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An Introduction To Thermodynamics

Sep 28,2017 by Edulab

Thermodynamics is the study of the behaviour of heat and thermal energy. Energy is the ability to bring about change or to do work. Historically, thermodynamics originated as a result of man’s endeavour to convert heat into work. In its simplest form, where P equals power, mgH equals work and t equals time, we have the following equation:

P = mgH/t

A thermodynamic system is one that interacts and exchanges energy with the area around it. If a thermodynamic system is in equilibrium, it can’t change its state or status without interacting with its environment.

Around 1850 Rudolf Clausius and William Thomson stated both the First and the Second Laws of Thermodynamics. Generally discussed prior to the First and Second Laws however, is the Zeroth Law. Although stated after the First and Second Laws, its importance dictates its position at the top spot of the list of the Laws.

The Four Laws of Thermodynamics

The Zeroth Law

This states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

More simply put, if systems one and two are each in equilibrium with system three, they, therefore, each have the same energy content as system three. If that is the case, then the values found in system three must match those in both systems one and two. Therefore, the values of one and two must also match, meaning that one and two have to be in equilibrium with each other.

The First Law

The first law is a little simpler. It states that when heat is added to a system, some of that energy stays in the system and some leaves the system. Energy can neither be created nor destroyed. It can only change forms. The energy that leaves the system interacts with the area around it. Energy that stays in the system creates an increase in the internal energy of the system.

For example, if you have a pot of water at room temperature and add some heat to it, firstly, the temperature and energy of the water increases. Secondly, the system releases some energy and it interacts with the environment around it. Possibly heating the air around the water and making the air rise.

For a thermodynamic cycle, the net heat supplied to the system equals the net work done by the system.

The Second Law

explains that it is impossible to have a cyclic process that converts heat completely into work. This means that no reaction is 100% efficient. Some amount of energy in a reaction is always lost to heat. Similarly, a system cannot convert all of its energy to working energy.

It is also impossible to have a process that transfers heat from cool objects to warm objects without using work. A cold body can’t heat up a warm body. Heat naturally wants to flow from warmer to cooler areas. Heat wants to flow and spread out to areas with less heat. If heat is going to move from cooler to warmer areas, the system must put in some work for it to happen.

The Third Law

Entropy is the measure of the random activity in a system. By random, it means energy that can’t be used for any work. The Third Law of thermodynamics states that the entropy of an object approaches to a constant as its temperature approaches to absolute zero.

Examples Of The Laws In Everyday Life

The laws of thermodynamics don’t exist in isolation. We can witness them in action every day. For example, on a hot day, someone might take an ice cube from the freezer to keep a drink cool. In doing so, they’ll witness the First and Second Laws of Thermodynamics.

Ice needs to be maintained at a temperature below the freezing point of water to remain solid. When an ice cube is put into a glass of lemonade, after a while, the ice will melt but the temperature of the lemonade will cool. The total amount of heat in the thermodynamic system has remained the same, but it has gravitated towards equilibrium. The ice cube, which is now water and the lemonade are the same temperature. This system isn’t completely closed however. The lemonade will eventually warm up again, as heat from the environment is transferred to the glass and its contents.

Similarly, the human body also obeys the laws of thermodynamics. Consider the experience of being in a small, crowded room, surrounded by lots of other people. In all likelihood, you’ll start to feel very warm and will start sweating. This is the process your body uses to cool itself down. Heat from your body is transferred to your sweat. As your sweat absorbs more and more heat, it evaporates from your body, becoming more disordered and transferring heat to the air. This, in turn, heats up the air temperature of the room. This is an example of both the First and Second Laws of Thermodynamics in action. No heat is lost. It is merely transferred and approaches equilibrium with maximum entropy.