In air pollution control for large industrial cities, dust and smoke particles suspended in air can be effectively removed in electrostatic precipitators by using which physical principle?

Introduction / Context:
This question comes from colloid chemistry and environmental engineering. It asks how dust particles in the air of large industrial cities are removed. Many power plants and factories use electrostatic precipitators to clean flue gases before they are released, and these devices rely on the principle of electrophoresis to separate charged particles from the gas stream.

Given Data / Assumptions:

• Dust and smoke particles are dispersed in air, forming an aerosol or colloidal system.
• We want a method used on a large scale in industrial settings.
• Options include sedimentation, electrophoresis, Tyndall effect, Brownian movement and simple filtration concepts.
• We assume modern practices such as using electrostatic precipitators in chimneys and exhaust systems.

Concept / Approach:
Electrophoresis is the movement of charged particles under the influence of an electric field. In electrostatic precipitators, dust and smoke particles in flue gas are given an electric charge by passing the gas between charged electrodes. The charged particles then migrate towards oppositely charged plates and stick to them. This removes particles from the gas stream, which can then be released much cleaner. Sedimentation and Brownian motion are natural processes but are too slow or random for efficient industrial dust removal, and the Tyndall effect only explains light scattering, not removal.

Step-by-Step Solution:
1) Recognise that the problem involves separating fine solid particles (dust, smoke) from a gas, which is often treated as a colloidal dispersion.2) In electrostatic precipitators, gas passes between wires and plates at high voltage, imparting charge to the dust particles.3) Once charged, these particles experience a force in the electric field and move (undergo electrophoresis) towards the oppositely charged collecting plates.4) The particles adhere to the plates, forming a layer that can later be removed mechanically.5) This process does not depend primarily on sedimentation or Brownian motion, but on directed movement of charged particles in an electric field, which is electrophoresis.6) Therefore, among the choices, electrophoresis best describes the principle used to remove dust particles from industrial exhaust air.

Verification / Alternative check:
Descriptions of large power station chimneys and industrial flue gas cleaning systems commonly mention electrostatic precipitators. These devices are specifically designed to utilise high voltage fields to charge and collect particles. The operating principle is explicitly described as the motion of charged particles in an electric field, that is, electrophoresis. In contrast, sedimentation and Brownian movement are not controllable enough for efficient large scale cleaning, and the Tyndall effect is just an optical phenomenon showing the presence of colloidal particles, not a removal method.

Why Other Options Are Wrong:
Sedimentation under gravity alone: Fine dust and smoke particles are too small and light to settle quickly by gravity in tall chimneys; the process would be inefficient.
Tyndall effect (light scattering): This effect helps detect colloidal particles by scattering light but does not help remove them.
Brownian movement of gas molecules: Brownian motion is random movement due to collisions with gas molecules and does not lead to systematic removal.
Simple filtration through cloth bags only: While bag filters are used in some systems, the classic answer for large industrial dust removal in the context of colloid chemistry is electrophoresis in electrostatic precipitators.

Common Pitfalls:
Students may choose Tyndall effect because it is a well known colloid phenomenon, but it is only an optical test. Others may think of sedimentation because it is used in water treatment, but air dust particles are too small. A good approach is to link the word industrial chimneys with electrostatic precipitators and remember that these devices rely on applying high voltage and using charged plates, which is clearly related to electrophoresis. Whenever dust removal in industrial gases is mentioned in colloid chemistry, electrophoresis is the key principle.

Final Answer:
Dust particles in the air of large industrial cities are removed in electrostatic precipitators by electrophoresis in an electric field.