Bihar Board - Class 11 physics - Chapter 12: Thermodynamics Handwritten Notes

The chapter "Thermodynamics" introduces the principles governing energy transformations in physical systems. Thermodynamics is crucial for understanding processes like heat engines, refrigeration, and various phenomena in physics and chemistry. This chapter covers the fundamental laws of thermodynamics, internal energy, entropy, and practical applications of thermodynamic principles.

Key Points

1. Laws of Thermodynamics

   • Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law defines temperature.

   • First Law of Thermodynamics: It states the principle of conservation of energy, which asserts that energy cannot be created or destroyed, only transferred or converted.

     $$\Delta U = Q - W$$
     

     where $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, and $W$ is the work done by the system.

   • Second Law of Thermodynamics: This law states that the entropy of an isolated system never decreases; it either increases or remains constant. It explains the direction of natural processes and why heat flows from hot to cold bodies.

     • Entropy ($S$) is a measure of the disorder or randomness in a system.

   • Third Law of Thermodynamics: As the temperature of a system approaches absolute zero (0 K), the entropy approaches a minimum constant value.

2. Internal Energy

   • The internal energy ($U$) of a system is the total energy contained within it, including kinetic and potential energies of particles.
   • Work and heat are the two primary ways to change the internal energy of a system.

3. Work and Heat

   • Work ($W$): In thermodynamics, work refers to the energy transferred when a force is applied to move a system (e.g., expansion of a gas).
   • Heat ($Q$): The transfer of energy due to temperature difference between a system and its surroundings.

4. Enthalpy

   • Enthalpy ($H$) is a thermodynamic quantity defined as: $$H = U + pV$$ where $p$ is pressure and $V$ is volume. It represents the total heat content of a system.

5. Reversible and Irreversible Processes

   • Reversible Process: A process that can be reversed without leaving any change in both the system and surroundings.
   • Irreversible Process: A process that cannot return to its original state without any external change.

6. Heat Engines and Efficiency

   • A heat engine is a device that converts heat into work, operating between two temperature reservoirs.
   • Efficiency of a heat engine is the ratio of work done to heat absorbed: $$\eta = \frac{W}{Q_{\text{in}}}$$ where $W$ is the work done by the engine and $Q_{\text{in}}$ is the heat supplied.

7. Carnot Engine

   • A theoretical engine that operates on the Carnot cycle, which is the most efficient cycle possible for a heat engine. The efficiency depends on the temperature of the heat reservoirs: $$\eta = 1 - \frac{T_{\text{cold}}}{T_{\text{hot}}}$$where $T_{\text{cold}}$ and $T_{\text{hot}}$ are the absolute temperatures of the cold and hot reservoirs.

8. Thermodynamic Cycles

   • Carnot Cycle, Otto Cycle, and Rankine Cycle are examples of thermodynamic cycles that describe the processes in heat engines.

9. Applications of Thermodynamics

   • Thermodynamics has a wide range of applications including refrigeration, air conditioning, engines, and even biological processes such as metabolism.

Conclusion

The chapter "Thermodynamics" delves into the fundamental laws governing energy transformation and heat flow. The principles of internal energy, entropy, work, and heat are vital to understanding not only mechanical systems but also natural processes in physics and chemistry.

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