Charles's Law
V/T = const at fixed P.
Key Notes
Charles's law: at constant pressure, V/T = constant for an ideal gas (T in Kelvin).
Equivalently: V₁/T₁ = V₂/T₂ at constant P.
DIRECT proportionality between V and absolute T.
On V-T graph: straight line through origin.
Extrapolating V to zero gives T = 0 K = −273.15°C = ABSOLUTE ZERO.
Microscopically: higher T ⇒ faster molecules ⇒ more momentum transfer ⇒ gas expands at constant P.
Discovered by Jacques Charles (1787) and later refined by Joseph Gay-Lussac.
Special case of ideal gas law at constant P and n.
Formulas
Charles's law
V ∝ T at constant P.
Two-state form
Same gas at constant P.
From ideal gas law
V linear in T at constant P.
Volume expansion coefficient (gas)
Linear T dependence.
Important Points
Charles's law uses ABSOLUTE temperature (Kelvin) — NOT Celsius.
Doubling T (in K) at constant P doubles V.
V vs T (Kelvin): straight line through origin. V vs T (Celsius): straight line, not through origin.
Extrapolation gives absolute zero: T = 0 K where V → 0 (classical limit).
Hot air balloons: heated air expands, becomes less dense, balloon rises.
Real gases deviate at low T (close to liquefaction) where molecular forces matter.
Charles's Law 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.