Physics · Thermodynamics · NEET
No. The first law is only about energy conservation: energy is neither created nor destroyed (Q = dU + W). It never says which direction a process goes. The second law adds the missing piece: it tells us the direction. For example, the first law allows heat to flow from cold to hot (energy would still be conserved), but the second law forbids it unless work is done from outside.
By the Kelvin-Planck statement, an engine must reject some heat to a cold sink. It cannot take heat from a hot source and turn ALL of it into work. Some heat is always thrown away to the sink. So efficiency = W/Q1 = 1 - Q2/Q1 is always less than 1 (less than 100%), because Q2 (heat given to the sink) is never zero.
They look different but they are equivalent. Kelvin-Planck is about engines (you cannot make an engine with only a source, no sink). Clausius is about refrigerators (heat cannot move from cold to hot by itself). If one statement were false, you could break the other too. So both describe the same single law from two angles.
The second law can also be stated using entropy: the total entropy (disorder) of an isolated system never decreases. For any real (irreversible) process entropy increases; for an ideal reversible process it stays the same. It never goes down on its own. This is the deepest form of the second law, but for NEET the Kelvin-Planck and Clausius statements are the main ones to remember.
Yes, but only when outside work is supplied. A refrigerator does exactly this: it moves heat from the cold inside to the hot room, but it needs electrical work from the compressor. On its own, with no work input, heat always flows from hot to cold. That is the Clausius statement.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No process is possible whose only result is the complete conversion of heat taken from a hot source into work. In simple words, an engine cannot turn 100% of absorbed heat into work; it must reject some heat to a cold sink.
No process is possible whose only result is the flow of heat from a colder body to a hotter body. Heat cannot move from cold to hot on its own; outside work is needed, as in a refrigerator.
No. The Kelvin-Planck and Clausius statements are equivalent. If one is violated, the other can also be violated. They are two forms of the same second law.
It explains why real heat engines have limited efficiency and why refrigerators need work input. These ideas lead directly to heat engine efficiency, the Carnot cycle, and coefficient of performance, which are frequent NEET numerical topics.
Yes. The first law allows heat to flow cold to hot and allows 100% conversion of heat to work, because energy would still be conserved. The second law forbids both of these when they happen with no other change.
