Question

Assertion : Reynold’s number is used to check whether the flow is laminar or turbulent.
Reason : Poiseuille’s law is valid only in conditions of laminar flow.
A. Assertion and reason both are correct and reason is the correct explanation for assertion.
B. Assertion and reason both are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

Hint: Reynold’s number is defined as the ratio of the internal forces and viscous forces of the flowing fluid. The value of the Reynold’s number determines the type of the flow of the fluid. Poiseuille’s law tells us about the factors affecting the laminar flow of a fluid.

Complete step by step answer:
Reynold’s number is a number used to understand the motion or characteristic of a flowing fluid. Reynold’s number is defined as the ratio of the internal forces and viscous forces of the flowing fluid.If the value of the Reynold’s number for the flowing fluid is less than 2100, then the flow of the fluid is laminar. If the value of the Reynold’s number for the flowing fluid is more than 4000, then the flow of the fluid is turbulent.Therefore, we can say that Reynold’s number is used to check whether the flow is laminar or turbulent.Hence, the given assertion is correct.

Let us now check the reason. Poiseuille’s law tells us about the factors affecting the laminar flow of a fluid.According to the law, the flow rate of a fluid flowing in a tube is directly proportional to pressure difference between two points and the radius of the tube. It is inversely proportional to length of the tube and the viscosity of the fluid.However, the Poiseuille’s law is only valid for laminar flow. Hence, the given reason is correct. But the reason is not the correct explanation for the assertion.

Therefore, the correct option is B.

Note:Since Reynold’s is a ratio of forces, it is a dimensional quantity. Therefore, Reynold’s number is just a number with no unit.When we say that the flow of the fluid is laminar, it means that it is flowing uniformly and smoothly.When we say that the flow of the fluid is turbulent, it means that it is flowing irregularly. It looks like the layers of the fluid are mixing with each other.