Thermal resistance is the hindrance offered by a body to the flow of heat.

Consider a conductor of length l, the area of cross-section A and thermal conductivity K. 

When the two ends of conductor are kept at temperature difference , then the rate of flow of heat is given by,

                       

i.e., for a given conductor, the rate of flow of heat is directly proportional to the temperature difference across the length of is conductance.

The reciprocal of thermal conductance i.e. l/KA is called thermal resistance.

The unit of thermal resistance is J^–1 Ks.

The dimensional formula is [M^–1L^–2T^–3K¹]

In transient state, the rate at which the rod gets heated is not the same for all the materials.

Heating depends not on the thermal conductivity but also depends on the thermal capacity per unit volume of material.

The rate at which the temperature of any part of rod increases is called thermometric conductivity or diffusivity. Thermal conductivity is equal to the ratio of thermal conductivity to the thermal capacity per unit volume of material. 

 

S.I. unit of thermal conductivity is Watt/m/K or J/s/m/K.

CGS unit of K is cal/cm/sec/K or ergs/cm/sec/K.

Dimensional formula of thermal conductivity is [M¹L¹T^–3K^–1]