Know that for an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy

2.3.3 Radiation – Energy Transfer & Temperature Balance

1. What is Radiation?

Radiation is the transfer of energy through electromagnetic waves. Unlike conduction or convection, it does not need a medium – it can travel through a vacuum. 🌌

2. How Energy Transfer Works

When an object is hotter than its surroundings, it emits radiation. The power radiated is given by the Stefan–Boltzmann law:

\$P = \sigma \epsilon A T^4\$

• \$P\$ = power radiated (W)
• \$\sigma\$ = Stefan–Boltzmann constant (\$5.67\times10^{-8}\,\text{W m}^{-2}\text{K}^{-4}\$)
• \$\epsilon\$ = emissivity (0–1)
• \$A\$ = surface area (m²)
• \$T\$ = absolute temperature (K)

3. Constant Temperature Condition

For a steady‑state temperature:

\$P{\text{received}} = P{\text{radiated}}\$

Imagine a kettle on a stove: the heat from the stove (received) equals the steam and heat lost to the air (radiated). If the stove is too hot, the kettle boils faster – the temperature rises. If the stove is cooler, the kettle stays at a lower temperature.

4. Real‑World Examples

1. Sunlight heating the Earth: Earth receives solar radiation and radiates infrared back to space. The balance keeps Earth’s average temperature stable. 🌞
2. Human body: We feel warm because our body radiates heat, but we also absorb heat from the environment. The body maintains a constant temperature by balancing these flows. 🧑‍⚕️
3. Microwave ovens: Food absorbs microwave radiation and heats up; the oven walls radiate heat back, but the system is designed so the food’s temperature rises to the desired level. 🍲

5. Energy Flow Table