Lecture Notes: First Law of Thermodynamics

Key Concepts

• First Law of Thermodynamics: Energy cannot be created or destroyed; it is transferred from one place to another.
  • Energy flows into/out of a system via heat and work.
  • Internal energy (symbol U) of a system changes when heat is added or when work is done on the system.

Energy Transfer

• Heat (Q): Energy transfer due to temperature difference.
• Work (W): Energy transfer when force is applied over a distance.

Internal Energy (U)

• Change in internal energy (∆U) is given by the equation:
  • Chemistry Perspective: ∆U = Q + W
  • Physics Perspective: ∆U = Q - W
• Sign conventions:
  • Q is positive when the system absorbs heat (endothermic processes).
  • Q is negative when the system releases heat (exothermic processes).
  • W is positive when work is done on the system (in chemistry).
  • W is negative when work is done by the system (in chemistry).

Types of Systems

• Open System: Both matter and energy can enter and leave.
• Closed System: Only energy can enter and leave; matter is contained.
• Isolated System: Neither matter nor energy can enter or leave.

Sign Conventions in Different Disciplines

• Chemistry:
  • Focuses on the system’s perspective.
  • Work done by the system results in a decrease in internal energy (W is negative).
  • Work done on the system increases internal energy (W is positive).
• Physics:
  • Focuses on the surroundings’ perspective.
  • Work done by the system is positive as the surroundings gain energy.
  • Work done on the system is negative as the surroundings lose energy.

Practical Examples

• Money analogy:
  • Like energy, money is transferred from one account (system) to another (surroundings).
  • Gain in one account means a loss in another.

Recap of Processes

• Endothermic: System absorbs heat, Q is positive.
• Exothermic: System releases heat, Q is negative.

Upcoming Content

• Practice problems on calculating change in internal energy using heat and work will be provided in another video.