What's the Difference Between Conduction, Convection, and Radiation?

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where $k$ is thermal conductivity, $A$ is cross-sectional area, and $\Delta T/\Delta x$ is the temperature gradient. The minus sign shows that energy is transferred down the temperature gradient.

For an ideal black body, the power emitted by radiation is modelled by the Stefan and Boltzmann law:

$L=\sigma AT^4.$

Convection is treated qualitatively in B.1: explain it using temperature changes, expansion, density differences, and bulk fluid motion.

A common misconception is that radiation requires particles. It does not: electromagnetic radiation can cross empty space, which is how energy travels from the Sun to Earth. Convection, however, cannot occur in a solid or vacuum because it requires bulk movement of a fluid.

Exam technique: For an Explain question, name the mechanism and link it to its physical cause. Do not simply state that “heat rises”; explain that warmer fluid becomes less dense and rises while cooler, denser fluid sinks.