Heat And Work Transfer In Thermodynamics

When we speak about energy in thermodynamics, it can be looked at in two ways; 

1. Energy In Transit (Heat, work, via mass flow) 

2. Energy In Storage (Internal Energy/Enthalpy)

Whenever there is a change in a system causing energy transfer, the transfer of energy into or out of the system is called the "Energy In Transit". Whenever a system has energy being transferred into or out of it, it can be of two forms, either in the form of heat, or in the form of work (or carried in/out along with mass). 

As we know, the system and its surroundings (or two different systems) interact with each other in the form of mass and energy transfer across the boundary. And "The transfer of energy between the system and the surrounding takes place in two ways, namely heat and work". 

In general, both heat and work differ in behavior. But in thermodynamics, both of these are the ways of energy transfer between the system and the surrounding. Knowing the difference between heat and work is important in the field of thermodynamics. 

Heat is the transfer of thermal energy between system and surrounding (or between two systems) Whereas on the other hand, Work is the transfer of mechanical energy between system and surrounding (or between two systems). So, we can say that heat and work are not the types of energy but are the processes related to transfer of energy. 

In this article, we will learn about these transfers of energy namely Heat and Work Transfer in details.

Ways Of Energy Transfer Between System & Surrounding 

Heat and work both appear and disappear at the system's boundary. Thus, they both are the "Transfer Phenomena". This means that they are observed when the energy crosses the boundary of the system. 

Let us now understand in detail about heat and work transfer;

Work Transfer

In Physics (Or In Practice), 

Work is performed whenever a force acts through a distance. In a simple words, "when a force is acting on an object and due to this force, the object is displaced from one place to another in the direction of the force (or may be in the opposite direction), then the product of force and displacement shows the amount of work done on the object". 

Mathematically;

Work-done ＝ force × displacement

➩ { Work-done, dW ＝ F × dz }

Where,

       F is the amount of force (or a component of the force acting along the line of displacement 'dz').

When integrated, this equation produces the function of a finite process. By convention, the value of work can be positive, negative and zero depending on the displacement of an object. 

             

Whereas In Thermodynamics;

"Work is a way of energy transfer between the system and the surrounding". A common example of work-done in thermodynamic is the compression or expansion of a fluid (or gas) in a cylinder resulting from the movement of a piston. In thermodynamic, work is given by a formula,

Work-done for expansion process is negative because the change in volume of gas confined in a cylinder is positive whereas work-done for compression process is positive because the change in volume of gas confined in a cylinder is negative.

➝ At constant volume (dV=0),

➩ W = 0

➝ At constant pressure (dP=0),

➩ W = P. (V₂-V₁)

The SI unit of work is the "Newton-meter" or "Joule". While in metric engineering system, the unit often used is "kilogram force- meter".

The above figure shows expansion work on PV-diagram where the pressure is increased from 'P₁' to 'P₂' and the volume is decreased from 'V₁' to 'V₂'. And the work-done is calculated by finding the area under the curve.

☛ In thermodynamics, the four classical types of mechanical work are -

1. Moving system boundary work.

2. Rotating shaft work.

3. Elastic work.

4. Surface tension work

Heat Transfer

In Practice, 

"Heat is a form of energy that is believed to be stored within a system". It tells us whether a system (or body) is hot and cold. This means that it measures the hotness and coldness of a system (or a body). The more heat present within a system, the hotter it is and the less heat present inside the system, the cooler it is. 

Heat is regarded as the temperature of the system.

"When a hot object comes in contact with a cold object, the hot object cools down and the cold object becomes hot. A reasonable view is that something transfers from a hot object to a colder object and we call that something heat". 

Thus, heat always transfers from an object of higher temperature to an object of lower temperature. Heat transfer is proportional to the temperature difference between two objects.

In our day to day activities, we see heat flow taking place surrounding us. Let us understand this with an examples of Hot Coffee.....

you can see that cup also gets heated when hot coffee is poured into it. This happens because the heat is transferred from the inner side of the cup to the outer side of the cup.

While In Thermodynamics

"Heat is the way of energy transfer between the system and surrounding (or between two systems)". 

It is never regarded as being stored within the system. The transfer of energy between the system and its surroundings in the form of heat occurs only when there is a difference in the temperature of the system and its surroundings.

➝ Heat transfer to a system – Increases the energy of system

➝ Heat transfer by a system – Decreases the energy of system

When the driving force between the system and the surrounding is the temperature difference, energy is transferred between them in the form of heat. And when the driving force between the system and the surrounding is something other than the temperature difference, the energy is transferred between them in the form of work. 

Sign Conventions

During the calculation of heat energy transfer and work energy transfer, we must understand the sign conventions used for heat and work energy transfer in thermodynamics.

Let us see here the sign conventions used for heat energy and work energy transfer...............

(1)           (Expansion work)

If work is done by the system over its surrounding, quantity of work energy transferred will be considered as positive.

             (Compression work)

If work is done over the system from its surrounding, quantity of work energy transferred will be considered as positive.

(2) If heat energy is added to the system from surrounding, quantity of heat energy transferred will be considered as positive. If heat energy is rejected from the system to the surrounding, quantity of heat energy transferred will be considered as negative.

Hope you have found this article helpful!!

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