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86 ENERGY AND THE FIRST LAW OF THERMODYNAMICS

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water at the bottom and top of the waterfall, finding it to be about 1 Fwarmerat
the bottom. In Joule’s own words, ‘A [water]fall of 817 feet [249 m] will generate
one degree [Fahrenheit] of temperature’. This result is not attributable to colder air at
the top of the waterfall, nor due to friction or viscous drag, or other effects occurring
during the water’s descent, but is wholly due to a change in internal energy. The
water was simply changing its altitude.
The potential energy of a raised object is given by the expression

potential energy = mgh (3.4)

where m is the mass, g is the acceleration due to gravity and h the height by which it
is raised. The potential energy of the water decreases during descent because its height
decreases. This energy is liberated; and, as we have noted several times already, the
simplest way to tell if the internal energy has increased is to determine its temperature.
Joule showed the temperature of the water of the waterfalls had indeed increased.
We could summarize by saying that thermodynamic work w is energetically equiv-
alent to the lowering or raising of a weight (like the water of the waterfall, above),
as discussed below.

Why is it such hard work pumping up a bicycle tyre?

Thermodynamic work

No one who has pumped up a bicycle tyre says it’s easy. Pumping a car tyre is harder
still. It requires a lot of energy, and we really have to work at it.
We saw in Chapter 1 how increasing the amount of a gas causes
The pressure inside a its volume to increase. This increase in volume is needed to oppose
party balloon is higher any increases in pressure. It also explains why blowing into a party
than the external, balloon causes it to get bigger. By contrast, a car tyre cannot expand
atmospheric pressure, greatly during pumping, so increasing the amount of gas it contains
as evidenced by the will increase its internal pressure. In a fully inflated car tyre, the
way it whizzes around a internal pressure is about 10 times greater than ‘standard pressure’
room when punctured. O O 5
p , where p has a value of 10 Pa.
The first law of thermodynamics states that energy may be con-
Work is a form of ener- verted between forms, but cannot be created or destroyed. Joule
gy. The word ‘energy’ was a superb experimentalist, and performed various types of work,
comes from the Greek each time generating energy in the form of heat. In one set of exper-
en ergon, meaning iments, for example, he rotated small paddles immersed in a water
‘from work’. trough and noted the rise in temperature. This experiment was
apparently performed publicly in St Anne’s Square, Manchester.
Joule discerned a relationship between energy and work (symbol w). We have to
perform thermodynamic work to increase the pressure within the tyre. Such work
is performed every time a system alters its volume against an opposing pressure or
force, or alters the pressure of a system housed within a constant volume.