The first law of
thermodynamics relates the input of heat energy to the mechanical work that may be
obtained from it. It says nothing about the way that this conversion may take place, however,
nor does it put any restrictions on it.
The second law of thermodynamics states the
way in which these changes of energy may take place. It is considered by many eminent
scientists to be one of the most fundamental laws of Physics and yet in one of its forms it
may be stated in the following very simple manner:
To study the
second law further we have to define a new quantity known as the entropy of a body.
In all heat engines energy is taken in as 'high grade energy' and only some of it is
converted into useful work, the remainder being emitted as 'low grade energy' at a lower
temperature.
The fundamental idea of an increase of the entropy of a system can
give us a way of checking the passage of time. Physicists call this the 'arrow of time' that tells
us that time is passing and in 'which direction'. For example if you watch a film of a pile of
bricks falling over it is easy to tell if the film is being run backwards since in only one case is
the entropy or disorder increasing. Mixing hot and cold water to give a bucket of lukewarm
water shows an entropy increase - you would not expect the lukewarm water to 'unmix' itself!
It is important to refer back to the original definition of the second law and notice the
word 'isolated'. Human intervention is 'not allowed' to rebuild the pile of bricks. However even
with human help it is not easy to see how we could 'unmix' the water!
We can put this idea on a cosmic scale. If
we look into the universe we see discrete sources of energy - the stars. As time passes their
energy is being spread out throughout space and if we could return many aeons in the future
we would find this energy smeared out through the universe. Hot spots and cold spots have
degenerated into a warm sea of energy. The entropy and hence the disorder of the universe
would have increased. The second law of thermodynamics therefore relates to what
physicists call the 'heat death' of the universe.