First Law of Thermodynamics
ΔU = Q − W — heat in, work out.
Key Notes
First law of thermodynamics: ΔU = Q − W. Energy conservation applied to a thermodynamic system.
Q = heat ADDED to the system (positive when heat enters).
W = work done BY the system on surroundings (positive when gas expands).
ΔU = change in internal energy.
Alternative sign convention: ΔU = Q + W where W = work done ON the system. Use one convention consistently.
For an ideal gas: ΔU = nC_v·ΔT, regardless of process.
Different processes redistribute Q and W differently — but ΔU = nC_v·ΔT always (for ideal gas).
Cannot create or destroy energy — only transform between heat, work, and internal energy.
Formulas
First law
Q in (positive), W out (positive). ΔU = change in internal energy.
Sign convention
Be consistent throughout problem.
Differential form
For infinitesimal changes.
Ideal gas (any process)
Always — U depends only on T for ideal gas.
Important Points
Δ U = Q − W (or ΔU = Q + W_on_system; sign convention varies).
Free expansion of ideal gas: Q = 0, W = 0 (no resistance), so ΔU = 0 ⇒ T unchanged.
Adiabatic: Q = 0 ⇒ ΔU = −W. If gas expands (W > 0), T drops.
Isothermal (ideal gas): ΔU = 0 ⇒ Q = W. All heat in becomes work out.
Energy conservation: every joule of heat is accounted for as work or internal-energy change.
1st law is universal — applies to engines, refrigerators, chemical reactions, biological systems.
First Law of Thermodynamics notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.