Munir Attaullah

Start with the scientific concept of ‘work’. Fortunately, it is not far removed from its everyday usage, but we need to be a touch more rigorous. So think of ‘work’ as the orderly and collective motion of atoms against an opposing force

To write a short column on a
scientific subject in non-technical language is no easy task. Einstein’s guideline, ‘keep it simple but don’t simplify’, sets a high standard. Regardless, from time to time, I enjoy this challenge. So, today I will discuss what is known as the Second Law of Thermodynamics, one of a handful of the most fundamental of physical laws that govern and drive our universe.

The second law is the explanation for how and why most of the physical changes we incessantly witness, occur at all: from how life is sustained (but decay is inevitable), to why some processes happen spontaneously but others do not (e.g. a hot cup of tea slowly cools but not vice versa); and why air will spread uniformly in the whole room and not all bunch up in a corner. The second law is so basic because it deals with the concept of ‘energy’, the driving force of all change.

But first, to anchor our discussions, I need to define a few concepts in simple language.

Start with the scientific concept of ‘work’. Fortunately, it is not far removed from its everyday usage, but we need to be a touch more rigorous. So think of ‘work’ as the orderly and collective motion of atoms against an opposing force. In lifting a bucket, or pushing a piston in the cylinder of an engine, or a man walking, all the atoms constituting the bucket, the piston, or the human body, are moving together against an opposing force (gravity, inertia etc.). ‘Work’ is being done.

What makes work possible? It is something physicists call energy. What is energy and what is its origin? A technical answer is beyond the scope of this column. But a rough answer would be that energy is a combinational property of mass in motion, and it arises because all atoms (as well as their more fundamental particle constituents) are permanently and inherently in ceaseless motion of some kind (jiggling, spinning, vibrating, etc.).

But for our purposes, eschewing technical complexity, think of energy as not some mysterious thing but — like height and speed which are also not things but quantitative physical descriptions — simply as a quantitative measure of the capacity of an object (or a system) to do work.

There is a fixed amount of energy in the universe. Energy can neither be created nor destroyed (that is the first law of thermodynamics). But it can be transferred. Work is therefore the ordered transfer of energy.

We need to know some more preliminary basics. The first is that every atom, at any given temperature, can exist in countless different energy states (the state with the lowest intrinsic energy being called the ground state). At any given temperature the zillions of atoms comprising, say, a little water, are distributed in these billions of discrete and different energy states in characteristic statistical numbers. The higher the temperature the more atoms will be found in the higher energy states (and, conversely, vice versa). Scientifically, the temperature of something is, again, not some mysterious quality but a bulk property of matter that precisely mirrors the statistical distribution of atoms in different energy states in the given sample of matter.

Transferring energy to or from a sample of matter causes the migration of a certain number (zillions) of atoms to higher (or lower) energy states. So what happens when all the atoms of a given sample are in the ground energy state? Why, we have reached the lowest possible temperature (the absolute zero, which corresponds to about -273 C).

Finally, I come to the concept of heat. For once the common usage of the word (with which we are unfortunately stuck now, for historical reasons) is totally misleading. Heat is not a quality, nor is it a form of energy, but a process. It is the process by which energy is transferred from something by the random and disorderly motion of the atoms surrounding that something.

What happens when a hot cup of tea slowly cools? Wherever the tea is in contact with the cup and the atmosphere, the more vigorously moving tea atoms pass on some of their energy of motion to the atoms of the cup and the atmosphere, through constant and haphazard colliding and jostling. The tea atoms slow down somewhat as a result, and more of its atoms fall to lower energy levels. The reverse happens to the atoms of the atmosphere and the cup. Energy is transferred in a disorderly manner. Heat seems to flow.

Eventually, when all the atoms in contact with each other are moving equally rapidly, the amount of energy absorbed, or given up, by the atoms in contact is, on average, the same. Equilibrium is reached, and the process of spontaneous energy transfer comes to a halt.

Heat, the disorderly and random transfer of energy, has long been known to man. The industrial revolution began when scientists began to ask if such a disorderly transfer of energy could be tamed and harnessed into ordered energy transfer (i.e. to do work). The answer was — in the form of the steam engine — yes.

The steam engine exploits the temperature difference (just as a water-mill exploits the height difference of water flowing down a gradient to turn paddles) between a hot source (the steam) and a colder sink (usually, the surrounding atmosphere). A part of the energy spontaneously transferred (as the steam cools) is used to drive a piston to and fro in a cylinder, while the rest is dumped into the sink as heat.

Can all the energy so given up be captured as work? The answer is — universally and always — no. Nature always extracts a whopping tax. And there is the second law of thermodynamics for you in simple words. Mathematically, the maximum proportion of the total energy transferred that can theoretically be extracted to do work always depends only on the temperature difference between the hot source and the cold sink and can be calculated by dividing the temperature of the cold sink (A) by the temperature of the hot source (B). As A is numerically always less than B, the answer is always some fraction less than one. Nature insists, absolutely, that the rest of the energy must always be dissipated into the cold sink.

Some of you might have heard the word ‘entropy’ in the context of the second law. That is because the more technical (but equivalent) expression of the law states that, ‘the entropy of a system always increases in the course of any spontaneous change’. I am not going to get scientific and define entropy (though you may think of it roughly as a precise mathematical measure of disorder). But whatever it is, a change in entropy — defined as the amount of heat energy supplied divided by the temperature at which the transfer occurred — is easy to understand and calculate.

Why is refrigeration not a natural process? Suppose that an object at room temperature is to cool further by spontaneously transferring energy to the surroundings. Calculate the change of entropy as a result by the above formula and call it X. Similarly, calculate the change in entropy of the surroundings as a result of this energy transfer to it and call it Y. Because the numerator is the same in both cases while the denominator is greater in the second case, Y is less than X. This means that the total change in entropy of the whole system (the object plus the surroundings taken together) has decreased. This would violate the second law.

But we can achieve cooling if we do some work on the system in an appropriate way (e.g. use a compressor) that, when taken into account, will increase the overall change in entropy of this larger system taken together as a whole.

Thus, even though we can, temporarily and partially, create order locally (as when our bodies synthesize proteins from the energy released by metabolizing food), this can only be achieved by creating even greater disorder elsewhere. It follows that the overall fate of our universe as a whole is ever-increasing disorder and dissipation.

To understand the imperious reach of the Second Law of Thermodynamics over nature’s vast domain, and the ruthless enforcement of its implacable writ over most of what goes on therein, is humbling in its enormity.

The writer is a businessman. A selection of his columns is now available in book form. Visit munirattaullah.com