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7. Principles of Thermodynamics
The laws of thermodynamics are simple to state. Do you know that the human body obeys the laws of thermodynamics? We start to sweat and feel warm when we’re in a room full of people and the sweating becomes excessive if the room size is small. This happens because your body is trying to cool off hence heat transfers from your body in the form of ‘sweat’. This entails the first law of thermodynamics. Interesting? Let us study more.
If two thermodynamic systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other. This is named zeroth law because it is basic law that is formulated after the three laws are formulated. The above is the actual statement for Zeroth law of Thermodynamics. This can be understood as follows:
When two bodies, of which one is at a higher temperature than the other, are kept in contact, heat energy flows from the one at the higher temperature to the one at lower temperature.
The first law of thermodynamics states that the total energy of an isolated system is constant. Energy can be transformed from one form to another, but can neither be created nor destroyed.
According to this law, some of the heat given to system is used to change the internal energy while the rest in doing work by the system. Mathematically, ΔQ=ΔU+ΔW, where,
- ΔQ = Heat supplied to the system
- ΔW = Work done by the system.
- ΔU = Change in the internal energy of the system.
If Q is positive, then there is a net heat transfer into the system, if W is positive, then there is work done by the system. So positive Q adds energy to the system and positive W takes energy from the system. It can also be represented as ΔU=ΔQ−W.
Limitations of First Law of Thermodynamics
- The limitation of the first law of thermodynamics is that it does not say anything about the direction of flow of heat.
- It does not say anything whether the process is a spontaneous process or not.
- The reverse process is not possible. In actual practice, the heat doesn’t convert completely into work. If it would have been possible to convert the whole heat into work, then we could drive ships across the ocean by extracting heat from the water of the ocean.
The first law of thermodynamics applied to metabolism. Heat transferred out of the body (Q) and work done by the body (W) remove internal energy, while food intake replaces it. (Food intake may be considered as work done on the body.)
Plants convert part of the radiant heat transfer in sunlight to stored chemical energy, a process called photosynthesis.
If you ever drop a glass and watch it shatter, you know there is no way of going back in time and getting back the unbroken glass. This is irreversibility. The second law of thermodynamics states that the heat energy cannot transfer from a body at a lower temperature to a body at a higher temperature without the addition of energy. This is why running an air conditioner for a long period of time, costs you money.
Total conversion of heat into work is not possible i.e 100% efficiency is not possible as it will lead to a negative change in entropy of universe which is not valid according to the Second Law of Thermodynamics. No heat engine can be 100% efficient
Let us see what is entropy, and its relation to the second law of thermodynamics. The entropy of the system is measured in terms of the changes the system has undergone from the previous state to the final state. Thus the entropy is always measured as the change in entropy of the system denoted by ∆S. If at all it is necessary to measure the value of the entropy at a particular state of the system, then zero value of entropy is assigned to the previously chosen state of the system.
Causes of increase in entropy of the closed system are:
- In a closed system, the mass of the system remains constant but it can exchange the heat with surroundings. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system.
- Due to internal changes in the movements of the molecules of the system, there is disturbance inside the system. This causes irreversibilities inside the system and an increase in its entropy.
The Third law of thermodynamics is sometimes stated as follows, regarding the properties of closed systems in thermodynamic equilibrium: The entropy of a system approaches a constant value as its temperature approaches absolute zero.
This constant value cannot depend on any other parameters characterizing the closed system, such as pressure or applied magnetic field. At absolute zero (zero kelvin) the system must be in a state with the minimum possible energy. Entropy is related to the number of accessible microstates, and there is typically one unique state (called the ground state) with minimum Energy. In such a case, the entropy at absolute zero will be exactly zero. If the system does not have a well-defined order (if its order is glassy, for example), then there may remain some finite entropy as the system is brought to very low temperatures, either because the system becomes locked into a configuration with non-minimal energy or because the minimum energy state is non-unique. The constant value is called the residual entropy of the system.
Do you know what a Carnot engine is? It’s an engine that is found in your refrigerators and air conditioners. It involves two reversible isothermal transitions and two reversible adiabatic transitions.
The Carnot cycle is reversible representing the upper limit on the efficiency of an engine cycle. Practical engine cycles are irreversible and thus have inherently lower efficiency than the Carnot efficiency when operating at the same temperatures. One of the factors determining efficiency is the addition of to the working fluid in the cycle and its removal. The Carnot cycle achieves maximum efficiency because all the heat is added to the working fluid at the maximum temperature.
The Carnot engine cycle when acting as a heat engine consists of the following steps:
- Reversible isothermal expansion of the gas at the “hot” temperature.
- Isentropic (reversible adiabatic) expansion of the gas.
- Reversible isothermal compression of the gas at the “cold” temperature.
- Isentropic compression of the gas.