First law of thermodynamics – what principle does it embody? Which conservation principle is expressed by the first law of thermodynamics for closed and open systems?

Introduction / Context:
The first law of thermodynamics is the formal energy accounting rule for systems undergoing work and heat interactions. It generalizes the concept that energy can change form but the total amount is conserved.

Given Data / Assumptions:

• Closed systems: ΔU = Q − W (sign convention: heat into the system positive, work by the system positive).
• Open systems (control volumes): steady-flow energy equation includes enthalpy, kinetic, and potential terms.
• No creation or destruction of energy occurs.

Concept / Approach:
The first law is a statement of conservation of energy. For closed systems it relates heat and work to changes in internal energy. For flowing systems it tracks energy transport with mass and across the boundary as shaft work and heat.

Step-by-Step Solution:
Identify energy forms: internal, kinetic, potential, flow work, and shaft work.Apply the first law: total energy entering − leaving equals the change in stored energy.Conclude that energy is conserved though converted among forms; it is never created or destroyed.

Verification / Alternative check:
Engineering energy balances (e.g., turbines, compressors, heat exchangers) directly implement the first law and predict performance consistent with energy conservation.

Why Other Options Are Wrong:

• Conservation of mass and momentum are separate laws (continuity; Newton’s 2nd law control-volume form).
• “Conservation of heat” is incorrect; heat is energy in transit, not a conserved property.
• Entropy is not conserved; it is generated in irreversible processes (second law).

Common Pitfalls:
Confusing heat with internal energy; heat is a mode of transfer, while internal energy is a state property.

Final Answer:
conservation of energy