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Macroscopic kinetic energy, temperature, and light

  1. Jan 20, 2012 #1
    Hi all,

    The way I understand it is that temperature is a measure of the random kinetic of molecules and atoms.

    When you heat something up, eventually it will start to emit visible light once it gets hot enough. When a macroscopic object is moving through a vacuum however, why does it not glow like a heated object does, even if moving at high speeds?
  2. jcsd
  3. Jan 22, 2012 #2
    Welcome to PF, Lightspeed5 – Yours is a simple question with a not-so-simple answer—which is probably why you haven’t seen too many responses.

    1. First of all, the parameter “temperature” means different things under different circumstances.

    2. For ideal gases under conditions of equilibrium, the gas temperature measures the kinetic energy of translation along a single axis of movement—the axis that is normal to the sensing surface.

    3. Keep in mind that “measurement” always involves the characteristic being measured, the device doing the measurement, and the observer observing that device.

    4. When a conductive thermometer measures the temperature of a gas, what it actually measures is the mean magnitude of the kinetic energy of the “impulse” transferred to the sensing surface of the thermometer by the impacts of the gas molecules upon that surface. It does not directly measure the speed of the molecules.

    5. The same thing is true when molecules impact upon one another in their random translatory motions. Their reactions to these impacts are reactions to the kinetic energies involved, not simply to the individual speeds at the instant of impact.

    6. It is the mean magnitude of these intermolecular impacts that influences the nature of the photons subsequently emitted by the gas molecules.

    7. The “temperature” of a macroscopic object traveling through space is unrelated to its speed through space, but is a function of its internal kinetic energies.

    8. I hope this answers your question.
  4. Jan 22, 2012 #3


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    An object that is heated to a high temperature emits light because the various particles that make up the material are all vibrating, rotating, and moving around against each other. When charged particles are accelerated, which is what happens when they "bounce" off of each other, they emit EM Radiation, which is what Light is. This also happens when electrons move between energy levels within atoms and molecules.

    In fact, you yourself are emitting EM Radiation right now, but since you are at a much lower temperature than something glowing Red Hot, you only emit a much lower frequency radiation, mainly infrared, since it takes less energy to make it than it does to make visible light.

    When an object is moving, it's composite particles are not moving any more or less relative to each other, so they don't have any more or less energy relative to each other and the temperature stays the same as it would if they were not moving.
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