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Temperature (the very basics)

  1. Sep 2, 2015 #1
    Is the following correct?

    Temperature is the measure of how hot or cold something is; not how much thermal energy it has.

    Examples; 1 kg of boiling water has a temperature of 100 degrees C and has heat energy of 4.184 kJ (the specific heat capacity of water).

    1 kg of lead heated to a temperature of 100 degrees C has the same temperature as the kg of boiling water but only has 128 J of thermal energy.

    But temperature is also a measure of how much kinetic energy the particles of a substance has (or is this just another way of saying how hot or cold it is?).
  2. jcsd
  3. Sep 2, 2015 #2
    I believe you are correct.

    Temperature and heat are related by Q = cmΔT with c being the heat capacity, m the mass of the object, and ΔT the change of temperature.
  4. Sep 3, 2015 #3
    If substance getting more hoter the molequlas in the subatance getting higher kinetic energy,
    there is only one position that molequles will be with non kinetic energy- in 0 kelvin degrees that is equlized ≈ -271.3 celsius degrees but scientist never suceed to achieve this low temprature, they get very close to this value...

    In addition cold is just way to say that matirial is have less thermal energy.
  5. Sep 3, 2015 #4


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    Staff: Mentor

    Indeed, temperature is not a measure of a system's thermal energy, it is the propensity of the system to exchange energy with another system. Two systems are at the same temperature when, on average, the exchange of thermal energy between the two cancels out (A gives as much to B per unit of time as B gives to A).

    For an ideal gas, there is a direct relation between its kinetic energy an temperature. But it can have more thermal energy than just kinetic energy (internal energy).
  6. Sep 3, 2015 #5


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    Staff: Mentor

    0 K is exactly -273.15 °C. The 3rd law of thermodynamics says that absolute zero can never be reached.

    Not exactly. See above.
  7. Sep 3, 2015 #6


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    Heating 1kg of water from 0C to 100C means you have increased its energy by 100 * 4.184kJ = 418kJ. That's not the same as saying it "has" 418kJ. You could get more than 418kJ of energy out by cooling it to -10C.

    The greatly oversimplified view is that temperature is a measure of the translational KE where as heat energy is a measure of the KE stored all "degrees of freedom" (translational, rotational etc). Thats far from the whole story.

    https://www.quora.com/Why-do-different-materials-have-different-specific-heat-capacities [Broken]
    Last edited by a moderator: May 7, 2017
  8. Sep 3, 2015 #7
    Thanks. What does Q stand for in your equation?
  9. Sep 3, 2015 #8
  10. Sep 3, 2015 #9
    Sorry forgot to mention it. Q is heat (or energy, joules).
  11. Sep 3, 2015 #10
    Also, this equation works at constant volume.

    Often when you add heat to something, it expands. If it expands, then thermal energy density decreases, thus temperature doesn't rise as much as if the volume didn't change.
  12. Sep 3, 2015 #11
    Thanks. Perhaps you could help with a related question. I read the following on gcse.science.com;

    "We say that energy is transferred when one form of energy is changed into another form."

    What then is the term for heat energy (or any energy) going from one object to another but the energy not changing form, eg, heat energy going from a warm hand into a cooler hand?
  13. Sep 3, 2015 #12
    I was reading the other post just now. :) Not sure this has a name. I guess you could say energy flows or propagates. If this has a specific name I don't know it.

    I am currently studying electromagnetism and, when energy "moves" in space via electromagnetic waves, we talk about the rate of energy flux density because we measure it per square meter.
  14. Sep 3, 2015 #13


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    Staff: Mentor

    You have a thread on this already. Please keep questions regarding it in that thread.
  15. Sep 4, 2015 #14
    Thanks again. I'll find out what the name for it is but at least I know for now that it's not 'energy transfer' (where the energy has to change form).
  16. Sep 4, 2015 #15
    Thanks. Will do.
  17. Sep 4, 2015 #16
    Have I got the following correct?

    When an object is (hypothetically*) at 0 degrees Kelvin (absolute zero) it has zero kinetic energy - even at the atomic and subatomic level – and has zero energy of any kind? So any increase in temperature from there is an increase in KE?

    A cube of ice therefore is relatively warm? It is relatively energetic?

    Q. Can one cube of ice be colder than another?

    Q. If we add heat energy to an ice cube it will melt and become liquid water (and then gas) but what happens to it when we take heat energy out of it? Can we take heat energy out of it? Can it get close to absolute zero (-273.15 degrees C)? If so, what would it look like? Would it still be a solid (just a much colder ice cube than the ones in our regular freezers)?

    Q. And, finally, if absolute zero can never be reached according to the 3rd law of thermodynamics, does that mean that the universe can never end? The particles that make it up would still have that small amount of KE due to them not reaching absolute zero?
  18. Sep 4, 2015 #17


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    No, it will still have zero-point energy.

    If the system can have energy in more than just translation degrees of freedom, then it will gain energy not just in the form of kinetic energy. (See post #6 above.)

    I don't undertand your use of "relatively".

    I'm quite sure that the ice cube sitting outside at -2°C is warmer than the one in my freezer.

    Yes, you can make it colder.

    That would depend on the pressure also. The crystalline phase would probably change

    It could reach the point where there is not enough free energy for any process to occur: the heat death of the universe.
  19. Sep 5, 2015 #18
    “I'm quite sure that the ice cube sitting outside at -2°C is warmer than the one in my freezer.”

    Thanks, I've never thought of ice as getting colder and colder. I just Googled average temperatures of freezers and the first two results suggest keeping a domestic freezer at around -18C (and fridges at around 4C). So the ice cubes in those freezers are 10 times colder than the one outside at -2C.

    “It could reach the point where there is not enough free energy for any process to occur: the heat death of the universe.”

    Thanks. I just read the opening paragraph of the link. Would this ‘death’ of the universe be, in a sense, like the death of a person in that when a person dies they carry on existing (as a corpse; decomposed matter; worm/bacteria food; etc etc)? If energy can’t be destroyed the ‘corpse’ of the universe will go on existing eternally (even if the energy that it consists of is no longer ‘free’)?
  20. Sep 5, 2015 #19


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    That last statement is incorrect. You can't make such comparisons using the Celsius scale, which is a relative scale. What about the difference between ice at 0°C vs ice at -18°C. Is the latter infinitely colder? To make such comparisons, you need to use the Kelvin scale.

    Death is only a metaphor. The universe would continue to exist, but there no be no more "interesting" process going on.
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