Lesson Overview

Unit 4 covers temperature, thermometers, thermal expansion, gas laws, kinetic theory, specific heat capacity, latent heat, humidity, and the laws of thermodynamics. This unit forms the basis for understanding how heat flows, how matter behaves with temperature changes, and the principles governing thermal processes.

1. Core Concepts (Short Notes)

4.1 Temperature & Thermometers

• Temperature: Degree of hotness or coldness.

• Thermometers depend on physical properties that vary with temperature.

• Fixed points: Ice point (0°C) and steam point (100°C).

• Celsius, Kelvin scales.

4.2 Thermal Expansion

• Solids, liquids expand on heating.

• Types: Linear, area, and volume expansion.

• Water has anomalous expansion (4°C → maximum density).

4.3 Gas Laws & Ideal Gas Equation

• Boyle’s law: P ∝ 1/V.

• Charles’ law: V ∝ T.

• Pressure law: P ∝ T.

• Combined gas law: PV/T = constant.

• Ideal gas equation: PV = nRT.

4.4 Kinetic Theory of Gases

• Gas molecules in constant random motion.

• Pressure due to collisions with walls.

• Temperature proportional to average kinetic energy.

4.5 Specific Heat Capacity & Cooling

• Specific heat capacity: Heat to raise 1 kg by 1°C.

• Newton’s law of cooling: Rate of cooling ∝ (Temperature difference).

4.6 Latent Heat & Phase Changes

• Latent heat: Heat absorbed/released without temperature change.

• Fusion, vaporization.

• Heating/cooling curves.

4.7 Humidity & Vapour Pressure

• Humidity: Water vapour in air.

• Dew point: Temperature at which condensation starts.

• Relative humidity (%) = (Actual vapour pressure / Saturated vapour pressure) × 100.

4.8 Thermodynamic Processes

• Isothermal: T constant.

• Adiabatic: No heat exchange.

• Isochoric: V constant.

• Isobaric: P constant.

4.9 Heat Transfer

• Conduction

• Convection

• Radiation

2. Detailed Notes for Each Section

4.1 Temperature & Thermometers

Temperature Scales

• Celsius → water-based scale.

• Kelvin → absolute scale starting at absolute zero.

• Conversion: T(K) = T(°C) + 273.

Thermometric Properties

• Volume of gas

• Electrical resistance

• Pressure of gas

Types of Thermometers

• Mercury/alcohol thermometer

• Resistance thermometer

• Thermocouple

• Constant-volume gas thermometer

4.2 Thermal Expansion

Linear Expansion

ΔL = αL₀ΔT

Area Expansion

ΔA = βA₀ΔT (β = 2α)

Volume Expansion

ΔV = γV₀ΔT (γ = 3α)

Applications & Problems

• Railway tracks (gaps provided)

• Bimetallic strips

• Thermometers

Anomalous Expansion of Water

• Water contracts from 0°C to 4°C.

• Expands beyond 4°C.

• Density maximum at 4°C.

4.3 Gas Laws

Boyle’s Law

Charles’ Law

Pressure Law

Ideal Gas Equation

• R = 8.31 J/mol·K

4.4 Kinetic Theory of Gases

Key Ideas

• Molecules are in constant motion.

• Collisions are elastic.

• Pressure arises from collision forces.

• Temperature ∝ average kinetic energy (½mv²).

4.5 Specific Heat Capacity & Cooling

Specific Heat Capacity

Newton’s Law of Cooling

Applications

• Hot beverages cool faster in windy conditions.

• Black surfaces emit more radiation → faster cooling.

4.6 Latent Heat & Phase Changes

Latent Heat of Fusion/Vaporization

Heating Curves

• Temperature constant during phase change.

• Energy used to break/form bonds.

4.7 Humidity & Vapour Pressure

Important Definitions

• Saturated vapour pressure: Max pressure at given temperature.

• Relative Humidity %: (Actual/Saturated vapour pressure) × 100.

• Dew point: When RH = 100%.

Effects of Humidity

• High humidity slows evaporation.

• Comfort decreases.

4.8 Thermodynamic Processes

Processes

• Isothermal: PV = constant.

• Adiabatic: PV^γ = constant.

• Isobaric: P = constant.

• Isochoric: V = constant.

Graphs

• PV diagrams for each process.

4.9 Heat Transfer

Conduction

Q/t = kAΔT/L

Convection

• Movement of fluids.

• Natural & forced convection.

Radiation

• Emission of electromagnetic waves.

• Dark & rough surfaces absorb/emit more.

3. Formula Summary for Unit 4

• ΔL = αL₀ΔT

• ΔV = γV₀ΔT

• PV = nRT

• c = Q/mΔT

• Q = mL

• Relative humidity = (Actual/Saturated) × 100

• Newton’s law: Rate ∝ (T – Tₛ)

• Conduction: Q/t = kAΔT/L

4. Common Mistakes to Avoid

• Mixing Celsius and Kelvin without conversion.

• Using wrong expansion coefficient.

• Not converting volume/pressure units.

• Forgetting that phase changes occur at constant temperature.

5. Exam Tips

• Always convert to Kelvin in gas laws.

• Draw neat heating/cooling curves.

• Check if problem involves expansion of solids or liquids.

• State assumptions clearly in thermodynamic processes.

• Label axes correctly in PV diagrams.

6. Quick Revision Table