Question

Fill in the blank, Thermodynamics is the branch of science concerned with ________ and _________, and their relation to energy and work.
A) Heat, temperature
B) Temperature, pressure
C) Heat, volume
D) Volume, pressure.

Answer 1

Thermodynamics is a branch of science in which we study the relationship between heat and other forms of energy. Heat is a form of energy and by the first law of thermodynamics energy can be changed from one form to another but it cannot be created or destroyed. To study this change from one form to another we divide the universe into two parts, namely, system and surrounding. The system refers to that part of the universe in which observations are made and the remaining part constitutes the surrounding.

Complete solution:
Step 1: The first term is heat. Thermodynamics deals with the energy changes when we observe a particular system. Energy may be in the form of heat, electrical energy, mechanical energy, etc. since heat is a form of energy therefore thermodynamics is concerned with heat. For example, if we take some water in a test tube and add some amount of sodium, it releases a very much amount of heat.
Here the test tube containing water and sodium is a system and other than that everything is surrounding. The heat released from the system to the surrounding was the chemical energy which is changed into heat. The internal energy of a system can be changed by doing some work on it or by it. For example, if we take water in a closed thermal beaker which is not allowing any amount of heat to be transferred from the system to the surrounding or surrounding to the system.
Now if we do some mechanical work by rotating a set of small paddles and thereby churning water. It will change the temperature of the system and consequently the internal energy. We can do electrical work on the system by an immersion rod that will also change the temperature of the water and the internal energy will also be changed.

Step 2: The second term is temperature. Temperature is a scalar quantity. It helps us to distinguish whether something is hot or cold. Hot and cold are relative terms. By the second law of thermodynamics, heat will always flow spontaneously from hotter substances to colder ones. In the first step we saw that when we do some work on the system or some work is done by the system, this process changes the internal energy of the system. This internal change in energy is because of the heat flow.
The immersion rod in the example above was converting electrical energy into heat and when the rod was hotter than the water, the heat started to flow from rod to water. This changed the temperature of the water and consequently its internal energy. Therefore temperature is also what we study deeply in thermodynamics.

Hence, the correct answer is option A.

Note: In a thermodynamic closed system if we change volume and pressure it will not change the internal energy of the system. Although pressure and volume both are directly related to temperature, if we change volume or pressure in a system the transfer of energy from the system to the surrounding or vice-versa does not take place.