Question

ASSERTION: The coefficient of viscosity of a liquid is the viscous force acting on the area of a liquid layer having unit velocity gradient perpendicular to the direction of flow.
REASON: Viscous force is directly proportional to the thickness of liquid layer and velocity gradient.
A: Both assertion and reason are correct and reason is the correct explanation of the assertion.
B: Both assertion and reason are correct and reason is not the correct explanation of the assertion.
C: Assertion is correct but the reason is incorrect.
D: Both the assertion and the reason are incorrect.

Answer 1

Hint:
To approach this question, initially we have to compare the actual definition of the concept mentioned here, to the definition given in our assertion. Only when the assertion is correct can we proceed to the reason. Further, we have to check the correctness of the reason. If it is correct then we should analyse whether the reason justifies the assertion.

Complete step by step answer:
In physics, the coefficient of viscosity is defined as the force of friction that is necessary to retain a difference of unit velocity between parallel layers of the given fluid.
Newton’s law of viscosity gives the relation between the shear stress and the shear rate of a fluid subjected to mechanical stress which acts tangentially. Coefficient of viscosity can also be defined as the ratio between shear stress and strain for a given temperature and pressure.
\[\tau =\dfrac{F}{A}=-\eta \dfrac{dZ}{dV}\]
From this equation, we can see that viscous force is proportional to thickness of the liquid layer or the area, and the velocity gradient.
Hence our assertion and reason is correct and the reason is the correct explanation for our assertion.

Therefore, option A is the correct answer among the given options.

Note:
Practical applications of viscosity include blood circulation in the body, hydraulic brakes and oil in lubricants. The Millikan experiment to find the charge of electrons is also based on viscosity.