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Toy electromagnet Overuntity?

  1. Sep 24, 2006 #1
    If a toy electromagent holds 200 lbs on 2 d cells is that overunity?


    If not could someone explain it without just stating the 2nd law of thermal dynamics.

    Also if anyone knows the push against and armature in an electric motor compared to the pull of an electromagnet built for lifting theres supposed to be quite a diffirence so why do we make motors this way?
     
  2. jcsd
  3. Sep 24, 2006 #2

    russ_watters

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    Same as in the other thread: force and energy are not the same thing.
     
  4. Sep 24, 2006 #3
    Right

    Force X speed = power. With an electromagnet speed is not a problem.

    We just to plug is some numbers like lifting a 100 pound steel plate 4 inches in 1/100 of a second then convert that to HP or watts. Then look at the watts used. Say 12 volts and 1 amp that 12 watts to lift 100 pounds sounds like overuntity to me but unless some one calculates it well never know.

    Found a conversion site

    100 lb ft per second is 135 watts
    1/2 second 270 watts
    1/4 second 540 watts
    1/10 second 1350 watts

    So even if its 20 volts at 20 amps its still overuntiy the magnet lifts it 1/10 of a second fast!
     
    Last edited: Sep 24, 2006
  5. Sep 24, 2006 #4

    russ_watters

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    No. Just no.
     
  6. Sep 24, 2006 #5
    What is 'overunity'? :confused:
     
  7. Sep 25, 2006 #6

    russ_watters

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    Greater than 100% efficient.
     
  8. Sep 25, 2006 #7
    You're completely misrepresenting the physics involved.
     
  9. Sep 25, 2006 #8

    russ_watters

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    My big problem here (which is starting to annoy me) isn't the lack of understanding - that's what this site is here for - its the lack of effort. Outrunnersarecool, you are just plain not listening to the explanations of the physics involved. Worse, you are substituting made-up numbers for real calculations.
     
  10. Sep 25, 2006 #9
    I think I can explain it without the second law. It's a matter of amp-turns. The strength of an electromagnet is dependent on how many amp-turns you wrap around the core. Each turn of wire adds to the magnetic field. You can keep adding turns and strengthening the magnet untill the core is saturated. Then you can chuck it and get a bigger core, and wrap more turns around that, and have a more powerful magnet.

    What's the limit here? If your power source is 2 D cells they are drained faster and faster the more powerful the magnet. At some point you'll reach a level where they drain so quickly there's no point in using something as expensive as a D cell. There's no magic source of power here: it's all coming from the batteries. More powerful electromagnets use that limited amount of power up more quickly.
     
  11. Sep 26, 2006 #10
    Thanks

    Can a round purpose built electromagnet produce a rotary force against a permanent magnet? If so then do we use such weak electromagnets known as armatures in common DC motors?
     
  12. Sep 27, 2006 #11
    What is a "round purpose built electromagnet"?

    Also: did you mean to write: "If so,then why do we use such weak electromagnets..."

    It would be much better if you took care to write grammatically correct, coherent sentences so people can understand what you're asking.
     
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