Section 2.4: Laws of Thermodynamics
The laws of thermodynamics govern the behavior of energy and heat in physical systems. They describe constraints on energy transfer and provide the foundation for understanding engines, refrigerators, and natural processes.
- First Law (Conservation of Energy): Energy cannot be created or destroyed, only transformed. For a system: \[ \Delta U = Q - W \] where \( \Delta U \) = change in internal energy, \( Q \) = heat added to the system, \( W \) = work done by the system.
- Second Law: Entropy of an isolated system never decreases. Heat cannot spontaneously flow from a colder to a hotter body. - Introduces the concept of entropy \( S \).
- Third Law: As temperature approaches absolute zero, the entropy of a perfect crystal approaches zero: \[ \lim_{T \to 0} S = 0 \]
Example: Heat and Work
A gas absorbs 500 J of heat and does 200 J of work. Determine the change in internal energy.
Using the first law: \( \Delta U = Q - W = 500 - 200 = 300 \, \text{J} \)
Practice Problems
- A system receives 800 J of heat and loses 350 J as work. Calculate its change in internal energy.
- Explain why a perpetual motion machine of the second kind is impossible.
- State the significance of entropy in the second law.
- What happens to the entropy of a perfect crystal as it approaches absolute zero?
- Give a real-life example illustrating the first law of thermodynamics.