Second law of thermodynamics: Statement, examples and applications

The second law of thermodynamics states that heat can flow spontaneously from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object. Carnot engine, heat engine are some examples of second law of thermodynamics.

Why we need  2nd law of thermodynamics?

The first law of thermodynamics would not be violated if any of these processes occurred in reverse. To explain this lack of reversibility scientists in the latter half of the nineteenth century formulated a new principle known as the 2nd law of thermodynamics.

Examples of the second law of thermodynamics

The first law of thermodynamics states that energy is conserved. There are, however, many processes we can imagine that conserve energy but are not observed to occur in nature. For example, when a hot object is placed in contact with a cold object, heat flows from the hotter one to the colder one, never spontaneously from colder to hotter. If heat were to leave the colder object and pass to the hotter one, energy could still be conserved. Yet it does not happen spontaneously.

As a second example, consider what happens when you drop a rock and it hits the ground. The initial potential energy of the rock changes to kinetic energy as the rock falls. When the rock hits the ground, this energy, in turn, is transformed into internal energy of the rock and the ground in the vicinity of the impact; the molecules
move faster and the temperature rises slightly.

But have you seen the reverse happen—a rock at rest on the ground suddenly rises up in the air because the thermal energy of molecules is transformed into the kinetic energy of the rock as a whole?
Energy could be conserved in this process, yet we never see it happen.

Before initiating the discussion on the formal statement of the 2nd law of thermodynamics, let us analyze briefly the factual operation of an engine. The engine or the system represented by the block diagram absorbs a quantity of heat Q 1 from the heat source at temperature T 1. 

It does work W and expels heat Q  to low temperature reservoir at temperature T 2. As the working substance goes through a cyclic process, in which the substance eventually returns to its initial state, the change in internal energy is zero. Hence from the first law of thermodynamics, the network done should be equal to the net heat absorbed.

W=Q 1_Q 2

In practice, the petrol engine of a motor car extracts heat from the burning fuel and converts a fraction of this energy to mechanical energy or work and expels the rest into the atmosphere. It has been observed that petrol engines convert roughly 25% and diesel engines 35 to 40% of available heat energy into work.
The second law of thermodynamics is a formal statement based on these observations. It can be stated in a number of different ways.

According to Lord Kelvin’s statement based on the working of a heat engine.
“It is impossible to devise a process which may convert heat, extracted from a single reservoir, entirely into work without leaving any change in the working system.”

This means that a single heat reservoir, no matter how much energy it contains, can not be made to perform any work. This is true of oceans and our atmosphere which contains a large amount of heat energy but can not be converted into useful mechanical work. As a consequence of the 2nd law of thermodynamics, two bodies of different temperatures are essential for the conversion of heat into work. Hence for the working of a heat engine, there must be a source of heat at a high temperature and a sink at a low temperature to which heat may be expelled. The reason for our inability to utilize the heat contents of oceans and the atmosphere is that there is no reservoir at a temperature lower than any one of the two.


  • Steam engine
  • Heat engine
  • Cooler engine

Watch the video to learn this law visually:

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