According to my knowledge, heat is the energy that is stored in form of kinetic energy of molecules in Brownian motion. However, in a macroscopic view, a rigid body seem to be "stable" but still store heat. I am wandering does it mean in a microscopic view, the molecules of solid matter are still moving?
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Let us understand Brownian motion in liquids before we look at the motion in solids. If you observe a glass of water at rest on a table, it "appears" to be motionless. However, all we need is a magnifying glass to observe the random, incessant motion of water on the surface. This random motion is a manifestation of heat.
The same thing happens in a solid. A block of steel "appears" to be at rest. But its atoms are constantly vibrating. They cannot wander off too much, unlike water molecules, because they are bound by the lattice. These vibrational modes are always present at room temperature. As you cool the solid, they will gradually vanish. However, even at absolute zero, you will still observe "zero-point vibrations".
Please note that atoms/molecules in a solid at rest are always exchanging energy with their surrounding. They absorb photons and emit them. They are always "active" and in motion.
Amey Joshi
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