Second Law of Thermodynamics
We all have noticed the hot body getting cold in the atmosphere.
while the cold body does not get heated in the same atmosphere.
Suppose, when we put a hot cup of tea in the open, the tea gets cooled after some time.
But, when we put a cold cup of tea in the same environment, it does not get heated up.
This phenomenon can be explained by the second law of thermodynamics.
Let's discuss the Second law of thermodynamics.
There are various statements given by many scientists to explain the second law of thermodynamics.
But those given by Kelvin and the Clausius are more important ones.
Let's understand them one by one.
The first is the Kelvin-Plank statement which States that "It is impossible to convert all of the heat energy which is extracted from a body into work".
Now, let's consider a hot body continuously giving the heat energy
to the working substance.
Now, the working substance converts some part of
into work and expel remaining part
to the surrounding (Sink).
The sink is the body which is at a lower temperature than the hot body.
The first law of thermodynamics is based on the law of conservation of energy.
Now, from Kelvin-Plank statement
will be always greater than zero.
And, in percentage, the efficiency will be less than hundred percent.
Now, let's take the example of an automobile when we use petrol as fuel in our car.
The engine extracts heat energy by burning of petrol and that heat energy cannot be converted back to work completely.
Now, we try to understand the Clausius statement.
According to this "It is impossible to transfer heat energy from cold body to a hot body without an external work".
Let's consider two bodies at two different temperatures, one is at a higher temperature
while the other one is at a lower temperature
Now, heat can easily flow from the hot body to the cold body. But, from the cold body to the hot body, it is not possible without any external work.
The Refrigerator is a good example to understand more about this statement.
The refrigerator is the heat engine which works in the reverse direction.
In the refrigerator, working substance takes heat from a cold body (items present in the refrigerator).
And some amount of work done on the working substance by the electric power supply, some heat energy is expelled to the surrounding.
Hence, there is no refrigerator invented so far which can transfer heat from a cold body to the hot body without external work.
Hence, the cup of cold tea which was kept at room temperature didn't heat up.
When we put a hot cup of tea at room temperature. Then the cup of tea gets cold after some time.
But, when we put a cold cup of tea at room temperature. Then the cup of tea not gets heat up.
No engine has been invented so far which can give hundred percent efficiency.