First Law of Thermodynamics

Why it matters

The First Law of Thermodynamics is crucial for understanding energy conservation in engineering systems. It helps engineers design efficient engines, refrigerators, and other systems by accounting for energy input, output, and losses.

Key ideas

• Energy Conservation: The First Law states that energy cannot be created or destroyed, only transformed from one form to another.
• Internal Energy: The microscopic energy of the system, excluding the kinetic energy of its bulk motion and macroscopic potential energy.
• Work and Heat: Energy can be transferred into or out of a system as work or heat.
• Closed System: A system where mass remains constant, but energy can be exchanged with the surroundings.
• Open System: A system where both mass and energy can cross the boundary.

For a closed system the general balance is ΔU + ΔKE + ΔPE = Q - W, with Q into and W out of the system positive. The reduced relation below assumes negligible changes in macroscopic kinetic and potential energy. Heat and work are path-dependent transfers, while internal energy is a state property. For open systems include energy carried by flowing mass.

Formulas

• ΔU = Q - W
  • ΔU: Change in internal energy (Joules)
  • Q: Heat added to the system (Joules)
  • W: Work done by the system (Joules)

Worked example

Given: A closed system receives 500 J of heat and does 200 J of work, with no change in bulk kinetic or potential energy.

1. Identify the known values:

   • Q = 500 J
   • W = 200 J

2. Apply the First Law of Thermodynamics:

   • ΔU = Q - W

3. Substitute the known values:

   • ΔU = 500 J - 200 J

4. Calculate the change in internal energy:

   • ΔU = 300 J

Final Answer: 300 J

Common mistakes

• Confusing the signs of Q and W: Remember that heat added to the system is positive, and work done by the system is positive.
• Forgetting to convert units to SI units.
• Misidentifying the type of system (closed vs. open).

For GATE ME

Questions often involve calculating changes in internal energy, work done, or heat transfer in closed systems. Practice problems involving different types of processes (isothermal, adiabatic, etc.) to strengthen understanding.

Quick check

1. What does the First Law of Thermodynamics state?
2. How is work defined in a thermodynamic system?
3. What is the unit of internal energy?

Answers: 1. Energy conservation; 2. Energy transfer due to force; 3. Joules.