First Law of Thermodynamics MCQ – Objective Questions with Answers
Mechanical Engineering › Thermodynamics | Free practice MCQs with detailed explanations
Last Updated: June 2026
📌 About this MCQ Set
The First Law of Thermodynamics applies conservation of energy to a thermodynamic system: energy is neither created nor destroyed, only transferred or converted. For a closed system, Q = ΔU + W.
These MCQs cover the sign convention, cyclic and non-flow processes, the steady flow energy equation, and the conceptual traps students fall into.
10 questions • every answer comes with a worked explanation. Click Show Answer to check yourself.
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First Law of Thermodynamics MCQs
Q1. The First Law of Thermodynamics is essentially a statement of the conservation of:
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Answer: C. Energy
The First Law is conservation of energy applied to thermodynamic systems: energy cannot be created or destroyed, only converted between heat, work and internal energy.
Q2. For a closed system undergoing a process, the First Law is written as:
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Answer: B. Q = ΔU + W
With heat added positive and work done BY the system positive, Q = ΔU + W.
Q3. For a complete thermodynamic cycle, the change in internal energy is:
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Answer: C. Zero
Internal energy is a state function; over a cycle the system returns to its initial state so ΔU = 0, giving ∮δQ = ∮δW.
Q4. Internal energy of a system is a:
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Answer: B. Point (state) function
Internal energy depends only on state, not path, so it is a point/state function with an exact differential.
Q5. Heat and work are:
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Answer: C. Path functions
Heat and work are energy in transit and depend on the path of the process; they are path functions (inexact differentials δQ, δW).
Q6. In a free (unresisted) expansion of an ideal gas in an insulated rigid vessel, ΔU is:
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Answer: C. Zero
Q = 0 (insulated) and W = 0 (expansion into vacuum), so ΔU = Q − W = 0; temperature of an ideal gas is unchanged.
Q7. A system absorbs 200 kJ of heat and does 80 kJ of work. The change in internal energy is:
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Answer: A. 120 kJ
ΔU = Q − W = 200 − 80 = 120 kJ.
Q8. A perpetual motion machine of the first kind (PMM-1) is impossible because it violates the:
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Answer: B. First Law
A PMM-1 would deliver work with no energy input, creating energy from nothing — a direct violation of the First Law.
Q9. The Steady Flow Energy Equation (SFEE) is the First Law applied to a:
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Answer: B. Open (control volume) system
SFEE applies to a control volume with mass crossing the boundary, including flow work, kinetic and potential energy terms.
Q10. For an isolated system, the First Law implies the total energy is:
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Answer: C. Constant
No heat or work crosses the boundary of an isolated system, so its total energy stays constant.
Frequently Asked Questions
What is the First Law of Thermodynamics in simple words?
Energy cannot be created or destroyed, only converted. For a system, heat added equals the rise in internal energy plus the work it does: Q = ΔU + W.
What is the sign convention?
Heat added to the system is positive, heat rejected negative; work done by the system is positive, work done on it negative.
Why is internal energy a property but heat and work are not?
Internal energy depends only on the current state (point function); heat and work depend on the path taken (path functions).