Physics Ch11 Thermal Properties of Matter Notes Class 11

Thermal Properties of Matter explains how heat and temperature influence the behavior of solids, liquids, and gases. This chapter from cbse syllabus class 11 physics builds the connection between microscopic particle motion and macroscopic thermal effects such as expansion, heat transfer, and change of state.

It is an important chapter for CBSE board exams as well as competitive exams because it combines theory with numerical applications involving formulas and laws.

Thermal Properties of Matter Topics

Thermal properties of matter deal with how different materials respond to changes in temperature, how heat flows from one body to another, and the nature of heat itself.

This chapter explains the concepts of temperature, heat transfer, expansion of solids, liquids and gases, calorimetry, change of state, thermal conduction, convection, radiation, and related laws.

Temperature

Every body consists of atoms and molecules in constant motion. The motion can be translational, rotational, or vibrational, depending on the phase of the substance. The faster the motion of the particles, the greater their average kinetic energy.

This average kinetic energy is directly related to what we call temperature. It determines the direction of heat flow.

• Heat flows from higher temperature to lower temperature.
• For an ideal gas, temperature is proportional to the average kinetic energy of molecules.

The SI unit of temperature is kelvin (K), and that of heat is joule (J). Commonly, temperature is also measured in degrees Celsius (°C). Relation between scales: T(K) = t(°C) + 273.15

Note: 0 K is called absolute zero — the lowest possible temperature where molecular kinetic energy is minimum (not zero due to quantum effects).

Heat

Heat is the form of energy transferred between bodies due to temperature difference. SI unit: joule (J)

Important distinction:

• Heat is energy in transit.
  
• A body does not “store heat”; it stores internal energy.

Measurement of Temperature and Thermometers

To measure temperature, various thermometers are used, which work based on the thermal expansion of substances such as mercury or alcohol in glass thermometers. Modern thermometers may also use electrical resistance (resistance thermometers) or thermoelectric effects (thermocouples).

• The two most common temperature scales are the Celsius scale and the Kelvin scale.
• The relation between them is ⇒ T(K) = t(°C) + 273.15

The Kelvin scale is the absolute scale of temperature, where 0 K is absolute zero, the lowest possible temperature, at which the kinetic energy of particles theoretically becomes minimum.

Thermal Expansion

Most materials expand when heated and contract when cooled. This property is called thermal expansion. It arises due to an increase in the average separation between atoms and molecules when the internal energy of the system increases.

Expansion of Solids

• Linear expansion: If the length of a rod increases with temperature, the fractional change in length per degree rise of temperature is called the coefficient of linear expansion (α). ΔL = L₀ α ΔT
• Area expansion: Similarly, the fractional change in area per degree rise in temperature is given by the coefficient of area expansion (β). ΔA = A₀ β ΔT with β ≈ 2α
• Volume expansion: For solids, the fractional change in volume per degree rise in temperature is the coefficient of volume expansion (γ). ΔV = V₀ γ ΔT 

In solids, α, β, and γ are small but characteristic for each material.

Expansion of Liquids

Liquids also expand on heating. Since a liquid is measured in a container, the observed expansion is affected both by the expansion of the liquid itself and by that of the container.

• Apparent expansion is the observed expansion of a liquid in a container.
• Real expansion is the actual expansion of the liquid itself.

The relation is ⇒ γ_real  = γ_apparent + γ_container

Anomalous Expansion of Water

Water behaves abnormally between 0°C and 4°C.

• When heated from 0°C to 4°C → it contracts
• Above 4°C → it expands normally
• Density of water is maximum at 4°C

Importance:

• Ice floats on water.
• Aquatic life survives in winter.

Expansion of Gases

Gases show the maximum expansion on heating. For an ideal gas, the volume expansion is governed by Charles’ Law, which states that at constant pressure, the volume of a gas is directly proportional to its temperature in kelvin.

Specific Heat Capacity

The amount of heat required to raise the temperature of a unit mass of a substance by one degree is called its specific heat capacity (c). Its unit is J kg^–1 K^–1

Q = m cΔT 

• Molar specific heat capacity (C) is the heat required to raise the temperature of one mole of the substance by one degree.
• For gases, we distinguish between two types:
  
  • C_p: Specific heat at constant pressure
  • C_v: Specific heat at constant volume

They are related by Mayer’s relation ⇒ C_p − C_v = R; where R is the universal gas constant.

Note: Water has a very high specific heat, which makes it useful in moderating temperatures in nature

Calorimetry

The study of heat transfer during mixing of substances is known as calorimetry. The principle is based on the law of conservation of energy: heat lost by hot bodies = heat gained by cold bodies (if there is no heat loss to the surroundings).

m₁c₁  (T₁ − T_f) = m₂c₂ (T_f − T₂)

A calorimeter is a device used for measuring heat transfer, typically made of copper with a known water equivalent.

Change of State

Matter exists in three states: solid, liquid, and gas. Heat transfer can cause change from one state to another. The important feature is that during a change of state at constant pressure, the temperature remains constant until the entire substance has changed its phase.

Latent Heat: The amount of heat required to change the state of a unit mass of a substance without change in temperature is called latent heat. Q = mL

• Latent heat of fusion (L_f): heat required to convert unit mass of a solid into liquid at its melting point.
• Latent heat of vaporization (L_v): heat required to convert unit mass of a liquid into vapor at its boiling point.

For example, ice at 0°C requires latent heat of fusion to melt into water at 0°C without rise in temperature

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Heat Transfer Mechanisms

Heat can be transferred by three modes: conduction, convection, and radiation.

Conduction

Conduction occurs mainly in solids due to transfer of energy from particle to particle without actual movement of matter.

Fourier’s law of heat conduction states ⇒ Q = kA (T₁−T₂) / L;  where k is the thermal conductivity of the material.

For example, metals have high conductivity, while insulators like wood and glass have low conductivity. The concept of thermal resistance is used when layers of different materials are involved.

Convection

Convection occurs in fluids (liquids and gases) where heat transfer takes place by actual movement of particles. Warmer, lighter fluid rises while cooler, denser fluid sinks, setting up convection currents.

For example, sea breezes, heating of water in a vessel, and atmospheric circulation.

Radiation

Radiation is the transfer of heat in the form of electromagnetic waves (infrared). It does not require any medium.

• Stefan–Boltzmann Law: The total energy radiated per unit area per unit time by a body is proportional to the fourth power of its absolute temperature. E = σT⁴; where σ is Stefan’s constant.
• Newton’s Law of Cooling: The rate of cooling of a body is proportional to the difference of temperature between the body and its surroundings, provided the difference is small. dT/dt ∝ (T − T_env)
  
• Kirchhoff’s Law: At a given temperature, the emissivity of a body equals its absorptivity for thermal radiation. A perfect blackbody is a perfect absorber and emitter.

Practical Applications

Thermal heat is used in diverse, everyday, and industrial applications, primarily through conduction, convection, and radiation

• The high specific heat of water helps regulate Earth’s climate.
• Conduction explains the use of thermos flasks, cooking utensils with copper bottoms, and insulating materials.
• Convection is applied in household ventilation and oceanic currents.
• Radiation is crucial in solar energy, greenhouse effect, and thermal imaging.

Conclusion

Thermal Properties of Matter connects microscopic molecular motion with macroscopic thermal behavior such as expansion, heat transfer, and phase change.

Understanding concepts like specific heat, latent heat, thermal conductivity, and radiation laws allows students to explain natural phenomena and solve practical numerical problems. This chapter strengthens the conceptual base required for thermodynamics and higher physics studies.

FAQs

Q1. What is calorimetry?

Ans. Calorimetry is the technique used to measure the amount of heat exchanged in physical or chemical processes using a calorimeter.

Q2. What is the principle of calorimetry?

Ans. It is based on the law of conservation of energy: heat lost by a hot body is equal to the heat gained by a cold body, neglecting losses.

Q3. What is latent heat?

Ans. Latent heat is the amount of heat required to change the state of a substance (like melting or vaporization) without a change in temperature.

Q4. What is thermal conduction?

Ans. Thermal conduction is the process of heat transfer through a material without movement of particles, due to temperature difference.

Q5. What is the difference between conduction, convection, and radiation?

Ans. Conduction occurs through direct contact, convection occurs by movement of fluids and radiation transfers heat in the form of electromagnetic waves without a medium.