Fourier's law or heat transfer by conduction.

Conduction:

Heat transfer by molecular contact (collision) is conduction.

Conduction is the heat transfer by molecular collision.

Conduction takes place in solids, liquids, and gasses but works best in solids. 

Why does conduction occur best in solids?

Because in the case of solids, atoms or molecules are located closer together. That’s why the transfer of kinetic energy from a hot system to a cold is easier than liquids and gases. So, conduction occurs best in conduction.

Mechanism of heat conduction in solid:

In solids, heat is conducted by 2 methods.

• By lattice vibration

• By transport of free electron

In the case of gases, the heat conduction mechanism is simple. The molecules of gases are in continuous random motion exchanging energy and momentum. When a molecule from the high-temperature region (with high kinetic energy) collides with a molecule from the lower temperature region (with low kinetic energy) it loses energy by collision and heat transfer occurs.

In the case of liquids, the mechanism of heat transfer is like that of gas. However, the molecules are more closely spaced.

Fouriers law

The rate of heat transfer by conduction can be calculated by Fourier's law.

Fourier’s law states that the rate of heat transfer is directly proportional to temperature difference, directly proportional to the area, and inversely proportional to thickness.

The negative sign indicates that heat flows from the hot to the cold body.

On which factors does the heat transfer by conduction depend?

It depends on 

1. Thermal conductivity

2. Area

3. Temperature difference

4. And wall thickness

Thermal conductivity:

Thermal conductivity ‘k’ is the ability of the material to conduct thermal energy or measure how fast heat flows through the material.

The unit of thermal conductivity is watt/m. C

On which factors does thermal conductivity depend?

It depends on

1. Temperature difference

2. Nature of material

Thermal resistance:

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow.

 Thermal resistance is the reciprocal of thermal conductance.

R= dx/ A k

Thermal diffusivity:

how fast heat is diffused through a material.

K (how well does a material conduct heat?)

Pcp (how much energy a material can store per unit volume?)

The larger the thermal diffusivity, the faster heat flows into the medium.

The smaller the thermal diffusivity, the heat absorbed by the material and the small amount of heat further conducted.