Know that thermal energy transfer by thermal radiation does not require a medium

2.3.3 Radiation – Thermal Energy Transfer Without a Medium

What is Thermal Radiation?

All objects that have a temperature above absolute zero emit electromagnetic waves called thermal radiation. The hotter an object, the more intense its radiation. Think of the Sun as a giant light bulb that sends heat and light across the vacuum of space to Earth. 🌞

Key Properties

• Does not need air, water, or any other material to travel.
• Can travel through the vacuum of space.
• Energy carried depends on temperature and surface area.
• All objects emit radiation, but cooler objects emit less and at longer wavelengths.

Stefan‑Boltzmann Law

The total power emitted per unit area by a black body is given by:

\$P = \sigma T^4\$

where \$\sigma = 5.67 \times 10^{-8}\,\text{W m}^{-2}\text{K}^{-4}\$ and \$T\$ is the absolute temperature in kelvin. This shows that a small increase in temperature leads to a large increase in emitted power.

Wien’s Displacement Law

The wavelength at which the emission is strongest is inversely proportional to temperature:

\$\lambda_{\text{max}} = \frac{b}{T}\$

with \$b = 2.90 \times 10^{-3}\,\text{m K}\$. Hotter objects peak at shorter wavelengths (blue light), cooler objects at longer wavelengths (infrared).

Practical Example: The Sun and Earth

The Sun, at about 5800 K, emits a huge amount of visible light and ultraviolet radiation. This radiation travels through the vacuum of space and warms the Earth’s surface. The Earth, at ~300 K, re‑radiates energy mainly in the infrared. The balance between absorbed solar radiation and emitted infrared keeps Earth’s climate stable. 🌍

Exam Tip Box

Quick Quiz

1. Which of the following can transfer heat through a vacuum? (A) Conduction, (B) Convection, (C) Radiation
2. Given a black body at 400 K, calculate the ratio of power emitted to that of a black body at 800 K.

Summary Table