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What is thermal energy ?

Thermal Energy: A specialized term that refers to the part of the internal energy of a system which is the total present kinetic energy resulting from the random movements of electrons, atoms, and molecules.


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          The ultimate source of thermal energy available to mankind is the sun, the huge thermo-nuclear furnace that supplies the earth with the heat and light that are essential to life. The nuclear fusion in the sun increases the sun's thermal energy. Once the thermal energy leaves the sun (in the form of radiation) it is called heat. Heat is thermal energy in transfer. Thermal energy is part of the overall internal energy of a system. Read about the physics of thermal energy
          At a more basic level, thermal energy comes from the movement of atoms and molecules in matter. It is a form of kinetic energy produced from the random movements of those molecules. Thermal energy of a system can be increased or decreased.
           When you put your hand over a hot stove you can feel the heat. You are feeling thermal energy in transfer, otherwise known as heat. The atoms and molecules in the metal of the burner are moving very rapidly because the electrical energy from the wall outlet has increased the thermal energy in the burner. We all know what happens when we rub our hands together. Our mechanical energy increases the thermal energy content of the atoms in our hands and skin. We then feel the consequence of this - heat. Laws of Thermodynamics
On the left you can see 3 hypothetical thermodynamic systems. I've labeled system C as air for simplicity and the other systems could be anything really. We'll pretend they are solid objects though, two pieces of round iron. One is cold, one is hot. The air we'll make a temperature the same as system A. Instantaneously we put all three into a sealed box. Technically the the box now becomes System D but for simplicity we will leave it out.

By convection and radiation the fast moving atoms of system B impact the movement of the atoms of the air. So the transfer of thermal energy begins. As the air "heats up" the faster moving air atoms and molecules now start hitting the thermodynamic boundary of system B and transfering that energy (thermal energy). All this will occur spontaneously until an equilibrium temperature is reached among all 3 systems. What is the lowest that the equilibrium temperature can be? Can it reach Absolute Zero?

thermal energy diagram