First law of thermodynamics / internal energy | Thermodynamics | Physics | Khan Academy

Introduction

This tutorial will guide you through the First Law of Thermodynamics and the concept of internal energy, as explained in the Khan Academy video. Understanding these principles is essential for grasping how energy is conserved and transformed in physical systems.

Step 1: Understand the First Law of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle can be summarized with the equation:

• ΔU = Q - W

Where:

• ΔU = Change in internal energy
• Q = Heat added to the system
• W = Work done by the system

Practical Tips

• Recognize that if heat is added to a system (Q > 0), the internal energy increases.
• Conversely, if work is done by the system (W > 0), internal energy decreases.

Step 2: Define Internal Energy

Internal energy is the total energy contained within a system, including kinetic energy of molecules and potential energy from molecular interactions.

Key Points

• It is a state function, meaning it depends only on the current state of the system, not how it got there.
• Changes in internal energy can occur due to heat transfer or work being done.

Step 3: Explore Heat Transfer and Work

Heat transfer and work are two critical processes that influence a system's internal energy.

Heat Transfer

• Heat (Q) can be transferred to or from a system, affecting its internal energy.
• There are three methods of heat transfer:
  • Conduction
  • Convection
  • Radiation

Work Done

• Work (W) can be done on a system or by a system.
• Common forms of work include:
  • Mechanical work (e.g., expansion or compression of gases)
  • Electrical work (e.g., moving charges)

Step 4: Apply the Concepts

To apply the First Law of Thermodynamics in real-world scenarios, consider the following examples:

• Heating a Gas: When a gas is heated, it absorbs heat energy (Q > 0), which increases its internal energy (ΔU > 0). If the gas expands, it does work on its surroundings (W > 0), potentially offsetting some of that energy gain.
• Cooling a System: If a system loses heat (Q < 0) and does no work (W = 0), its internal energy decreases (ΔU < 0).

Conclusion

The First Law of Thermodynamics is foundational for understanding energy conservation in physical systems. By grasping the relationship between internal energy, heat transfer, and work, you can better analyze various thermodynamic processes. As a next step, consider exploring more complex systems or the implications of these laws in real-world applications. For further learning, check out additional lessons on thermodynamics offered by Khan Academy.