Cyclic Process

Cyclic Process 

The term “cyclic process” refers to a process in which a system’s initial and final states are the same.

The power cycles turn a certain amount of heat into mechanical output, while the heat pump cycles use work as a mechanical input to move heat from low to high temperatures.

Formula for Cyclic Process

Because the internal energy is a state variable in the cyclic process,

 ∆U=0,

 i.e., the internal change is zero. Internal energies are equal at the start and at the end.

When we apply the first law of thermodynamics to a cyclic process, we get the following formula:

∆Q=∆U+W 

Since, ∆U=0

∆Q=W 

As a result, the work done by the system in the cyclic process is equal to the heat absorbed by the system. The total work done in the cyclic process is the area encompassed in a P-V graph, where P is on the Y-axis and V is on the X-axis. If the cycle is anti-clockwise, work on the system is done every cycle. The Carnot Engine in a refrigerator or air conditioner, for example, goes through a cyclic process.

PV diagram for a Cyclic Process

A closed curve represents the cyclic process in the PV diagram.

Allow the gas to proceed through a cyclic process, returning to its original state after expansion and compression, as depicted in Figure below:

Let W1 represent the work done by the gas during expansion from V1 to V2. It is the same as the area under the CBA graph, as shown in Figures a and b below:

Figure (a): W for path CBA

Figure (b): W for path ADC

Let W2  to represent the work done on the gas during compression from V2 to V1. It’s the same as the area under the ADC graph in Figure (b)

As illustrated in Figure below, the total work done in this cyclic process is

 W1-W2 i.e., shaded area in the loop:

Heat Engines

A heat engine is a device that causes a system to go through a cyclic process, converting heat to work. A heat engine has the following characteristics:

• A heat engine’s system is made up of a working substance. In a gasoline or diesel engine, for example, the working substance is a mixture of vapour and fuel. Similarly, steam is used to create the working substance in a steam engine.

• In a cycle, the heat engine’s working ingredient goes through numerous steps. A total amount of heat Q1 is absorbed from an external reservoir at a high temperature T1 in some of these processes.

• In some other cycle operations, the system releases a total amount of heat Q2 to an external reservoir at a lower temperature T2.

• Through some mechanism, the work W done by the system in the cycle is transmitted to the environment.

The cycle is repeated several times in order to complete some meaningful activity for a certain objective. The fundamental science of heat engines is akin to thermodynamics. The method for converting heat into work differs from one engine to the next.

Refrigerator & Heat Pump

A refrigerator is the opposite of a heat engine. At a temperature of T2, the working substance in a refrigerator extracts heat Q2 from a cold reservoir. External work W is performed on the system, and heat Q1 is delivered to the hot reservoir at temperature T1 . 

On the other hand, a heat pump is similar to a refrigerator. The device’s lifespan is determined by its function. It’s named a refrigerator if the device’s purpose is to cool a place while the reservoir temperature is higher; it’s dubbed a heat pump if the device’s purpose is to pump heat into space while the outside environment is cooler. 

Conclusion

The system may convert heat from a warm source into productive work and even dispose of the remaining heat to a cold sink during the entire process of going through a working fluid cycle. As a result, it acts as a heat engine. The basic or major alternative is to reverse the cycle and utilise labour to transport heat from a cold source to a warm sink, so operating as a heat pump.

We can suppose that the system is in thermodynamic equilibrium at every point in the cycle. As entropy is a state function, we can conclude that the cycle is reversible, and that its entropy change is zero. The system returns to its initial thermodynamic condition of pressure and temperature after a cycle is closed. Work and heat, for example, are process-dependent quantities or path quantities.