Thermal Properties of Matter: CBSE Class 11 Physics Chapter 11 Notes

Key Definitions for Thermal Properties of Matter

Temperature

A measure of the degree of hotness or coldness of a body. It determines the direction of heat flow. SI unit: Kelvin (K).

Heat

The form of energy that is transferred between systems or objects with different temperatures. SI unit: Joule (J).

Specific Heat Capacity (c)

The amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius or one Kelvin. Formula: c = Q / (mΔT).

Latent Heat (L)

The heat energy absorbed or released by a substance during a change of state (e.g., solid to liquid or liquid to gas) at a constant temperature. Formula: Q = mL.

Thermal Expansion

The tendency of matter to change its shape, area, and volume in response to a change in temperature.

Coefficient of Linear Expansion (α)

The fractional change in length per unit change in temperature. Formula: α = (ΔL/L) / ΔT.

Thermal Conductivity (K)

A measure of a material's ability to conduct heat. Materials with high K are good conductors, while those with low K are good insulators.

Black Body

An idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.

Wien's Displacement Law

States that the wavelength (λ_m) corresponding to the maximum energy emission of a black body is inversely proportional to its absolute temperature (T). Formula: λ_m * T = b (Wien's constant).

Key Formulas & Must-Remember Points

• Temperature Scales: Relation between Celsius (T_C), Fahrenheit (T_F), and Kelvin (T_K) is T_K = T_C + 273.15 and (T_F - 32)/9 = T_C/5.
• Thermal Expansion: Linear: ΔL = αL₀ΔT; Areal: ΔA = βA₀ΔT; Volumetric: ΔV = γV₀ΔT.
• Relation between Expansion Coefficients: For isotropic solids, β ≈ 2α and γ ≈ 3α.
• Anomalous Expansion of Water: Water contracts on heating from 0°C to 4°C. It has its maximum density at 4°C.
• Heat Capacity (S) and Specific Heat (c): S = mc. Heat capacity is for an object, specific heat is for a substance.
• Principle of Calorimetry: For an isolated system, Heat Lost by hot body = Heat Gained by cold body.
• Heat Transfer during Temperature Change: Q = mcΔT (where ΔT is the change in temperature).
• Heat Transfer during Phase Change: Q = mL (where L is the latent heat of fusion or vaporization).
• Heat Conduction (Fourier's Law): Rate of heat flow, H = dQ/dt = KA(T₁ - T₂)/L.
• Newton's Law of Cooling: Rate of cooling dQ/dt is proportional to the temperature difference (T - T_s) between the body and its surroundings. dQ/dt = -k(T - T_s).
• Stefan-Boltzmann Law: The total energy radiated per unit surface area of a black body per unit time is directly proportional to the fourth power of its absolute temperature. E = σT⁴.
• Wien's Displacement Law: λ_m * T = b, where b is Wien's constant (2.898 × 10⁻³ m·K).

Modes of Heat Transfer

Heat transfer is the process by which thermal energy moves from a hotter object to a colder one. There are three distinct modes through which this can occur: conduction, convection, and radiation.

1. Conduction: This is the primary mode of heat transfer in solids. It occurs when heat is transferred through a medium from one particle to the next without any actual movement of the particles from their mean positions. In metals, which are excellent conductors, this transfer is facilitated by the movement of free electrons. In insulators like wood or glass, the transfer happens much slower, through vibrations passed along from atom to atom. The rate of conduction is governed by Fourier's Law of Heat Conduction, which states that the rate of heat flow is proportional to the area and the temperature gradient.

2. Convection: This mode is characteristic of fluids (liquids and gases). Convection involves the transfer of heat by the bulk movement of the fluid itself. When a part of the fluid is heated, it becomes less dense and rises. The cooler, denser fluid from the top sinks to take its place, gets heated, and rises. This creates a continuous circulation pattern called a convection current. Examples include boiling water, sea breezes, and land breezes.

3. Radiation: Unlike conduction and convection, radiation does not require a medium for heat transfer. Heat is transferred in the form of electromagnetic waves. All objects with a temperature above absolute zero (0 K) radiate thermal energy. The heat we feel from the Sun is a prime example of radiation, as it travels through the vacuum of space. The rate of energy radiation is described by the Stefan-Boltzmann Law.

Conduction vs. Convection vs. Radiation

Worked Example: Calorimetry Problem

• {"title":"Calculating Heat for Temperature and Phase Change","bodyMarkdown":"Problem: Calculate the total heat required to convert 100 g (0.1 kg) of ice at -10°C to water at 20°C.\n\nGiven:\n Specific heat of ice (c_ice) = 2100 J kg⁻¹ K⁻¹\n Latent heat of fusion of ice (L_f) = 3.34 × 10⁵ J kg⁻¹\n Specific heat of water (c_water) = 4200 J kg⁻¹ K⁻¹\n\nSolution:\nThe process involves three steps:\n1. Heating ice from -10°C to 0°C:\n Q₁ = m c_ice ΔT = 0.1 kg 2100 J kg⁻¹ K⁻¹ (0 - (-10)) K = 2100 J\n\n2. Melting ice at 0°C to water at 0°C:\n Q₂ = m L_f = 0.1 kg 3.34 × 10⁵ J kg⁻¹ = 33400 J\n\n3. Heating water from 0°C to 20°C:\n Q₃ = m c_water ΔT = 0.1 kg 4200 J kg⁻¹ K⁻¹ * (20 - 0) K = 8400 J\n\nTotal Heat Required (Q_total):\nQ_total = Q₁ + Q₂ + Q₃ = 2100 J + 33400 J + 8400 J = 43900 J or 43.9 kJ"}

Exam Traps and Scoring Tips

Unit Consistency is Key!

• Always check units. If Specific Heat is in J/kg·K, mass must be in kg. If it's in J/g·°C, mass must be in g.
• For laws like Stefan-Boltzmann (E = σT⁴) and Wien's Law (λ_m T = b), the temperature must be in Kelvin (K). T(K) = T(°C) + 273.15.
• However, for formulas involving temperature change (ΔT), the value is the same in Celsius and Kelvin (e.g., ΔT of 10°C = ΔT of 10 K). So, for Q = mcΔT, you don't need to convert if specific heat is also given per °C.
• Don't confuse Specific Heat (Q=mcΔT) with Latent Heat (Q=mL). Specific heat is for temperature change, latent heat is for phase change at a constant temperature. Problems often combine both.

Practice Questions with Solutions

• Why does a metal chair feel colder than a wooden chair at the same room temperature? Metal has a much higher thermal conductivity than wood. It conducts heat away from your body much faster, making it feel colder, even though both are at the same temperature.
• What is the physical significance of Wien's Displacement Law? It tells us that hotter objects emit radiation at shorter wavelengths (appearing bluer), while cooler objects emit at longer wavelengths (appearing redder). This is why a heated iron rod first glows red, then orange, then yellowish-white as its temperature increases.
• State the principle of calorimetry. In a thermally isolated system, the total heat lost by the hotter object(s) is equal to the total heat gained by the colder object(s) until thermal equilibrium is reached.
• Why are two thin blankets generally warmer than a single thick blanket of the same total thickness? The layer of air trapped between the two thin blankets acts as an excellent insulator. Air has very low thermal conductivity, which significantly reduces heat loss by conduction.

Frequently Asked Questions

Frequently Asked Questions