A system absorbs 50 J and does 20 J of work. Using ΔU=Q−W, find ΔU.
Answer and explanation
30 J
Subtract work done by the system from heat entering it.
Subjects
University thermal
Choose questions individually, practise online, or create a worksheet with matching answers. Counts include generated variants, not only authored originals.
Make a worksheet8 question instances
Sample exercises
Which temperature scale belongs in Tc/Th for Carnot efficiency?
- Celsius directly
- Kelvin
- Any shifted scale without conversion
- Fahrenheit directly
Answer and explanation
Kelvin
The ratio requires absolute thermodynamic temperatures.
A reversible engine operates between 600 K and 300 K. Find its Carnot efficiency as a fraction.
Answer and explanation
0.5
1−300/600=0.5.
In a free expansion of an isolated ideal gas into vacuum, what is the external boundary work?
- Infinite
- Equal to pressure times final volume
- Zero
- Always positive
Answer and explanation
Zero
The external resisting pressure is zero; isolation also makes Q zero.
For a reversible isothermal heat transfer of 40 J at 20 K, find entropy change.
Answer and explanation
2 J/K
ΔS=Qrev/T for this stated constant-temperature reversible transfer.
Why does Q=0 not imply ΔS=0 for every process?
Answer and explanation
An irreversible adiabatic process can generate entropy internally. Entropy transfer and entropy production are distinct.
Credit internal entropy generation and the restriction to reversible adiabatic paths for constant entropy.
Study notes
First law, ideal gases and entropy
Use ΔU=Q−W, where W is work done by the system. A different sign convention is possible only when used consistently.
UniversityFirst law, ideal gases and entropy · revision notes
Use ΔU=Q−W, where W is work done by the system. A different sign convention is possible only when used consistently.
UniversityRelated courses
Physics4 lessons
University mechanics and waves
Connect rotation, oscillations, central forces and continuous wave models.
32 quiz questions · University
Physics4 lessons
Fields, circuits and thermal physics
Work with Gauss laws, electrical field models, thermodynamic laws and electromagnetic waves.
32 quiz questions · University
Physics4 lessons
Introduction to quantum physics and relativity
Compare relativistic measurement with quantum states, the Schrödinger equation and uncertainty.
32 quiz questions · University
Physics3 lessons
Analytical and statistical mechanics
Use Lagrangian and Hamiltonian methods, then interpret canonical ensembles.
24 quiz questions · Advanced university
Physics4 lessons
Advanced physics methods and spacetime
Explore perturbation theory, scattering, variational quantum estimates and a conceptual bridge to general relativity.
32 quiz questions · Advanced university