This concluding topic is a bit of everything and is a great check of so much that you have already studied:
- conservation of energy
- waves
- electromagnetic spectrum
- nuclear fusion
- electron energy levels
This page is a quick summary, but for fuller details, go to:
Key Concepts
Black body radiation
When a body is heated it gives out energy in the form of EM radiation. A perfect black body emits all radiation that is incident upon it, making e = 1.
\(P=e\sigma AT^4\)
This is the Stefan-Boltzmann law.
The constant σ = 5.67 x 10-8 Wm-2K-4
Emissivity (e)
Not all bodies are perfect black bodies.
The emissivity is the ratio of radiation emitted by a body to that which is incident. A body with emissivity 0.5 will emit half the radiation of a black body.
The black body spectrum
A black body spectrum shows the intensity at each wavelength. The distinctive shape of this curve is the same, no matter what the body's temperature or surface area.
Wien's displacement law
This gives the peak wavelength for a given temperature:
\(\lambda_{\text {peak}}={0.00289\over T}\)
Inverse square law
The radiation from the sun spreads out in an ever-increasing sphere.
Intensity is defined as power per unit area:
\(I={P\over 4\pi r^2}\)
The power per unit area at the Earth is 1400 Wm-2 , where r is the total distance from the Sun to the Earth.
Radiation absorbed by the Earth
The Earth absorbs energy only on the side facing the sun so the total power absorbed = 1400 x πr2
When energy is absorbed by a body the increase in temperature depends upon the specific heat capacity of the material and its mass. When heat is absorbed by the earth the temperature rise depends on the surface heat capacity and the area.
The average surface heat capacity is 4 x 108 Jm-2K-1 if the intensity of radiation is 350 Wm-2 .
Radiation reflected by the Earth
The fraction of radiation reflected (or scattered) depends on the albedo of the surface (this is related to the colour):
EM radiation from the sun is mainly in the visible region. This passes through the atmosphere without being absorbed. The radiation lands on the Earth where some is reflected. The rest is absorbed, causing the temperature to rise. The Earth emits radiation in the infrared region of the spectrum which is absorbed by the atmosphere and re-radiated in all directions. Some of the radiation goes back to the Earth causing the temperature to rise: the greenhouse effect.
How much of Global thermal energy transfer have you understood?