Thermodynamics

Thermodynamics is the science of heat, work and energy, and how they change into one another. Its handful of laws are among the most powerful in all of physics: they govern car engines, refrigerators, power stations and even the direction in which time seems to flow. The laws apply to everything from a steam engine to the whole universe.

Two foundational laws come first:

• The Zeroth Law says that if two bodies are each in thermal equilibrium with a third, they are in equilibrium with each other. This lets us define temperature and use a thermometer. Bodies at the same temperature exchange no net heat.
• The First Law is the conservation of energy extended to heat. The heat added to a system goes into increasing its internal energy and doing work: heat = change in internal energy + work done. Energy is never created or destroyed.

The Second Law captures a deep truth about nature: processes have a preferred direction.

• Heat flows naturally from hot to cold, never the other way on its own.
• No engine can convert heat completely into work. Some heat is always wasted.

The law is often expressed through entropy, a measure of disorder. In any natural process, the total entropy of a system and its surroundings tends to increase. That is why a broken cup never reassembles itself and why perpetual-motion machines are impossible.

The laws of thermodynamics explain the machines that run modern life:

• A heat engine (like a car or steam engine) takes in heat from a hot source, converts part of it into useful work, and rejects the rest to a cold sink. Its efficiency is always less than 100%, as the second law demands.
• A refrigerator (or air conditioner) works in reverse. It uses work (electrical energy) to move heat from a cold inside to a warm outside. This pushes heat "uphill", which nature will not do on its own.

These devices are everyday proof of the thermodynamic laws at work.

Heat engines need a heat source, and the most concentrated source known is the atomic nucleus. Nuclear fission splits a heavy nucleus, usually uranium, and releases enormous heat. A nuclear power station is at heart a thermodynamic plant: fission heat boils water, the steam drives a turbine, and the turbine generates electricity. Nuclear fusion is the opposite process. It joins light nuclei, as in the Sun, and promises far more energy with little long-lived waste. The International Thermonuclear Experimental Reactor (ITER), a multinational fusion experiment in France, aims to prove that fusion can be harnessed on Earth. India is a full member, so a successful ITER would let India build fusion reactors for power generation.

Inside a fission reactor, a few components matter for exams:

• Fuel: natural or enriched uranium. Power reactors need only low-enriched uranium, about 3 to 5 percent. Enrichment near 90 percent is for weapons, not electricity.
• Moderator: a substance that slows down fast neutrons to thermal speeds so they can sustain the fission chain reaction. Heavy water (D2O) and graphite are common moderators. Slowing neutrons is the moderator's job, not cooling or stopping the reaction.
• Control rods: absorb neutrons to regulate or stop the chain reaction.
• Coolant: carries the heat away to raise steam.

India has uranium deposits, yet coal still produces most of its electricity because uranium reserves are modest and coal infrastructure is vast. The long-term answer is thorium, the centrepiece of India's three-stage nuclear programme. Thorium is far more abundant in nature than uranium, India holds some of the world's largest reserves. Per unit mass of mined mineral, thorium can generate more energy than natural uranium, since thorium-232 breeds fissile uranium-233. It also produces less harmful, shorter-lived radioactive waste. A different device often confused with a reactor is the radioisotope thermoelectric generator (RTG). An RTG is not a miniature fission reactor. It converts the heat of radioactive decay directly into electricity using thermocouples, with no chain reaction and no moving parts. RTGs power the onboard systems of spacecraft on long deep-space missions, and they can use plutonium-238, a by-product of weapons development.