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 basic idea of energy spreading
out as time goes by 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 disorder increasing. Mixing hot and cold water to
give a bucket of lukewarm water shows an increase in disorder - you would not expect the
lukewarm water to un-mix 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. Although we could stack up the pile of bricks again it is not easy to
see how we could 'un-mix' the water!
We can put this idea on a cosmic scale. If we look into the universe we
see isolated 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 turned into a
warm sea of energy. The disorder of the universe would have increased. The spreading out
of energy therefore relates to what physicists call the 'heat death' of the universe.