Thermal Properties of Matter Class 11 Notes: Key Concepts & Formulas

Key Terms and Definitions

Temperature

A measure of the degree of hotness or coldness of a body. It determines the direction of heat flow between two bodies in thermal contact. SI unit: Kelvin (K).

Heat

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

Thermal Expansion

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

Specific Heat Capacity (s or c)

The amount of heat required to raise the temperature of a unit mass of a substance by one degree. Formula: s = (1/m) * (ΔQ/ΔT). Unit: J kg⁻¹ K⁻¹.

Latent Heat (L)

The heat energy absorbed or released by a substance during a change of state (like melting or boiling) at a constant temperature. Formula: Q = mL. Unit: J kg⁻¹.

Conduction

The mode of heat transfer through a substance from a region of higher temperature to a region of lower temperature without any actual movement of the particles.

Convection

The mode of heat transfer by the actual bulk movement of matter. It occurs only in fluids (liquids and gases).

Radiation

The mode of heat transfer through electromagnetic waves. It does not require a medium and can occur through a vacuum.

Black Body

An idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. It is also a perfect emitter of thermal radiation.

Must-Remember Formulas

• {"point":"Temperature Conversion","description":"Relation between Celsius (C), Fahrenheit (F), and Kelvin (K): C/5 = (F-32)/9 = (K-273.15)/5. Note: ΔT in °C = ΔT in K."}
• {"point":"Linear Expansion","description":"Change in length: ΔL = αL₀ΔT, where α is the coefficient of linear expansion."}
• {"point":"Area & Volume Expansion","description":"ΔA = βA₀ΔT and ΔV = γV₀ΔT. For isotropic solids, β ≈ 2α and γ ≈ 3α."}
• {"point":"Heat Transfer (Calorimetry)","description":"Heat required to change temperature: Q = msΔT, where 's' is the specific heat capacity."}
• {"point":"Change of State","description":"Heat required for phase change: Q = mL, where 'L' is the latent heat (Lf for fusion, Lv for vaporization)."}
• {"point":"Principle of Calorimetry","description":"When two bodies at different temperatures are mixed, Heat Lost by hot body = Heat Gained by cold body (assuming no heat loss to surroundings)."}
• {"point":"Conduction (Fourier's Law)","description":"Rate of heat flow: dQ/dt = H = kA(T₁-T₂)/d, where 'k' is thermal conductivity and 'd' is the thickness."}
• {"point":"Newton's Law of Cooling","description":"The rate of loss of heat of a body is directly proportional to the temperature difference between the body and its surroundings: dQ/dt ∝ (T - T_s)."}
• {"point":"Stefan-Boltzmann Law","description":"For a perfect black body, the energy radiated per unit area per second is E = σT⁴, where σ is the Stefan-Boltzmann constant (5.67 x 10⁻⁸ W m⁻² K⁻⁴)."}
• {"point":"Wien's Displacement Law","description":"The wavelength (λ_m) at which a black body radiates most strongly is inversely proportional to its absolute temperature: λ_m T = b (Wien's constant)."}

Modes of Heat Transfer Explained

Heat is energy in transit, and it always flows from a region of higher temperature to one of lower temperature. This transfer occurs through three distinct mechanisms: conduction, convection, and radiation. Understanding the differences is key to solving problems.

Conduction is the transfer of heat through stationary matter by physical contact. In solids, this happens when atoms vibrate and collide with their neighbors, passing thermal energy along. In metals, which are excellent conductors, heat is also transferred by the movement of free electrons, making the process much faster. This is why a metal spoon in hot soup quickly becomes hot to the touch. The rate of conduction is governed by Fourier's Law, which depends on the material's thermal conductivity (k), the cross-sectional area (A), the temperature gradient (ΔT/Δx).

Convection involves the transfer of heat by the bulk movement of fluids (liquids or gases). When a part of a fluid is heated, it expands, becomes less dense, and rises. The cooler, denser fluid sinks to take its place, gets heated, and rises in turn. This creates convection currents that distribute heat throughout the fluid. This is how water boils in a pot and how rooms are heated by radiators. Convection can be natural (due to density differences) or forced (using a fan or pump).

Radiation is the transfer of heat in the form of electromagnetic waves, primarily in the infrared spectrum. Unlike conduction and convection, radiation does not require a medium and can travel through the vacuum of space. This is how we feel the heat from the Sun, a bonfire, or an incandescent bulb. All objects with a temperature above absolute zero (0 K) radiate thermal energy. The rate of radiation is described by the Stefan-Boltzmann Law, which states that the energy radiated is proportional to the fourth power of the absolute temperature (T⁴).

Conduction vs. Convection vs. Radiation

Worked Mini-Examples

• {"title":"Calculating Heat Energy","description":"How much heat is required to raise the temperature of 5 kg of iron from 20°C to 120°C? (Specific heat of iron, s = 450 J kg⁻¹ K⁻¹)\n\nSolution:\n- We use the formula Q = msΔT.\n- m = 5 kg\n- s = 450 J kg⁻¹ K⁻¹\n- ΔT = T_final - T_initial = 120°C - 20°C = 100°C = 100 K\n- Q = 5 kg × 450 J kg⁻¹ K⁻¹ × 100 K\n- Q = 225,000 J or 225 kJ"}
• {"title":"Calculating Thermal Expansion","description":"An aluminum rod is 20 m long at 10°C. What is its length at 100°C? (α for aluminum = 23 × 10⁻⁶ °C⁻¹)\n\nSolution:\n- We use ΔL = αL₀ΔT.\n- L₀ = 20 m\n- α = 23 × 10⁻⁶ °C⁻¹\n- ΔT = 100°C - 10°C = 90°C\n- ΔL = (23 × 10⁻⁶) × 20 × 90 = 0.0414 m\n- Final Length L = L₀ + ΔL = 20 m + 0.0414 m = 20.0414 m"}

Exam Traps and Scoring Tips

A common trap is confusing temperature difference (ΔT) with absolute temperature (T). For formulas like Q = msΔT and ΔL = αL₀ΔT, a temperature difference in Celsius is the same as in Kelvin (e.g., a change of 10°C is a change of 10 K). However, for radiation laws like the Stefan-Boltzmann Law (E = σT⁴), you MUST convert the temperature to Kelvin (K = °C + 273.15) before applying the formula, as it uses the absolute temperature raised to a power. Forgetting this conversion is a frequent source of error. Always check your units! Ensure all quantities are in SI units (Joules, kilograms, meters, Kelvin) before calculation unless the question specifies otherwise.

Quick Revision Check

• Q: Why are cooking pots made of metal but have handles made of wood or plastic? A: Metals are good conductors of heat (high thermal conductivity), which allows for efficient transfer of heat to the food. Wood and plastic are poor conductors (insulators), which prevents heat from reaching the hand holding the handle.
• Q: What is the significance of the anomalous expansion of water? A: Water has its maximum density at 4°C. As it cools from 4°C to 0°C, it expands. This means ice is less dense than water, causing it to float. This insulates the water below, allowing aquatic life to survive in frozen lakes and ponds.
• Q: Why do we wear light-coloured clothes in summer and dark-coloured clothes in winter? A: Light-coloured surfaces are poor absorbers and good reflectors of heat radiation. Dark-coloured surfaces are good absorbers of heat. Thus, light clothes keep us cool in summer by reflecting sunlight, while dark clothes keep us warm in winter by absorbing it.
• Q: On a cold day, why does a metal bench feel colder to touch than a wooden bench at the same temperature? A: Both benches are at the same ambient temperature. However, metal is a much better conductor of heat than wood. When you touch the metal bench, it conducts heat away from your hand much faster than the wooden bench does, creating the sensation of being colder.

Frequently Asked Questions

Frequently Asked Questions