Page 48 - Physical chemistry understanding our chemical world

THE PRACTICE OF THERMODYNAMIC MEASUREMENT 15

platinum–iridium alloy on which two deep marks were scratched
‘Ductile’ means the
1 m apart. It was used in exactly the same way as King Edgar’s
ability of a metal to
yardstick 10 centuries earlier.
be drawn to form a
Unfortunately, platinum–iridium alloy was a poor choice, for it wire, or to be worked.
has the unusual property of shrinking (albeit microscopically) with Ductile is the opposite
time. This SI metre rule is now about 0.3 per cent too short. King of ‘brittle’.
Edgar’s yardstick, being made of gold, would still be the same
length today as when it was made, but gold is too ductile, and
In vacuo is Latin for
could have been stretched, bent or re-scored.
‘in a vacuum’. Many
In 1960, the SI unit of length was redefined. While keeping
properties are mea-
the metre as the unit of length, it is now defined as 1 650 763.73
sured in a vacuum to
wavelengths of the light emitted in vacuo by krypton-86. This is a avoid the complication
sensible standard, because it can be reproduced in any laboratory of interference effects.
in the world.

How fast is ‘greased lightning’?

Other SI standards

In comic books of the 1950s, one of the favourite phrases of super-heroes such as
Superman was ‘greased lightning!’ The idea is one of extreme speed. The lightning we
see, greased or otherwise, is a form of light and travels very, very fast. For example,
−1
8
it travels through a vacuum at 3 × 10 ms , which we denote with the symbol c.
But while the speed c is constant, the actual speed of light may not be: in fact, it
alters very slightly depending on the medium through which it travels. We see how
a definition of time involving the speed of light is inherently risky, explaining why
we now choose to define time in terms of the duration (or fractions and multiples
thereof) between static events. And by ‘static’ we mean unchanging.

SI ‘base units’

Time is one of the so-called ‘base units’ within the SI system, and so is length.
Whereas volume can be expressed in terms of a length (for example, a cube has a
3
2
volume l and side of area l ), we cannot define length in terms of
something simpler. Similarly, whereas a velocity is a length per unit There are seven base
time, we cannot express time in terms of something simpler. In fact, SI units: length, time,
just as compounds are made up of elements, so all scientific units mass, temperature,
are made up from seven base units: length, time, mass, temperature, current, luminous
current, amount of material and luminous intensity. intensity and amount
of material.
Table 1.1 summarizes the seven base (or ‘fundamental’) SI phys-
ical quantities and their units. The last unit, luminous intensity, will
not require our attention any further.
The SI unit of ‘time’ t is
The SI unit of ‘time’ t is the second. The second was originally
the second (s).
defined as 1/86 400th part of a mean solar day. This definition is