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A cyclic process is an underlying principle for an engine. If the cycle goes counterclockwise, work is done on the system every cycle. In a cyclic process, the initial and the final points are the same. Thus, the system returns to the same thermodynamic state from which it had started. In this process, a given amount of power is converted into work. This work is again converted into heat by the use of heat cycles. In a cyclic system, the work done (W) is equal to the heat absorbed (Q) by the system, i.e., W = Q.
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What is Cyclic Process?
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Cyclic process refers to the process where the initial and the final state of a system remain the same. The cycles of power convert some quantity of heat into work or mechanical output and the pump of heat cycles transfer heat from low temperatures to high temperatures, by the use of work as mechanical input.
Cyclic Process
Examples of Cyclic Process
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Cyclic Process Formula
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In Cyclic Process, since the internal energy is a state variable, ΔU = 0, i.e., the internal change is zero. The initial and final internal energies remain equal.
By applying the first law of thermodynamics to a cyclic process, the formula we get is:
| ΔE = Q+W Where, ΔE = 0, W = Q |
Therefore, the work done by the system in the cyclic process is equal to the heat that the system absorbs. In a P-V graph, where P is on the Y-axis and V is on the X-axis, the net work involved in the cyclic process is the area enclosed in the diagram. If the cycle goes anticlockwise, then work is done on the system in every cycle. A good example is a refrigerator or an air conditioner, where the Carnot Engine undergoes a cyclic process.
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PV Diagram of Cyclic Process
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In the PV diagram, cyclic process is shown by a closed curve. Let the gas undergo a cyclic process in which it returns to the initial stage after expansion and compression as shown below:
PV Diagram of Cyclic Process
Work done in Cyclic Process
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The power of thermodynamic cycles provides a basis for the operation of all heat engines which supply most of the world’s electricity power and runs a vast majority of motor vehicles.
Diagram of Net Work done in a Cyclic Process:-
Net work done in cyclic process
Heat Engines
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A heat engine is a device that makes a system undergo the cyclic process, resulting in the conversion of heat to work. The characteristics of a heat engine are:
- A working substance forms the system of a heat engine. Example: In a gasoline or diesel engine, the working substance is formed of a mixture of vapour and fuel. Similarly, in a steam engine, the working substance is formed from steam.
- The working substance in the heat engine undergoes several processes in a cycle. In some of these processes, a total amount of heat Q1 is absorbed from an external reservoir at a high-temperature T1.
- In some other processes of the cycle, a total amount of heat Q2 is released by the system to an external reservoir at a lower temperature T2.
- The work W done by the system in the cycle is transferred to the environment by the means of some arrangement.
The cycle is repeated many times to get some useful work done. The basic discipline of heat engines is similar to the principle of thermodynamics. The mechanism of heat conversion into work varies from engine to engine.
Heat Engine Working Principle
Refrigerator and Heat Pump
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A refrigerator can be defined as the reverse of a heat engine. The working substance in a refrigerator extracts heat Q2 from a cold reservoir at a temperature of T2. External work W is done on the system and the heat Q1 is released at temperature T1 to the hot reservoir. The workflow of a refrigerator can be represented as:
Refrigerator workflow
A heat pump, on the other hand, is the same as a refrigerator. The term of the device depends on its purpose. If the purpose of the device is to cool a space, while the reservoir temperature is higher, it is called a refrigerator; if the purpose is to pump heat into space while the outside environment is cooler, it is called a heat pump. Working process of a heat pump can be represented as:
Heat pump workflow
Working of the refrigerator and heat pump is explained as:
- A sudden expansion of gas from high pressure to low pressure. This results in the cooling and conversion of the gas into a vapour-liquid mixture.
- Absorption by a cold fluid of heat from the region to be cooled. This results in conversion into vapor.
- Heating up of the vapor as a result of external work done on the system.
- Release of heat by the vapour into the surroundings, which brings it to the initial state, as a result, completing the cycle.
The coefficient of performance ‘a’ by a refrigerator is given by the formula,
a= Q2/W
In a heat engine, the heat energy cannot be fully converted into work. In the same way, a refrigerator cannot work without a minimum external work done on the system, that is, the value of the coefficient of performance cannot be infinite.
Compression Cycle
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Things to Remember
- A cyclic process is one in which the system returns to the same thermodynamic state from where the system had started.
- In a cyclic process, the work done is equal to the amount of change in heat energy in the system.
- A heat engine is a device that makes a system undergo the cyclic process, resulting in the conversion of heat to work.
- A refrigerator is a device in which the heat is removed from the substance.
- The coefficient of performance for refrigerator is given by: a= Q2/W
- Except for the ideal cycle, the energy in all other cycles cannot be fully converted into work done.
Sample Questions
Ques: What are the steps of the Cyclic Process? (1 mark)
Ans. Cyclic Process works in four main steps called Isothermal expansion, Isochoric Expansion, Isothermal Compression, and Isochoric process.
Ques: Where is the Cyclic Process seen in real life? (1 mark)
Ans. The cyclic process is seen in expansion at a constant temperature, Removal of heat at a constant temperature, Compression at a constant temperature, and Addition of heat at a constant temperature.
Ques: Give some examples for cyclic process? (1 mark)
Ans. Heat engines and refrigerators work under the principle of cyclic process.
Ques: What is the total work done in a cyclic process? (1 mark)
Ans. The work done in a cyclic process is zero as the system remains in the same state where it started by the end of the process.
Ques: Why is it called Cyclic process? (1 mark)
Ans. As the components involved in the system come back to the same state in which they started, it is called a cyclic process. In a cyclic system, the work done is always equal to the energy absorbed.
Ques: What is the Cyclic Process which is zero? (1 mark)
Ans. In every energy in a Cyclic process, the change is zero since the initial and final states are the same. The quantity of heat gained and the work done in the process are therefore denoted with opposite signs that are R=-Q.
Ques: Is the net work of a Cyclic Process positive or negative? (1 mark)
Ans. A Cyclic system has a positive loop with a larger area than the negative loop which is why the net work for each cycle is positive.
Ques: The PV diagrams for a thermodynamical system is given in the figure below. Calculate the total work done in each of the cyclic processes shown. (3 Marks)
The PV diagrams for a thermodynamical system
Ans. In case (a) the closed curve is anticlockwise. So the net work done is negative, implying that the work done on the system is greater than the work done by the system. The area under the curve BC will give work done on the gas (isobaric compression) and the area under the curve DA (work done by the system) will give the total work done by the system.
Area under the curve BC = Area of rectangle BC12 = 1 × 4= − 4J
Area under the curve DA = 1 × 2= + 2J
Net work done in cyclic process = −4 + 2= −2 J
Ques: Explain the limitations of the first law of thermodynamics with examples. (3 Marks)
Ans. The first law of thermodynamics explains well the interconvertibility of heat and work. But it does not indicate the direction of change.
Examples:
- When a hot object is in contact with a cold object, heat always flows from the hot object to the cold object but not in the reverse direction. According to the first law, it is possible for the energy to flow from hot object to a cold object or from a cold object to a hot object. But in nature, the direction of heat flow is always from higher temperature to lower temperature.
- When brakes are applied, a car stops due to friction, and the work done against friction is converted into heat. But this heat is not reconverted to the kinetic energy of the car.
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