Chapter 8 – Thermodynamics (JEE Physics)
1. What is Thermodynamics?
Thermodynamics is the branch of physics that deals with the relationship between
heat, work, temperature and internal energy of a system.
Thermodynamics does NOT deal with time or rate of process.
It only studies initial and final states.
2. Thermodynamic System
A system is a part of the universe under study.
- Open system – exchange of mass and energy
- Closed system – exchange of energy only
- Isolated system – no exchange of mass or energy
3. Thermodynamic Variables
Variables that define the state of a system:
- Pressure $(P)$
- Volume $(V)$
- Temperature $(T)$
- Internal energy $(U)$
4. Equation of State
For an ideal gas:
$$PV = nRT$$
- $P$ = pressure
- $V$ = volume
- $T$ = temperature
- $n$ = number of moles
- $R$ = gas constant
5. Zeroth Law of Thermodynamics
If two systems are in thermal equilibrium with a third system,
they are in thermal equilibrium with each other.
This law defines the concept of temperature.
6. Heat and Work
- Heat – energy transfer due to temperature difference
- Work – energy transfer due to mechanical interaction
Work done by gas:
$$W = \int P\,dV$$
7. First Law of Thermodynamics
$$\Delta Q = \Delta U + W$$
- $\Delta Q$ = heat supplied
- $\Delta U$ = change in internal energy
- $W$ = work done by system
First law is the law of conservation of energy.
8. Internal Energy of Ideal Gas
Internal energy of an ideal gas depends only on temperature.
$$\Delta U = nC_v\Delta T$$
9. Isothermal Process
Temperature remains constant.
$$PV = \text{constant}$$
$$W = nRT\ln\left(\frac{V_2}{V_1}\right)$$
For isothermal process:
$$\Delta U = 0 \Rightarrow Q = W$$
10. Adiabatic Process
No heat exchange with surroundings.
$$PV^\gamma = \text{constant}$$
$$TV^{\gamma-1} = \text{constant}$$
$\gamma = \dfrac{C_p}{C_v}$
11. Isochoric Process
Volume remains constant.
$$W = 0 \Rightarrow Q = \Delta U$$
12. Isobaric Process
Pressure remains constant.
$$W = P(V_2 - V_1)$$
$$Q = nC_p\Delta T$$
13. Specific Heat Capacities
$$C_p - C_v = R$$
This relation is valid only for ideal gases.
14. Second Law of Thermodynamics
Heat cannot flow from a colder body to a hotter body without external work.
This law introduces the concept of direction of processes.
15. Heat Engine
A heat engine converts heat into work.
Efficiency:
$$\eta = \frac{W}{Q_h} = 1 - \frac{Q_c}{Q_h}$$
16. Carnot Engine
Most efficient ideal heat engine operating between two temperatures.
$$\eta_{max} = 1 - \frac{T_c}{T_h}$$
17. Refrigerator
Coefficient of performance:
$$COP = \frac{Q_c}{W}$$
18. Entropy
Entropy is a measure of disorder.
$$\Delta S = \frac{Q_{rev}}{T}$$
19. Entropy Change in Isothermal Process
$$\Delta S = nR\ln\left(\frac{V_2}{V_1}\right)$$
20. Important JEE Points & Traps
- First law does not give direction of process
- Adiabatic curve is steeper than isothermal
- Entropy of isolated system always increases
- Carnot efficiency depends only on temperatures
21. Final Revision Checklist
You have mastered Thermodynamics if you can:
- Apply first law correctly in all processes
- Differentiate between all thermodynamic processes
- Solve efficiency and COP problems
- Handle entropy-based questions
- Interpret PV diagrams