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What is heat and cold?

  1. Jul 14, 2011 #1
    What is heat and cold in the atomic scale? Why do things sometimes feel hot and sometimes cold?
  2. jcsd
  3. Jul 14, 2011 #2
    Heat is the amount of thermal energy a system has. Cold just means that a system has lesser thermal energy as compared to a reference system.
    Atomic scale, it's the vibration of the particles in the system. An object feels hot because when you are in contact with the system, heat is conducted from the system to your hand, and vice versa for the cold, to achieve thermal equilibrium.

    This is just a very simplistic understanding of hot and cold.
  4. Jul 14, 2011 #3
    yeah just like thunder said, temperature is just how fast something is vibrating. The faster an atom or molecule vibrates/moves, the hotter it is, as the vibrations get slower, it gets closer. If you stop all motion then you get the temp down to absolute zero (0 K)

    basically a way to measure how fast something is moving.
    since hot and cold are subjective terms
    hot = fast compared to you
    cold = slow compared to you
  5. Jul 14, 2011 #4


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    Heat is the flow of thermal energy from one body to another. Objects feel hot or cold depending on how fast heat is transferring between the objects. The rate of heat flow is determined by both the temperature difference and the thermal conductivity between the two objects. For example, touching a steel pole at 32 F will feel colder than a wooden pole at 32F.
  6. Jul 14, 2011 #5
    Not entirely. For atmospheric gases at the temperatures normally encountered in the atmosphere, gas temperatures are purely a function of the kinetic energy of translation. They are almost entirely independent of molecular vibrations and other internal energies.
  7. Jul 14, 2011 #6
    Yes, but they are at exactly the same temperature. I believe that in Physics, temperature is an objective phenomenon, not a subjective one.

    In fairness, in the field of physiological climatology we do deal with subjective temperatures. However, we take pains to point out that these "sensible" temperatures are purely in the mind and do not exist in the environment.
  8. Jul 14, 2011 #7


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    There are two concepts here. What is heat, (and by extension cold) and Why do we feel hot and cold.

    Heat has been described here already so I'll avoid repeating that.

    I THINK this is an accurate description, but I could be wrong.

    The reason things feel hot and cold is that our nerves that tell us that depend on heat transfer to determine how hot or cold something is. For example, if I go outside in the cold 30 degree weather with no gloves on, when I come back inside I can turn on the cold water tap in my sink and the water will feel WARM to my hand. Why is this? Because at that point the temperature of the skin on my hand is colder than the water is. There is heat being transferred from the water and into my hand.

    Similarly coming into my house from outside where it is 105 degrees or so, and into the 78 degree AC feels MUCH colder than 78 degrees would feel if I had been in my house for a few hours.

    Now, none of this brings core body temperature into the issue. We are merely talking about the temperature of your skin. Your skin can easily be much colder or warmer than your core temperature without danger.
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