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B What is heat?

  1. Mar 7, 2016 #1
    I've recently begun studying thermal effects. I learned that thermal energy could be transferred within an object through conduction and from a distant object through radiation. (let me know if I'm wrong here) I also learned that when matter is heated, it's particles begin to vibrate, move apart and gain potential and kinetic energy. My question here is, what really makes these particles vibrate? Why would they begin vibrating and move apart? And when we touch something and it feels hot, is it because of the fact that the atoms within the object have a lot of internal energy? I'm sorry if I can't convey my problem properly. It's just that I'm not able to wrap my head around it. :/
  2. jcsd
  3. Mar 7, 2016 #2
    Thermal energy is talked about only when we have a system of large number of particles which you are calling molecules. When we consider the whole object as a whole then we talk only two kinds of energies kinetic and potential. There are certain why questions which are not answered in Physics.For example if you ask me why does earth attracts us or why does Sun attract earth. We have to take it as a postulate. So molecules of an object continually vibrate about their mean positions. They do not go away from each other in solids and liquids but in gases they tend to move away. when we consider the collision of two objects then we can think about some time in which collision occurs and then we can talk about exchange of energy momentum during the collision time. But in case of exchange of thermal energy it is a continuous tendency of mutual exchange due to conduction or radiation. Over a period of time either the internal energy of one has a tendency to lose or gain energy or at times the exchange may be equal. When the later happens we say that these systems are in thermal equilibrium. and teh individual systems are then said to have the same temperature.
  4. Mar 7, 2016 #3


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    Good thing you wonder about what heat is. Check some of the other threads at the bottom too.
    The continuous redistribution of forms of energy over degrees of freedom: collisions in gases, loose coupling of degrees of freedom in other materials, interaction with radiation. You're not wrong in your
    Again, this redistribution of energy. A main driving force is temperature difference. A part of a substance where nothing is moving or vibrating has a very low temperature. So for that part picking up energy is much more likely than giving off energy. After a while it is very unlikely that the temperature deviates more than infinitesimally from the equilibrium temperature.
  5. Mar 7, 2016 #4
    Energy is a concept which can be talked about in case of an object taken as a single unit or also as the total energy of its large number of molecules. Later is called internal energy. But there are certain concepts like temperature which has no meaning when only one entity is there. For temperature of a molecule is a meaning less attribute. This dichotomy of concepts such as temperature equilibrium which are meaningful only about so called macroscopic systems and are meaning less for its microscopic parts. But concept of "energy" is applicable throughout. This creates all the difficulties in understanding. I would suggest that we should refrain from talking about microscopic structure of macroscopic systems. You say that molecules of hot water are vibrating faster than cold water bit I would be satisfied by thinking to myself that internal energy of hot water is more than that of cold water of equal magnitude.
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