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A bar spinning with unlimited angular velocity

  1. May 28, 2014 #1
    Theoretical question about a bar spinning with unlimited angular velocity.

    Lets imagine we have a bar of indestructible metal and we attach this bar onto a motor which is capable of an infinite number of rpm. We have a dial next to the motor that allows us to increase the rpm of the motor. We start the motor off and gradually increase the rpm.

    First a low pitch hum is heard coming from the rotating bar which is attached at right angles to the motor shaft so that the center of the bar is in line with the center of rotation of the motor shaft. This hum gradually increases in pitch as the motor speeds up until it is a high pitch sound then at a frequency above human hearing. It is noted that the bar starts to heat up as it spins rapidly, first only an infrared camera can pick it up but soon the bar is glowing a dull red, then a cherry red, then orange, yellow, blue and then it is white hot.

    As you can see we started with low frequency sound waves then to high frequency sound waves then onto low frequency infrared electromagnetism and went through the spectrum until it was glowing white hot.

    My question is will the bar go on to emit x-rays and eventually gamma rays and would it at any point emit radio or microwaves
  2. jcsd
  3. May 28, 2014 #2


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    If the temperature is high enough, yes. Just plug the temperature int the black body formula and look at the peak of the spectrum.
  4. May 28, 2014 #3
    You are mixing up the mechanical waves with the electromagnetic waves. The IR waves are not a continuation of the high frequency sound waves. They are different phenomena. If for some reason the bar produces sound waves, there is no reason to assume that these will be replaced by EM waves above some frequency.

    And why would the ideal bar in the ideal experiment heat up? Friction with air or in the axle? Or you are looking at a different mechanism? The acceleration of charges in the bar?
  5. May 28, 2014 #4


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    Yes. The model needs to be defined better; just how 'ideal' are you wanting to be? Unlimited rotation speed implies no atmosphere around it so no sound, for a start and no increase in temperature. A relatively modest tip speed in air at sea level would produce a temperature of several thousand K. (Consider the temperature of impacting meteorites) So we're not talking in terms of unlimited speed - just 'high' speed.
  6. May 28, 2014 #5
    I am aware that sound waves are not electromagnetic however it is a well known fact to anyone who has played with a motor as a kid that increased rpm results in increased sound pitch. It is also a fact that a object spinning in an atmosphere will heat up, infrared is electromagnetic as is all the other colored lights emitted by the glowing bar.

    I suspect the bar would heat up even in the vacuum of space if spun fast enough. First the spinning bar should generate a magnetic field due to spin charge alignment
  7. May 28, 2014 #6
    I allowed unlimited speed so that nobody could say we cant spin it fast enough or there is no motor that could do that ;)

    What would happen to the bar as its angular velocity approached the speed of light for example, I'm sure the effect of spin would result in a different phenomena then an object traveling in a straight line approaching the speed of light.
  8. May 28, 2014 #7
    I don't know of any physics by which a neutral spinning bar generates a magnetic field. I hope this isn't a relativistic effect. And could you kindly outline the mechanism by which the bar would heat up even in the vacuum of space.
  9. May 29, 2014 #8
    The angular speed cannot approach the speed of light. Or any other linear speed.
    They are different quantities with different units. Does not make sense to compare them.
    I am afraid this may be just one of the points you need to clarify (at least for yourself).

    Also, what do you mean by "spin charge alignment"?
    By "spin" you mean the macroscopic spin of the bar or the microscopic quantity, the spin of the electron?

    Before you can think about an answer you need to understand the question. I am afraid we are not there yet.:)
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