Question

how can you say that heat is a form of energy?

Answer 1

Heya here's the ans.
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.Heat is a form of energy.
.Kinetic theory explains the phenomena of heat as the vibration of molecules/atoms of the material. So technically heat is vibrational energy at molecular level.
.Heat is often accompanied by radiation energy(light) which is release of photons from these vibrating molecules.
.Heat energy is always accompanied by increase in entropy. This means the entire energy in the form of heat can never be extracted. Never.
.Energy is a very weird term (in my opinion all of fundamental physics is :D).
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hope it helps