MOTION MOUNTAIN

electromagnetic effects and challenges 223
TA B L E 17 (Continued) Selected matter properties related to electromagnetism.
Property Example Definition
Challenge 215 e
Ref. 192
Challenge 216 s
All matter properties given in the list can be influenced by electromagnetic fields or
directly depend on them.This shows in detail:
⊳ The nature of all everyday material properties is electromagnetic.
In other words, electric charges and their interactions are an essential and fundamental
part of the structure of objects.The table shows so many different electromagnetic properties
that the motion of charges inside each material must be complex indeed. Most
effectsarethetopicofsolidstatephysics,*fluidphysicsorplasmaphysics.
Solid state physics is by far the most important part of physics, when measured by
theimpactithasonsociety.Almostallitseffectshave applications in technical products,
and give employment to many people. Can you name a product or business application
for any randomly chosen effect from the table?
In our mountain ascent however, we look at only one example from the above list:
thermal radiation, the emission of light by hot bodies.
All bodies emit radiation
Earnshaw’s theorem about the impossibility of a stable equilibrium for charged particles
at rest implies that the charges inside matter must be moving. For any charged particle
in motion, Maxwell’s equations for the electromagnetic field show that it radiates energybyemittingelectromagneticwaves.
In short, we predict that all matter must radiate
electromagnetic energy.
Interestingly, we know from experience that this is indeed the case. Hot bodies light
up depending on their temperature; the working of light bulbs thus proves that metals are
made of charged particles.Incandescence, as it is called, requires charges. Actually, every
body emits radiation, even at room temperature. This radiation is called thermal radiation;
at room temperature it lies in the infrared. Its intensity is rather weak in everyday
life; it is given by the general expression
I(T)=fT 4 2π 5 k 4
15c 2 h 3 or I(T)=fσT 4 with σ=56.7 nW/K 4 m 2 , (85)
wheref is a material-, shape-and temperature-dependentfactor, with a value between
zero and one, and is called theemissivity.The constantσ is called theStefan–Boltzmann
blackbodyradiation constant orblackbodyradiation constant. A body whose emissivity
is given by the ideal casef=1 is called ablackbody, because at room temperature such a
body also has an ideal absorption coefficient and thus appears black. (Can you see why?)
The heat radiation such a body emits is calledblack bodyradiation.
*Probablythebestandsurelythemostentertaining introductoryEnglishlanguagebookonthetopicisthe
onebyNeil Ashcroft & David Mermin,SolidState Physics,Holt Rinehart &Winston,1976.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net