Question

In the sedimentation process, the rate of settling (moving) particles is affected by
[A.] Gravitational pull 
[B.] Density of the particle 
[C.] Buoyancy force 
[D.] Size of the particle 
Choose the most appropriate answer from the options given below:

• A. A, B, C and D
• B. A, B and C only
• C. A, B and D only
• D. B, C and D only

Hint:

Think of Stokes' Law. Settling is a battle between gravity pulling a particle down and buoyancy/drag holding it up. Gravity depends on mass (size and density), buoyancy depends on fluid density, and drag depends on size and fluid viscosity. All listed factors are involved.

Answer 1

The Correct Option is A

Step 1: Recall the principles of sedimentation, governed by Stokes' Law for particles in the laminar flow regime. The terminal settling velocity ($v_t$) is given by: \[ v_t = \frac{g D_p^2 (\rho_p - \rho_f)}{18 \mu} \]
Step 2: Analyze how each factor in the options relates to the formula.
A. Gravitational pull (g): This is the primary driving force for settling and is present in the formula.
B. Density of the particle ($\rho_p$): The settling rate is proportional to the density difference between the particle and the fluid. Particle density is a key part of this.
C. Buoyancy force: This upward force is represented by the density of the fluid ($\rho_f$). The net downward force depends on the difference between particle density and fluid density ($\rho_p - \rho_f$). So buoyancy is crucial.
D. Size of the particle ($D_p$): The velocity is proportional to the square of the particle diameter. Larger particles settle much faster.
All four factors are fundamentally important in determining the settling rate.