Transfer of Heat

Heat energy flows from a hot body to a cold body. There are three ways by which transfer of heat takes place:
1. Conduction: Transfer of heat through matter without actual movement of particles (mostly in solids).
2. Convection: Transfer of heat by actual movement of molecules (in fluids - liquids and gases).
3. Radiation: Transfer of heat in the form of electromagnetic waves (needs no medium).

When one end of a solid is heated, atoms/molecules vibrate faster and bump into neighboring atoms, transferring energy. In metals, free electrons also help transfer heat quickly, making them good conductors.

The rate of flow of heat ($Q/t$) across the opposite faces of a meter cube of a substance maintained at a temperature difference of 1 Kelvin is called the thermal conductivity ($k$) of that substance.
Formula: $\frac{Q}{t} = \frac{kA(T_1 - T_2)}{L}$
Where $A$ is area, $L$ is length/thickness.
Unit: $Wm^{-1}K^{-1}$ (Watt per meter per Kelvin).

Definition: Transfer of heat by actual movement of molecules from hot place to a cold place is known as convection.
It occurs in fluids (liquids and gases).
Mechanism: When a fluid is heated, it expands, becomes less dense, and rises up. Cold fluid from sides moves in to take its place, setting up convection currents.

Sea Breeze: During variable day, land heats up faster than sea. Hot air over land rises, and cool air from sea moves towards land. (Blows from Sea to Land).
Land Breeze: At night, land cools faster than sea. Hot air over sea rises, and cool air from land moves towards sea. (Blows from Land to Sea).

Birds (like eagles) and Gliders use upward moving hot air currents (called thermals) to gain height and fly for hours without flapping wings. This is an application of convection currents in air.

Definition: The mode of transfer of heat from one place to another in the form of electromagnetic waves.
It does not require any material medium.
Example: Heat from Sun reaches Earth by radiation.

A Leslie's cube is used to compare thermal radiation from different surfaces.
Findings:
1. Dull Black surface: Best Emitter, Best Absorber.
2. Shiny Silver surface: Worst Emitter, Worst Absorber (Best Reflector).

Glass and Polythene allow short-wavelength solar radiation to enter but do not allow long-wavelength heat radiation (emitted by plants/soil) to escape. This traps heat inside, keeping the interior warm. This is called Greenhouse Effect.
Global Warming: $CO_2$ in atmosphere acts like glass, trapping earth's heat.

A double-walled glass vessel with vacuum between walls.
1. Vacuum: Stops Conduction and Convection.
2. Silvered walls: Reflect radiation (Stop Radiation).
Thus, it keeps hot things hot and cold things cold.