Physics Thermodynamics 1 — Questions and Answers
Question 1: What does the First Law of Thermodynamics state?
- Energy cannot be created or destroyed, only transferred or transformed. (Correct answer)
- Heat flows from a cold body to a hot body.
- The entropy of the universe is always increasing.
- The temperature of a system remains constant during heat exchange.
Correct answer: Energy cannot be created or destroyed, only transferred or transformed.
The First Law of Thermodynamics is a restatement of the conservation of energy, applied to thermodynamic systems. It says the change in internal energy (ΔU) of a system equals the heat (Q) added to the system minus the work (W) done by the system: ΔU=Q−W.
Question 2: A gas absorbs 500 J of heat and does 200 J of work. What is the change in its internal energy?
- 300 J (Correct answer)
- 500 J
- 700 J
- 1000 J
Correct answer: 300 J
Using the First Law of Thermodynamics (ΔU=Q−W): ΔU=500J−200J=300J.
Question 3: Which of the following processes describes heat transfer through direct contact?
- Conduction (Correct answer)
- Convection
- Radiation
- Evaporation
Correct answer: Conduction
Conduction occurs when heat is transferred directly through a material or between materials in physical contact, as a result of collisions between molecules or free electrons.
Question 4: If the temperature of an ideal gas doubles (in Kelvin scale) while the pressure remains constant, what happens to its volume?
- It stays the same.
- It doubles. (Correct answer)
- It halves.
- It quadruples.
Correct answer: It doubles.
For an ideal gas at constant pressure, the relationship between temperature and volume is given by Charles's Law: V 1/T 1 = T /2 If 𝑇2=2𝑇1, then 𝑉2=2𝑉1 .
Question 5: What is the efficiency of a heat engine that absorbs 500 J of heat from a high-temperature source and rejects 300 J to a low-temperature sink?
- 20%
- 40% (Correct answer)
- 50%
- 60%
Correct answer: 40%
The efficiency of a heat engine is given by: Efficiency= Heat Input/Work Output ×100. Efficiency= 200/500×100=40%.
Physics Thermodynamics 1
What does the First Law of Thermodynamics state?