Cool drinks and the inevitable arrow of time

The ice in your glass makes a pleasant clinking sound as you raise it to your lips. You know, of course, that your drink won’t stay cool for long – both the temperature of the liquid surrounding the ice and the warm air are slowly melting the ice. But what exactly is happening? Would you be surprised to find one of the deepest and far reaching concepts in physics?

Moving energy around

Thermodynamics is the branch of physics that deals with the transfer and conversion of energy in physical processes. The most fundamental principle of thermodynamics is that energy cannot be made or destroyed: it can change from form to form, but the total amount of energy always remains the same. For example, if you put your foot on the gas pedal when driving a car, the chemical energy of the fuel is converted to kinetic (movement) energy of the car. As you approach a red light, being a good driver, you press your foot to the brake - and the kinetic energy of the car is converted by friction between the wheel and brake pads into heat energy, which then dissipates in the atmosphere. Since you know that no energy has been made or destroyed, you could, if you knew the mass of the car and by how much it had slowed down, work out how much heat had been generated in the brakes. Scientists say that energy behaving in this manner is conserved, it may not be immediately obvious where it has gone but it persists in some form. The principle of conservation of energy is known as the First Law of Thermodynamics.

Ok. But where's my cool drink?

But the First Law is not the whole story. If you put an ice cube into water, the ice will melt and the water will cool down. This is perfectly permissible according to the First Law: heat energy flows out of the warmer water and into the ice, and so the water is left with less heat energy – it becomes colder – and the ice warms, up and becomes liquid water. You would be rather surprised, however, if the process reversed itself spontaneously, and cool water separated itself into hot water and ice.- would you be more surprised to learn that the First Law of Thermodynamics does not imply that this is impossible? To explain why one process will happen and the other does not, we need to introduce a much more difficult but equally important concept known as entropy.

The word entropy originates, as many scientific terms do, from Greek and the word ‘trope’ meaning ‘transformation’. Entropy is often described as a measure of disorder or randomness – but it also refers to the tendency for systems to transform from being more ‘organised’ to more ‘random’. This leads us to the Second Law of Thermodynamics – The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

Hold on... certainly a glass of ice and water isn't that complicated!