Lecture Notes: First Law of Thermodynamics

Introduction

• Key Concept: Energy cannot be created or destroyed; it is transferred from one place to another.
• System and Surroundings:
  • System: The area of focus.
  • Surroundings: Everything outside the system.
  • Energy can flow between system and surroundings via two methods: heat and work.

Internal Energy

• Internal Energy (U): It is the energy contained within the system.
• Energy transfer can occur through:
  • Heat (Q): Heat added to the system increases its internal energy.
  • Work (W): Work done on the system increases its internal energy.

Energy Transfer Example

• Joules Transfer: If the surroundings do 300 Joules of work on the system:
  • Internal energy of the system increases by 300 Joules (ΔU is positive).
  • Energy of the surroundings decreases by 300 Joules.

Money Analogy

• Similar to Money Transfer:
  • Gaining money means someone else loses it, illustrating that money (like energy) is transferred, not created or destroyed.

Types of Systems

• Open System:
  • Matter and energy can enter and leave the system.
• Closed System:
  • Only energy can enter or leave; matter cannot.
• Isolated System:
  • Neither matter nor energy can enter or leave; both are fixed.

First Law of Thermodynamics

• Equation: ΔU = Q + W
  • Q (Heat): Positive when heat is absorbed (endothermic), negative when released (exothermic).
  • W (Work): Negative when done by the system, positive when done on the system.

Chemistry vs. Physics Perspectives

• Chemistry Viewpoint:
  • Focus on the system.
  • W is negative when the system does work (internal energy decreases).
• Physics Viewpoint:
  • Focus on the surroundings.
  • W is positive when the system does work (surroundings gain energy).

Sign Conventions

• Q (Heat Transfer):
  • Positive: System absorbs heat (endothermic).
  • Negative: System releases heat (exothermic).
• W (Work Done):
  • Positive: Work is done on the system.
  • Negative: Work is done by the system.

Conclusion

• Upcoming content: Practice problems will be provided in a future video to calculate changes in internal energy using heat and work.