Section 6.1 · Worked examples · 4 of 5
Worked example 6.1.4: Sunlight on Earth and Mars
About the worked examples of Section 6.1
Five short calculations tied to the applications. They use only , , the polarisation vectors, Malus' law and the two-way Doppler shift, and no new physics. Numerical constants: , . The polarisation match of a wave and an antenna, with unit polarisation vectors written for the same , is , with for RHCP and LHCP along . A good way to use them: read the given quantities, produce the formula from the corresponding application or problem yourself, and only then put the numbers in.
Sunlight on Earth and Mars
#Given. Solar irradiance at the Earth () . Mars at . A solar panel, efficient, in on the ground. Sunlight is replaced by one equivalent plane wave in free space, .
Find. (a) and at the Earth, (b) and at Mars, (c) the panel's electrical power, (d) the average energy density of sunlight, from and from .
Adapted from: B. M. Notaroš, Electromagnetics, Example 9.11.
For a plane wave in free space in phase with , so . In empty space no power is lost, so the flux of through every sphere round the sun is the same: . Energy that crosses a unit area in one second fills a column of length : .
and .
.
From the power, . From the fields, and
because : the two routes in (d) are the same identity, the energy velocity, Eq. (6.1.52), in free space.
A kilovolt per metre sounds large, yet it carries only and stores a few microjoules per cubic metre: free space is a poor energy store but a fast carrier. Mars receives of the Earth's sunlight. Sunlight is broadband and incoherent, so is the amplitude of a single wave that carries the same , not a measured amplitude.
cf. Application 6.1.1