Page 59 - Physical chemistry understanding our chemical world

26 INTRODUCTION TO PHYSICAL CHEMISTRY

Lightning is quite a simple phenomenon. Just before a storm
‘Experiential’ means
breaks, perhaps following a period of hot, fine weather, we often
the way we notice
something exists fol- note how the air feels ‘tense’. In fact, we are expressing an expe-
lowing an experience riential truth: the air contains a great number of ions – charged
or sensation. particles. The existence of a large charge on the Earth is mirrored
by a large charge in the upper atmosphere. The only difference
between these two charges is that the Earth bears a positive charge and the atmosphere
bears a negative charge.
Accumulation of a charge difference between the Earth and the upper atmosphere
cannot proceed indefinitely. The charges must eventually equalize somehow: in prac-
tice, negative charge in the upper atmosphere passes through the air to neutralize
the positive charge on the Earth. The way we see this charge conducted between
the Earth and the sky is lightning: in effect, air is ionized to make it a conductor,
allowing electrons in the clouds and upper atmosphere to conduct through the air
to the Earth’s surface. This movement of electrical charge is a current, which we
see as lightning. Incidentally, ionized air emits light, which explains why we see
lightning (see Chapter 9). Lightning comprises a massive amount of energy, so the
local air through which it conducts tends to heat up to as much as a few thousand
degrees centigrade.
And we have already seen how air expands when warmed, e.g. as described math-
ematically by Charles’s law (Equation (1.6)). In fact, the air through which the
lightning passes increases in volume to an almost unbelievable extent because of
its rise in temperature. And the expansion is very rapid.
SAQ 1.8 Show, using the version of Charles’s law in Equation (1.8), how
a rise in temperature from 330 K to 3300 K is accompanied by a tenfold
increase in volume.

We hear the sensation of sound when the ear drum is moved by compression waves
travelling through the air; we hear people because their speech is propagated by subtle
pressure changes in the surrounding air. In a similar way, the huge increase in air
volume is caused by huge changes in air pressure, itself manifested as sound: we hear
the thunder caused by the air expanding, itself in response to lightning.
And the reason why we see the lightning first and hear the thunder later is because
light travels faster than sound. The reason why thunder accompanies lightning, then,
is because pressure p, volume V and temperature T are interrelated.

How does a bubble-jet printer work?

The ideal-gas equation

A bubble-jet printer is one of the more useful and versatile inventions of the last
decade. The active component of the printer is the ‘head’ through which liquid ink
passes before striking the page. The head moves from side to side over the page. When