Beginner
Thermodynamics: First Law
Energy can't be created or destroyed — only moved around as heat and work.
STEP 1 · CONCEPT
A system is the part of the universe we study. Energy crosses its boundary in two ways: heat (Q) and work (W).
STEP 2 · EXPLANATION
The change in the system's internal energy equals the heat added minus the work done by the system. It's bookkeeping for energy.
STEP 3 · SIGN CONVENTION
Heat added to the system is positive. Work done BY the system (e.g. gas expanding) is positive.
WORKED EXAMPLE
A gas receives 500 kJ of heat and does 200 kJ of work pushing a piston. ΔU = 500 − 200 = 300 kJ. Its internal energy rises by 300 kJ.
Formulas
First law (closed system)
ΔU = Q − W
U internal energy, Q heat added, W work done by system
Boundary work (constant P)
W = P (V₂ − V₁)
P pressure, V volume
Practice questions
Q1.In ΔU = Q − W, W is the work done…
Thermodynamics: First Law · Easy
Q2.In a rigid closed tank, 100 kJ of heat is added. Change in internal energy?
Thermodynamics: First Law · Medium
Q3.For an isolated system, which is true?
Thermodynamics: First Law · Medium