Key relationship
ΔU=Q−Wby; positive heat enters the gas; positive work leaves as work done by the gas.
Physics · Class 11
Balance heat, work and internal energy
The first law is an energy balance. Heat entering increases internal energy; work done by the gas decreases it. Compression has negative work-by and can raise energy even without heat.
Open the interactive experiment →Balance heat, work and internal energy
முதல் வெப்ப இயக்கவியல் விதி ΔU = Q − W. வாயு செய்த வேலை நேர்மறை; வாயுவின் மீது செய்த வேலை எதிர்மறை.
Key notes, relationships and model limits
Remember
The first law is an energy balance. Heat entering increases internal energy; work done by the gas decreases it. Compression has negative work-by and can raise energy even without heat.
Common trap
Work done on the gas and work done by the gas have opposite signs.
Model limits
Closed system with no kinetic or gravitational energy change. Work is supplied independently from heat; negative W means compression work done on the gas.
What you will learn
Use a consistent sign convention to audit energy transfer.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
Starting at U=2000 J, the final energy is 2300 J. For adiabatic compression with Q=0 and Wby=−200 J, ΔU=+200 J.
Why this matters
The model connects balance heat, work and internal energy with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
A gas receives 500 J and does 200 J of work. What is ΔU?
- 700 J
- 300 J
- −300 J
Read the explanation
ΔU=Q−W=500−200=300 J.
Course connection: Class XI Chapter 11. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.