Topic 6. The sources of electromagnetic radiation

General principle: The size of a transmitting antenna is about the same as the size of the wave it produces

For electromagnetic waves, it is common to use an antenna that is one-quarter of the wavelength of the wave to be generated.

(By comparison, sources of sound waves often generate many wavelengths simultaneously. They usually have a length that is

one-half or one-quarter of the longest wavelength (lowest frequency) that can be generated.)

a. Moving charges in wires and bulk materials are usually used at low frequencies

physical oscillation of the charges matches frequency of the radiated energy

typical wavelengths

AM radio (1 MHz) 300 m (Antennas are big!)

FM radio (100 MHz) 3 m

Cell telephone (900 MHz) 0.33 m (about 13 inches)

Microwave oven (3 GHz) 0.1 m (about 4 inches)

Large-scale movement of charges in wires and bulk materials becomes impractical at higher frequencies

b. the black body radiation function

electromagnetic energy generated by random motions of charges in a material

radiation is not at one single frequency

details of frequency spectrum are not obvious and not trivial to calculate – requires introducing quantum phenomena

Total power increases very rapidly with temperature (increases as T⁴), where T is measured in Kelvin (degrees centigrade+273).

Peak of radiation function moves to higher frequency/shorter wavelengths as temperature increases. The power radiated at visible wavelengths begins to become appreciable when the temperature reaches about 600 K (350° C), and objects at that temperature begin to appear red to the eye.

c. atomic and molecular transmitters and receivers

resonance behavior due to details of atomic and molecular structure – atoms and molecules absorb and emit energy

only at specific frequencies/wavelengths.

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