Laws of Thermodynamics

Keywords

The Second Law of Thermodynamics

The Zeroth Law of Thermodynamics

Second Law Thermodynamics

Second Law of Thermodynamics Examples

There were some phenomenon and concepts observed by scientist which cannot explain by using the zeroth law of thermodynamics and 1^st law. As necessity is the mother of discovery so it developed the second law of thermodynamics.

There are many formulations and applications of the 2^nd Law. To understand the law, batter to take some second law of thermodynamics problems like production of heat in a heater after passing the current (work) a wire (resistance).

Electricity (work) heat

Therefore as per first law of thermodynamics;

Q = W

But this law does say anything about the direction of the energy transfer. It means we can reverse the process to produces electricity by passing heat.

However, we know it’s not possible practically. So there must be some kind of directions followed by process.

Let’s take another example to understand Second Law Thermodynamics. Take a cold can of soda and placed in a warm room. According to 1^st law, heat will be transferred from surroundings to the soda. As system is in no motion, so work done will zero and Q will be positive as system is absorbing some energy.

But we cannot say anything for reverse process, transfer of energy to room and decreases the temperature of soda can. No doubt, from various experiments and everyday life, we know that heat can only transferred spontaneously from a warm system to cold one.

Now to understand the direction of various processes, there are two statements given by Clausius and Kelvin are as follows;

1. There is no such type of process, in which heat transferred from low temperature to high temperature.
2. In any thermodynamic process, the absorption of heat from a reservoir cannot convert completely into work.

There are generally two cycles studied for 2^nd law;

1. Heat engine: Converting heat to net work.
2. Refrigerator: transfer heat from low temperature to high temperature medium.

In both cycles; there is a thermal reservoir which can supply or absorb heat without changing temperature. The thermal reservoir with high temperature is called as source and with low temperature, known as sink. In a thermodynamic cycle, source can supply heat to the system and sink can absorb heat.

A heat engine receives heat from a high temperature source and a fraction of the heat converted into work. The remaining heat rejected to a low temperature sink.

According to 1^st law of thermodynamic; Q_NET = W_NET

While from 2^nd law; Q _out>0

Efficiency of heat engine can give by; η= W_net/Q_input

or η= Q_input –Q_output /Q_input

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