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: Remember the sedimentation principles, particularly Stokes' Law for laminar flow. The terminal settling velocity ($v_t$) is defined as:\[ v_t = \frac{g D_p^2 (\rho_p - \rho_f)}{18 \mu} \]
Step 2: Examine the connection of each factor to the settling velocity formula.
A. Gravitational pull (g): This force initiates settling, as shown in the formula.
B. Particle density ($\rho_p$): The settling rate increases with the difference between particle and fluid densities. Particle density is therefore significant.
C. Buoyancy force: This force counteracts gravity and is reflected by the fluid density ($\rho_f$). The effective downward force depends on ($\rho_p - \rho_f$), highlighting buoyancy's importance.
D. Particle size ($D_p$): Settling velocity scales with the square of the particle diameter, meaning larger particles settle considerably faster.
Consequently, gravitational pull, particle density, buoyancy, and particle size all significantly influence the settling rate.