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 him.
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 increased slightly).
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.