Geology Reference
In-Depth Information
Those reading this chapter for the first time may find
that Box 5.4 provides a useful short cut.
in their motion around the Sun. The attractive forces
in these two cases, although different in kind -
electrostatic and gravitational respectively - conform
to the same inverse square 'law' (Appendix A,
equations A8 and A9), so one would expect the
mathematical analysis to be the same. Nevertheless,
it has been clear for more than 80 years that classical
mechanics is not wholly applicable to physics on the
atomic and sub-atomic scale. Other influences seem
to be at work which, although having little obvious
impact on the macroscopic world, are paramount in
the physics of the atom.
Perhaps the most radical departure from everyday
experience is the notion that atomic particles such as
electrons possess some of the properties of waves, a
suggestion first made by the French physicist Louis de
Broglie in 1924. (Readers unfamiliar with the physics of
waves may find Box 5.2 helpful.) De Broglie's idea was
The mechanics of atomic particles
Mechanics is the science of bodies in motion. It describes
the motion of anything from billiard balls to space sat-
ellites and planets. Except for the twentieth-century
contributions of relativity and quantum mechanics, the
basic rules of mechanics have been known for more
than two centuries, since the days of Isaac Newton. His
contribution was of such importance that today we
often refer to the classical mechanics of the macroscopic
world as 'Newtonian mechanics'.
It is natural to expect the movement of electrons
around the nucleus in an atom to conform to the
principles of Newtonian mechanics, like the planets
Box 5.2 What is a wave?
A wave describes a periodic disturbance in the value of
some physical parameter. When a ripple travels across the
surface of an otherwise still pond, the physical parameter
being disturbed is the elevation of the pond surface: a twig
floating on the surface will be seen to bob up and down as
the ripple passes by, indicating that the pond surface is
periodically displaced from its equilibrium position. A wave
is periodic in both time and space: the twig bobs up and
down v times per second (we call v the frequency of the
wave, measured in units of s −1 ; v is the Greek letter 'nu'),
but if we take a snapshot at one instant we will see that
successive wave crests are spaced out at a constant dis-
tance from each other, known as the wavelength λ of the
wave (measured in metres; λ is the Greek letter 'lamda').
The ripple on the pond surface is the easiest type of
wave to visualize, because the disturbance affects the
position of a visible feature, the pond surface. The con-
cept of 'waves' recognized by physics has a much wider
scope, however, because physical quantities other than
position may undergo periodic oscillation. A good example
is a sound wave, in which it is the pressure of the air that
fluctuates as the wave passes by. These changes in air
pressure generate a periodic pressure difference across
the eardrum that we sense as sound; the more times the
pressure oscillates per second (the greater the frequency),
the higher the 'pitch' of the note that we perceive. In the
absence of air there is no pressure to fluctuate, which is
why sound cannot propagate through a vacuum.
An electromagnetic wave (e.g. light) can be visualized as
a train of 'ripples' in the intensity of electric and magnetic
fields. In a place that is remote from magnets and electro-
static charges, the average or equilibrium values of these
fields will both be zero, but the passage of a light wave
causes them each to oscillate between positive and neg-
ative values. Light waves are characterized by wavelength
and frequency in the same way as sound waves, although
the values are very different (Box 6.3).
The examples of waves so far considered have been
travelling waves which propagate energy from one place to
another (e.g. from the loudspeaker to the ear). A wave
confined within an enclosure of some kind, however,
behaves in a different way: when it is reflected from the
walls of the enclosure, the interaction between forward
and reflected waves makes the wave appear to stand still.
This stationary wave is simplest to see in the one-dimen-
sional example of a guitar string. An important property of
a stationary wave is that it has a clearly defined wave-
length determined by the dimensions of the  enclosure
(e.g. the length of the guitar string). This phenomenon is
exploited in organ pipes (sound) and lasers (light).
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