Some Like it Hot!
There are four of them but, in this article, we’re
going to take a closer look at just two: the first and second laws.
The first law of thermodynamics concerns the conservation of
energy, such that the total amount of energy in an isolated system remains
constant over time. This energy includes the ‘internal energy’ that resides in
the random motion and vibration of particles such as atoms and molecules, as
well as heat or ‘thermal energy’. Finally, there is ‘work’, which is a transfer
of mechanical energy such as when a gas is compressed.
So, this law means that, in an isolated system, energy can
change its location within that system, and it can change to a different type
of energy: for instance chemical energy from a battery can become electricity
which, in turn, can drive a motor to become kinetic energy. Importantly,
however, that energy can neither be created nor destroyed. It remains constant.
English brewer and Physicist James Prescott Joule was one of
the pioneers in this field and his experimental work led to the theory of
conservation of energy, which in turn led to the development of the first law
of thermodynamics. The SI derived unit of energy, the joule, is named after
In 1843 he carried out a series of experiments, the most
famous of which involved what is now called the ‘Joule apparatus’. He built a
gadget consisting of a descending weight attached to a string, which turned a
paddle wheel immersed in water. By knowing the mass of the weight and the
distance it descended, and also by monitoring the temperature of the water, he
was able to show that the gravitational potential energy lost by the weight in
descending was equal to the heat (thermal energy) gained by the water due to
the friction caused by the rotating paddle.
Along with the second law of thermodynamics, the first law
means that so-called ‘perpetual motion machines’ – ie ones that continue working
indefinitely without any external source of energy – are impossible, despite
many people having tried to invent them down the centuries.
So what’s the second law of thermodynamics all about? It says
that heat flows naturally from an object at a higher temperature to an object
at a lower temperature. Think of a cup of hot coffee left in a room: over time,
it will cool to approach the temperature of the room (which itself will have
At the same time, heat doesn’t flow in the opposite
direction of its own accord and this is again borne out by observation of our
everyday environment. For instance, if we want to chill food in a warm
environment we put it in a refrigerator, which needs electrical energy to
function. Similarly, we keep ourselves cool in hot climates with
air-conditioning, which also requires consumption of electrical energy.
The idea of ‘entropy’ is an essential concept in
thermodynamics. Essentially it is a measure of disorder and randomness in the
system. For instance, imagine a flask containing gas. If all the molecules stayed
at one end of the flask, with a vacuum in the rest of the container, then they
would be in a highly organised low state of entropy. As the particles move out to
fill the rest of the container, entropy increases as they take up random
positions within the flask.