Frequently asked intresting facts - 1

Hey friends, this blog does not contain any sequel of my lectures. It only contains answers of those questions which people asks from me very frequently. These are basically intresting facts and natural phenomenon which we observe in our daily life. So some of them are as follows-
1) wheather Carnot efficiency is 100% efficiency or not?
Most of the students are really confused with this. They consider that Carnot efficiency is always 100%, but it's not true. Actually Carnot efficiency can never be 100% for any particular system, 100% efficiency never exist, even in our theories also because concept of 100% efficiency violates Kelvin Planck statement of second law of thermodynamics. Kelvin planck states that it is impossible to construct an engine which exchange heat from only one heat source. Below is the diagram-

Above shown machine is a perpetual motion machine of second kind (PMM 2), which converts all the heat into work. This machine have 100% efficiency. So remember, no machine can have 100% efficiency because it violates second law of thermodynamics.
Now Carnot efficiency, it is the best possible efficiency we can get in a machine. Practically it is also not possible but concept of Carnot do not violates any law of thermodynamics. Let me explain carnot efficiency, let us consider an practical working engine ofcourse which will be Irreversible, as shown in below figure-

it's efficiency comes out to be 9.5%

Now if we consider this same engine to be reversible and calculate it's efficiency then we get

Now this efficiency is its maximum possible efficiency or we can say Carnot efficiency.
From here we can conclude that performance of above engine is poor because it's actual efficiency is far less than it's maximum possible efficiency, so this engine needs much improvement.
One more reason is there that why Carnot efficiency can never be 100% is described in next question 2.

2) Zero Kelvin temperature is possible or not - 
Temperature is a parameter which represents average random energy per unit molecule. If value of temperature reduces to 0 kelvin, than it means that there is no random energy per unit molecule so there is no random motion between molecules. Zero Kelvin temperature is not possible because at zero Kelvin temperature random motion of molecules get arrested totally. We can also say that when no motion of molecules is possible than only zero Kelvin temperature is achievable. But it's impossible to completely stop the random motion of molecules.
Existence of zero Kelvin temperature also violates second law of thermodynamics, Let us see how-
Assume for an instance that zero Kelvin temperature is possible and as we know it is minimum possible temperature. Now take a Carnot engine which is rejecting heat to the reservoir having zero Kelvin temperature. Now calculate it's efficiency-

Efficiency is 100% means it is violating Kelvin-planck statement. In above question1  I have explain that Carnot efficiency can never be 100%. Even 100% efficiency does not exist.
3) why water drops and bubbles are always spherical in shape?
For better understanding of this concept you guys can check my Lecture-8

4) What is difference between drag force, viscous force and lift force?
Viscous force is basically a friction which acts between 2 different layer of same fluid. Viscous force is the reason behind existence of viscosity. For detailed explanation of viscosity you guys can check my Lecture - 4.
Drag force is defined as resistance offered by fluid on the body which is in contact with that fluid. Basically drag force is friction act on the body by the fluid. Let me explain you with the help of example, consider a submarine moving undersea, here the resistance acting on submarine by the sea water is basically drag force. Direction of drag force is in the direction of flow of fluid.
Now let us understand lift force with the help of an example. Consider a aerofoil shape like wing of any airplane or blade of windmills. Cross section area of that wing is shown in below figure.

Upper camber is generally covered with teflon coating so that it become very smooth. Whereas lower camber is generally rough surface. The purpose for this is to make the air flow at faster rate on upper camber as compare to lower camber. When on upper camber airflow at faster rate then pressure will decrease, whereas on lower camber airflow at comparatively slower rate then pressure will increase. Due to this difference in pressure, a force will generate which act perpendicular to the direction of air flow and this force will be responsible for the lift of the aerofoil shape therefore it is called as lift force.

For having better grip on this topic, to know how pressure varries with change in velocity and for Practical example of generation of lift force check out my Lecture 10.