Thermal Properties Of Matter Class 11 Notes Physics Chapter 11 - CBSE

Chapter : 11

What Are Thermal Properties Of Matter ?

Heat

Heat is a form of energy, which produces in us the sensation of hotness and coldness.

Temperature

It is the degree of hotness of a body.

Thermometer

It is a device used to measure the temperature of a body.

Relations Between Different Temperature Scales

$$\frac{\text{T}_{c}-0}{100-0}-\frac{\text{T}_{F}-0}{212-32}\\=\frac{\text{T}_{R}-0}{80-0} =\frac{\text{T - 273.15}}{373.15 - 273.15}$$

Absolute Scale Of Temperature

The lowest possible temperature of 273.15°C at which a gas is supposed to have zero volume (and zero pressure) is called absolute zero of temperature.

Linear Expansion

When a solid rod of initial length l is heated through a temperature ∆T, its final (increased) length is given by l' = l(1 + a∆T) where a is coefficient of linear expansion.

Superficial Expansion

When a solid sheet of initial surface area S in heated through a temperature ∆T, its final (increasd) surface area is given by,

S’ = S(1 + β∆T)

Cubical Expansion

When a solid of initial volume V is heated through a temperature ∆T, its final (increased) volume is given by V’ = V(1 + γ∆T) where g is coefficient of cubical expansion.

Relation Between α,β And γ

$$\frac{\alpha}{1} =\frac{\beta}{2}=\frac{\gamma}{3}\\\text{or}\space \beta = 2\alpha\space \text{and}\space\gamma = 3\alpha$$

The units of α, β and Y are same, viz °C–1 or K–1.

Specific Heat

It may be defined as the amount of heat required to raise the temperature of unit mass of a substance through are degree.

$$\text{C} =\frac{Q}{m\Delta \text{T}}$$

It SI unit is Jkg–1 K–1

Molar Specific Heat

It is defined as the amount of heat required to raise the temperature of 1 mole of the substance through one degree.

Heat Capacity

It is defined as the amount of heat required to raise the temperature of a body through one degree.

Latent Heat

The amount of heat required ot change the state of unit mass of a substance at a constant temperature is called its latent heat.

Principle Of Calorimetry

When two bodies at different temperatures are placed in contact with each other, the heat lost by the hot body is equal to the heat gained by the cold body. This is the temperature of calorimetry of the principle of mixture.

Conduction

It is a process in which heat is transmitted from one part of a body to another at a lower temperature through molecular collisions, without any actual flow of matter.

Factors Of Which Conduction Of Heat Depends

$$\text{Q} =\frac{\text{KA}(\text{T}_{1} -\text{T}_{2})t}{x}$$

where A is area of cross section of faces of a slab, separated by distance x, forces are at temperature T1 and T2 (T1 > T2 ) Q is amount of heat that flows K is coefficient of thermal conductivity.

Its SI unit is Js–1 m–1 K–1.

Convection

It is the process by which heat is transmitted through a substance from one point to another due to the bodily motion of the heated particles of the substance.

Radiation

It is the process by which heat is transmitted from one place to another without heating the
invervening medium.

Absorptive Power

The absorptive power of a body for a given wavelength λ is defined as the ratio of amount of heat energy absorbed in a certain time to total heat energy incident on it in the same time within a unit wavelength range around the wavelength λ.

Emissive Power

The emissive power of a body, at a given temperature and for a given wavelength λ, is defined as the amount of radiant energy emitted per unit time per unit surface area of the body within a unit wavelength range around the wavelength λ.

Emissivity

$$\text{Emissivity} =\frac{e}{\text{E}}\\=\frac{\text{emissive power of a body}}{\text{emissive power of a blockbody}}$$

Black Body

A black body is one which neither reflects nor transmits but absorbs whole of the heat radiation incident on it.

Kirchhoff’s Law

It states that at any given temperature, the ratio of the emissive power (eλ) to the absorptive power (aλ) corresponding to certain wavelength is constant for all bodies and this constant is equal to the emissive power of the perfect black body (Eλ) at the same temperature and corresponding to the same wavelength.

$$\therefore\space \frac{e_{\lambda}}{a_{\lambda}}= E_{\lambda}\space\text{(constant)}$$

Wien ’s Displacement Law

It states that the wavelenght (λm) corresponding to which the energy emitted by a perfect blackbody is maximum and it is inversely proportional to the absolute temperature (T) of the black body. $$\text{i.e.\space} \lambda_{m}\propto\frac{1}{\text{T}}\\\text{or}\space\lambda_{m} =\frac{b}{\text{T}}$$ where b is a constant of proportionality and is called Wien's constant. Its value is b = 2.9 × 10–3 mK