Lecture on the First Law of Thermodynamics

Key Concepts

• First Law of Thermodynamics: Known as the law of energy conservation.
  • Relationship between internal energy (ΔU), work (W), and heat (Q).
  • Expressed by the equation: ΔU = Q - W
  • All quantities measured in joules.

Types of Processes

1. Isovolumetric Process

   • No change in volume, thus W = 0.
   • ΔU = Q
   • Internal energy changes due to heat transfer only.
   • Example: Bomb calorimeter.

2. Isothermal Process

   • No change in temperature, ΔU = 0.
   • Q = W
   • Heat transferred is used for work, not changing internal energy.
   • Example: Ideal car engine.

3. Adiabatic Process

   • No heat transfer, Q = 0.
   • ΔU = -W
   • Internal energy changes due to work done on/by the system.
   • Example: Atmospheric processes with air masses.

4. Isolated System

   • No heat transfer or work, Q = 0 and W = 0.
   • No change in internal energy.

Sign Conventions

• Heat (Q)
  • Positive: Heat absorbed by the system.
  • Negative: Heat lost by the system.
• Work (W)
  • Positive: Work done by the system.
  • Negative: Work done on the system.

Example Calculation

• Problem: 100 joules of compression work done on a system, internal energy increases by 74 joules.
• Solution: Rearrange equation to solve for Q: Q = ΔU + W.
  • ΔU = +74 joules (internal energy increases).
  • W = -100 joules (work done on the system).
  • Result: Q = -26 joules.
  • Interpretation: 26 joules of energy dissipate as heat.

Important Notes

• Correctly assign signs to Q and W to avoid calculation errors.
• Regular practice with these scenarios is crucial for mastering thermodynamics calculations.