire. However, I forgot where I read it and can't find a citation. As I recall, they were also inefficient compared to regular electric generators (with permanent magnets).
     
  6. Dec 8, 2012 #5
    Thanks for replying, but I know how magnets and generators work. I am just saying if magnets demagnetize over time, and would require electrical energy to create a magnetic field, to re align the magnet. Then any electrical energy produced in a wind tubines life would be the exact same amount as it would take to create a magnet/s.

    For example, if a wind turbine can create 500 MW in 10 years before its magnets become to weak to produce energy; am I right in thinking that they will recycle the material and create a new magnet or simply re align it to its full strength.

    And to do so will require a massive magnetic field to create a magnetic field in the original magnet. Would this energy be MORE or LESS than 500 MW.

    If it is LESS then inst this contradicting the 1st/ 2nd law of thermo dynamics, because what you put in to a system you will get out.

    And if it is LESS why cant we make a generator with a rotor and stator made of permanent magnets, spinning. And after a few years or so when it becomes too weak to spin it has generated 5 MW, accouriding to the wind turbine if it requires less energy than created in the generators life to to remagnetize the magnets.

    Shouldnt magnetic generators be able to work? And why not, becasue if they are able to repel each other and rotate fully without loosing speed, then the amount of energy created would be more than it takes to remagnize its own magnets so, you can off set some electrical energy to remagnitize its own magnets.
     
  7. Dec 8, 2012 #6
    Magnets don´t automatically demagnetize. A properly built PM generator can operate for hundreds of years, if its operated within its limits, without the PM demagnetize.

    To re-magnetize a ferrite magnet, look at its BH curve, and see how large a magnetic field you need. How much power is needed to produce such a magnetic field depends on the construction, and in a ideal case with lossless coils the active power needed is nearly 0 W. And no way near 500 MW, and not much energy (Joules).

    I think you have misunderstood the principle of PM generators, there is no active power (Watts) extraction from the magnets. Yes there is a certain amount of energy stored in the magnetic field from the magnets ( 142 Wh in 1m^3), and a 500 MW generator does not have to operate for many seconds to produce/convert such amount of energy.

    Don´t really know what to say next to help you in your chain of thought.
     
  8. Dec 8, 2012 #7
    First I beg to differ that magnets "have no energy" -- they definitely have stored energy. Otherwise how could they do the work of moving material, the Magenet establishes a magnetic field that can absolutely do work..
    However - this energy is not consumed in a typical motor / generator, this comes from a different principal of a conductor moving through a magnetic field, this field can be from electromagnet, as in a typical synchronous motor or induction motor or from a permanent magnet. This is Faraday's Law of Induction - and is an important concept for really any engineering discipline.
     
  9. Dec 8, 2012 #8

    russ_watters

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    No, I'm sorry but that implies you don't understand how generators work. The amount of energy generated is much, much larger than the amount required to create the magnet.
    No. What is put "in" to the system is mechanical energy. Wind energy. That's what is converted to electricity. You said you're a mechanical engineering student.....that's kinda the whole point of mechanical engineering (converting mechanical energy to other forms of energy and vice versa).
    It isn't possible to physically align magnets so their forces only push in one direction for rotation.
     
  10. Dec 8, 2012 #9
    You are right. The energy would be taken out of whatever spins the magnets. However, there is an additional flaw in what he says.

    Suppose it were possible to extract the potential energy from the polarization field of the permanent magnet, just as he hypothesizes. It takes energy to make a permanent magnet. So where is he getting the permanent magnets?

    One could use electricity to make permanent magnets. However, one would need a power source for that. Next, one could mine lodestone for power. I think we would run out of lodestone very rapidly.

    You don't get anything for nothing.
     
  11. Dec 8, 2012 #10

    sophiecentaur

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    Perhaps I could put you right in another way. A permanent magnet is not a form of battery! You don't 'charge it up' with magnetism and it does not discharge its energy into an electrical device. The energy from a permanent magnet generator (billions of Joules in its life) comes from the mechanical energy that's put into it and not from the few Joules used in the magnetising process.
     
  12. Dec 8, 2012 #11

    uart

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    There is a mistaken belief in the crack-pot world of perpetual motion free energy magnetic motors that goes something like this. That motors constructed using only arrangements of permanent magnets (with no electrical input) do work, but are limited in the amount of energy that they can produce (over their usable lifetime) due to demagnetizing of the magnets, and that is related to the energy used to originally magnetize the material. I think perhaps that this false belief is somehow a the origin of the OP's confusion here.

    MrSponge consider this analogy. I know that coal is used in steel production, and I have shovel which is made out of steel. Is it true therefore that it must be physically impossible to shovel more coal, over the usable lifetime of this shovel, than the amount of coal that originally was used to make the shovel? I'm sure you can see that is obviously not true, Similarly your contention re the magnet and total electricity produced is equally absurd.
     
    Last edited: Dec 8, 2012
  13. Dec 8, 2012 #12

    jim hardy

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    Work it backward.

    If i make a permanent magnet motor
    and use it to drive a fan blade
    is there a limit to how much air i can move with that fan?

    Of course the only limit is how much electrical energy i can hand to the motor in the century or two i plan to run the fan.
    The bearings will wear out before the permanent magnets. That's why they're called permanent.

    A magnet is a tool - we rearrange its atoms a little so it'll be a magnet,
    just as we forge another piece of nonmagnetic iron(or bronze) into fan blades or windmill blades, so they'll be a propeller.

    Magnets are not magic and anybody who tells you otherwise is, well,,,, a crackpot.
     
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