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WHAT IS PHYSICAL CHEMISTRY: VARIABLES, RELATIONSHIPS AND LAWS 7

Why does a radiator feel hot to the touch when ‘on’,
and cold when ‘off’?

Laws and the minus-oneth law of thermodynamics

Feeling the temperature of a radiator is one of the simplest of
A ‘law’ in physical
experiments. No one has ever sat in front of a hot radiator and
chemistry relates to
felt colder. As a qualitative statement, we begin with the excellent
a wide range of situa-
generalization, ‘heat always travels from the hotter to the colder
tions.
environment’. We call this observation a law because it is universal.
Note how such a law is not concerned with magnitudes of change
but simply relays information about a universal phenomenon: energy in the form of
heat will travel from a hotter location or system to a place which is colder. Heat
energy never travels in the opposite direction.
We can also notice how, by saying ‘hotter’ and ‘colder’ rather than just ‘hot’ and
‘cold’, we can make the law wider in scope. The temperature of a radiator in a living
◦
room or lecture theatre is typically about 60 C, whereas a human body has an ideal
◦
temperature of about 37 C. The radiator is hotter than we are, so heat travels to us
from the radiator. It is this heat emitted by the radiator which we absorb in order to
feel warmer.
Conversely, now consider placing your hands on a colder radiator having a tem-
◦
perature of 20 C (perhaps it is broken or has not been switched on). In this second
◦
example, although our hands still have the same temperature of 37 C, this time the
heat energy travels to the radiator from our hands as soon as we touch it. The direction
of heat flow has been reversed in response to the reversal of the relative difference
between the two temperatures. The direction in which the heat energy is transferred
is one aspect of why the radiator feels cold. We see how the movement of energy
not only has a magnitude but also a direction.
Such statements concerning the direction of heat transfer are
sometimes called the minus-oneth law of thermodynamics, which The ‘minus-oneth law
sounds rather daunting. In fact, the word ‘thermodynamics’ here of thermodynamics’
may be taken apart piecemeal to translate it into everyday English. says, ‘heat always
First the simple bit: ‘dynamic’ comes from the Greek word travels from hot to
dunamikos, which means movement. We obtain the conventional cold’.
English word ‘dynamic’ from the same root; and a cyclist’s
‘dynamo’ generates electrical energy from the spinning of a bicycle wheel, i.e. from a
moving object. Secondly, thermo is another commonly encountered Greek root, and
means energy or temperature. We encounter the root thermo incorporated into such
everyday words as ‘thermometer’, ‘thermal’ and ‘thermos flask’. A ‘thermodynamic’
property, therefore, relates to events or processes in which there are ‘changes in heat
or energy’.