Thermal Physics - Notes for Quick Revision 📚

Thermal physics is a branch of physics that studies heat, temperature, and their effects on matter. It combines concepts from thermodynamics, heat transfer, and kinetic theory, playing a vital role in understanding energy transformations in natural and engineered systems.

This article provides detailed notes on thermal physics, covering definitions, laws, key formulas, and applications.

Introduction to Thermal Physics

Definition: Thermal physics deals with the study of heat energy, temperature, and their effects on matter, including phase changes and the behavior of particles.

Key Concepts:

• Heat: A form of energy transfer due to temperature differences.
• Temperature: A measure of the average kinetic energy of particles in a substance.
• Thermal Equilibrium: When two objects in contact no longer exchange heat energy.

Heat and Temperature

1. Heat ($Q$)

Definition: Energy transferred between objects due to a temperature difference.

Units:

• SI Unit: Joule ($\text{J}$).
• Other Units: Calorie ($\text{cal}$), $1\text{cal}=4.186\text{J}$.

2. Temperature ($T$)

Definition: A measure of the average kinetic energy of particles in a substance.

Scales:

• Celsius ($\mathrm{°}\text{C}$).
• Kelvin ($\text{K}$): $\text{K}=\mathrm{°}\text{C}+273.15$.
• Fahrenheit ($\mathrm{°}\text{F}$): $\mathrm{°}\text{F}=\frac{9}{5}\mathrm{°}\text{C}+32$.

Specific Heat Capacity

Definition: The amount of heat required to raise the temperature of 1 kg of a substance by

• $1 \degree \text{C}$.

Formula: $$Q=mc\mathrm{\Delta }T$$ Where:

• $Q$: Heat energy.
• $m$: Mass of the substance.
• $c$: Specific heat capacity.
• $\mathrm{\Delta }T$: Change in temperature.

Thermodynamics

Zeroth Law of Thermodynamics

Statement: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.

Significance: Defines temperature as a measurable property.

First Law of Thermodynamics

Statement: The change in internal energy (

$\mathrm{\Delta }U$) of a system is equal to the heat added to the system ($Q$) minus the work done by the system ($W$).

Formula:

$$\mathrm{\Delta }U=Q-W$$

Applications:

• Heat engines.
• Refrigerators.

Second Law of Thermodynamics

Statement: Heat energy cannot spontaneously flow from a colder body to a hotter body without external work being done.

Concepts:

• Entropy ($S$): A measure of the disorder in a system.

Third Law of Thermodynamics

Statement: As the temperature of a system approaches absolute zero ($0\text{K}$), the entropy of the system approaches a minimum value.

Heat Transfer

1. Conduction

Definition: Transfer of heat through a solid due to particle collisions.

Formula: $$Q=\frac{kA\mathrm{\Delta }T}{d}t$$ Where:

• $k$: Thermal conductivity.
• $A$: Cross-sectional area.
• $\mathrm{\Delta }T$: Temperature difference.
• $d$: Thickness of the material.
• $t$: Time.

2. Convection

Definition: Transfer of heat through a fluid due to the movement of the fluid itself.

Examples:

• Boiling water.
• Atmospheric circulation.

3. Radiation

Definition: Transfer of heat through electromagnetic waves without requiring a medium.

Formula: $$Q=\sigma A{T}^{4}t$$ Where:

• $\sigma$: Stefan-Boltzmann constant ($5.67\times 10^{-8}{\text{W/m}}^2{\text{K}}^4$).
• $A$: Surface area.
• $T$: Absolute temperature.

Kinetic Theory of Gases

Postulates:

1. Gases consist of a large number of tiny particles in random motion.
2. Collisions between gas particles are elastic.
3. The average kinetic energy of gas particles is proportional to the absolute temperature.

Key Formulas:

Pressure of an Ideal Gas:

$$P=\frac{1}{3}\rho v_{\text{rms}}^2$$

Where:

• $\rho$: Density of the gas.
• $v_{\text{rms}}$: Root mean square velocity.

Kinetic Energy:

$$KE=\frac{3}{2}{k}_{B}T$$

Where:

• $k_B$: Boltzmann constant ($1.38\times 10^{-23}\text{J/K}$).

Ideal Gas Equation:

$$PV=nRT$$

Where:

• $P$: Pressure.
• $V$: Volume.
• $n$: Number of moles.
• $R$: Universal gas constant ($8.31\text{J/mol}\text{cdotp}\text{K}$).
• $T$: Temperature.

Phase Changes

Key Concepts:

Latent Heat ($L$):

• The heat required to change the phase of 1 kg of a substance without a change in temperature.
• Formula: $$Q=mL$$

Types of Latent Heat:

• Latent Heat of Fusion: Solid to liquid.
• Latent Heat of Vaporization: Liquid to gas.

Applications of Thermal Physics

Heat Engines:

• Converts heat energy into mechanical work.
• Efficiency: $$\eta =\frac{W}{Q_{\text{input}}}\times 100\mathrm{\%}$$

Refrigerators:

Transfers heat from a colder to a hotter region using external work.

Weather and Climate:

Convection and radiation explain atmospheric phenomena like winds and greenhouse effects.

Material Science:

Understanding thermal expansion and conductivity aids in designing buildings and machinery.

FAQs About Thermal Physics

What is the difference between heat and temperature?

Heat is the energy transfer due to temperature difference, while temperature measures the average kinetic energy of particles.

Why is specific heat capacity important?

Specific heat capacity determines how much heat a material can store, influencing temperature changes.

What is the significance of entropy?

Entropy measures the disorder in a system and determines the feasibility of thermodynamic processes.

How does radiation differ from conduction and convection?

Radiation transfers heat through electromagnetic waves, requiring no medium, while conduction and convection need a medium.

Thermal physics is a cornerstone of physics that helps us understand energy transformations and the behavior of matter under varying thermal conditions. From thermodynamics to heat transfer, this topic is fundamental for solving practical problems in engineering, meteorology, and material science. These comprehensive notes provide an excellent resource for mastering the concepts and their real-world applications.

Explore related topics:

• Calorimetry
• Heat Transfer
• Kinetic Theory of Gases