Radiation (Stefan–Boltzmann)
P = εσAT⁴ — blackbody vs polished surface.
Key Notes
Radiation: heat transfer by ELECTROMAGNETIC WAVES (mostly infrared at terrestrial T). Doesn't need a medium.
Stefan-Boltzmann law: power radiated per unit area P/A = εσT⁴, with σ = 5.67×10⁻⁸ W/m²·K⁴ and ε = emissivity (0 ≤ ε ≤ 1).
Black body: ε = 1, perfect absorber and emitter. Sun is approximately a black body at ~5800 K.
Net radiative exchange: P_net = εσA(T_body⁴ − T_surroundings⁴).
Wien's law: λ_max·T = 2.898 × 10⁻³ m·K. Peak wavelength of emitted radiation.
Sun's peak: λ_max ≈ 500 nm (visible). Human body (310 K): peak ~ 9.4 μm (infrared).
Radiation is how Earth gets energy from Sun (no medium between). Also how Earth loses heat to space.
Greenhouse effect: atmosphere absorbs outgoing IR, traps heat ⇒ Earth's surface stays warmer.
Formulas
Stefan-Boltzmann law
Power radiated by surface area A at temperature T.
Net exchange
Body at T_b in surroundings at T_s.
Wien's displacement law
Peak wavelength shifts inversely with T.
Solar constant
Solar power per m² at Earth's orbit, before atmosphere.
Important Points
Radiation does NOT need a medium — works through vacuum (unlike conduction & convection).
Power radiated ∝ T⁴ — doubles for only ~19% T increase. Hot objects radiate vastly more.
Wien's law: hotter ⇒ shorter peak wavelength (blue/UV); cooler ⇒ infrared.
Emissivity ε depends on surface: black 1, polished metal 0.05.
Earth would be ~33°C colder without greenhouse effect — water vapor and CO₂ trap IR.
All bodies at T > 0 K emit radiation continuously. We see colors because surfaces absorb/emit selectively.
Radiation (Stefan–Boltzmann) 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.