Thermodynamics Tricky Questions

Thermodynamics is one of the most dreaded topics by engineering aspirants. There are so many formulae, laws and theories promulgated by different scientists which makes this topic lengthy and complex. JEE Main and JEE Advanced aspirants need to have good command over thermodynamics as it is a very important and high scoring topic of Physics. Although there are different ways to approach the topic, one of the most effective ways is to try and solve tricky questions and assess one’s command over the topic.

Thermodynamics covers the relationship between temperature, heat, work and energy. The students who intend to score good marks in this section must have good understanding of these four first. Any lag or gap in any of these individual topics may prove to be a roadblock while studying thermodynamics. This is why thermodynamics is not only a lengthy topic but tricky as well!

What is Thermodynamics?

Thermodynamics is the study of the dynamics of heat, temperature, work and energy and the interactions between these quantities in a system. In short, heat is a form of energy, and this energy is equivalent to a definite amount of mechanical (physical movement or displacement of a body) work!

There are four Laws of Thermodynamics which summarise the whole concept of relationship between work, energy, heat and temperature. These are:

Zeroth Law: Two systems are considered to be in thermal equilibrium if they are individually in equilibrium with a third system. Practical Application: A Thermometer

First Law: This is also known as the Law of Conservation of Energy, which states that the change in internal energy of a system is equal to the work done by the system minus the heat added to it. Practical Application: Heat Engine

Second Law: Heat flows from a body at higher temperature to a body at lower temperature. This results in the change of entropy of the system. In the process, some energy loss takes place. Practical Application: Refrigerators

Third Law: This law allows for heat capacity measurements. The law states that the entropy of a perfect crystal (pure substance) approaches zero at absolute zero temperature.

And…Why are Some Questions of Thermodynamics Tricky?

The questions of Thermodynamics are tricky for obvious reasons. There are so many factors and variables at play when we talk about a system of bodies. Then there are different quantities like entropy, temperature, work, energy and its types, etc. which are inter-related. Since every law of thermodynamics has different applications and derivations, it is again tricky to solve the questions!

Some examples of complex questions of thermodynamics are:

Example 1:

A thermally insulating cylinder has a thermally insulating and frictionless movable partition in the middle, as shown in the figure below:

On each side of the partition, there is one mole of an ideal gas, with specific heat at constant volume, ? = 2?. Here, ? is the gas constant. Initially, each side has a volume ?0 and temperature ?o . The left side has an electric heater, which is turned on at very low power to transfer heat ? to the gas on the left side. As a result the partition moves slowly towards the right reducing the right side volume to ?0/2. Consequently, the gas temperatures on the left and the right sides become ?l and ?r, respectively. Ignore the changes in the temperatures of the cylinder, heater and the partition, Find the value of Tr/To and Q/RTo

Example 2:

Internal Energy of the thermodynamics system is given by which of the following formula:

U=∫TdS−PdV
U=∫TdS−PdV-∑iμidNi
U=∫TdS−PdV+∑iμidNi
U=∫TdS+PdV+∑iμidNi

So, How can I Solve These Questions?

The right approach to a problem is the way forward to solve tricky thermodynamics questions. But it is not easy to get through the topic and understand each and every concept in a limited time. One of the best ways is to practise as many questions as you can and try to solve a variety of questions of varying difficulty levels!

Stay tuned to Embibe to know more!