radiation

Meaning: Every object above absolute zero emits electromagnetic waves because of the thermal motion of its charged particles. When another object absorbs those waves, its internal energy rises. Conduction and convection need matter, but radiation does not, which is how sunlight reaches Earth. Hotter objects emit more power and their emission peaks at shorter wavelengths. Dark surfaces absorb and emit well. Shiny surfaces reflect more and emit less. An ideal blackbody has e = 1.

Equation: P = eσAT⁴, where P is the radiated power (W), e is emissivity (dimensionless, 0 to 1), σ is the Stefan–Boltzmann constant (5.67 × 10⁻⁸ W/(m²·K⁴)), A is surface area (m²), and T is absolute temperature (K).

On the MCAT: Expect questions that ask which transfer mode operates in a given setting, such as a vacuum-walled thermos with silvered surfaces or a person standing near a fire. Others ask how P changes when T or A changes, or why an object at steady temperature absorbs and emits equal power.

Common confusion: Students use °C in the T⁴ relationship. Use kelvin, or the ratios come out wrong. Others think radiation means only dangerous nuclear emissions or that it needs air. Thermal radiation comes from all matter above 0 K and crosses a vacuum. Nuclear radiation (alpha, beta, gamma) is a separate sense, tied to decay of unstable nuclei.