Case Study:

In many thermal power plants and cement factories, thick smoke containing fine dust particles is released during combustion. Engineers observed that when an electrostatic precipitator is switched on, the smoke leaving the chimney becomes significantly cleaner. In a study, dust particles of mass 2 x 10^-12 kg were found to acquire a negative charge of  3 x 10^-6 C when exposed to high voltage electrodes. These charged particles then moved toward large positively charged plates placed inside the device.

The plates created a uniform electric field of 2 x 10⁴ N/C. Due to the electric force acting on them, the particles accelerated toward the plates and got deposited, reducing air pollution. However, engineers noticed that some particles were not collected efficiently when their charge was too small or when the electric field strength decreased.

To visualize how these invisible forces actually work, you might find it helpful to look at these simple visual concepts of electric charges and fields before diving into the math.

This raised an important question: how do Coulomb’s Law, the electric field, and the electric force determine the motion and collection efficiency of these microscopic particles? Can adjusting the electric field strength improve pollution control?

While we focus on single particles here, remember that many pollutants behave like dipoles; make sure you've mastered the electric dipole derivations and common mistakes to avoid losing marks on similar board questions.

Ready to test your knowledge? Beyond this specific case, here is a collection of the most important electric field questions frequently seen in final exams.

CASE-BASED QUESTIONS

MCQ

Q1.  The force acting on a charged dust particle in an electric field depends on:
A. Only the mass of the particle
B. Only the electric field strength
C. Charge of particle and electric field strength
D. Distance between plates only

Q2. If the electric field inside the precipitator is doubled, the force on a dust particle will:
A. Remain unchanged
B. Become half
C. Double
D. Become four times

Assertion - Reason

Q3.  Assertion (A): Charged dust particles move toward the collecting plates in an electrostatic precipitator.
Reason (R): An electric field exerts a force on a charged particle.

A. Both A and R are true, and R is the correct explanation of A
B. Both A and R are true, but R is not the correct explanation
C. A is true, R is false
D. A is false, R is true

Application-Based

Q4. Calculate the electric force acting on a dust particle with charge 3 x 10^-6 C placed in an electric field of 2 x 10⁴ N/C.

Q5. Explain why particles with very small charges are not effectively collected in the precipitator.

Data/Logic-Based

Q6. If a particle of mass 2 x 10^-12 kg experiences the force calculated above, determine its acceleration.

ANSWER KEY WITH EXPLANATION

A1.C - Electric force is given by (F = qE). It depends on both charge and electric field strength.

A2. C - Since (F = qE), doubling (E) doubles the force.

A3. A -  Electric field exerts force on charges, causing motion toward oppositely charged plates.

A4. F =qE
       F = (3 x 10^-16)  (2 x 10⁴)
       F = 6 x 10-¹² N
Force depends directly on charge and electric field.

A5. Particles with a small charge experience a very weak electric force (F = qE), so they do not move effectively toward plates.

A6. a = F/m
        =  6 x 10^-12 / 2 x 10^-12 
           = 3  m/s² 
Acceleration is determined using Newton’s second law.

Mastered this case? Keep the momentum going with these targeted practice materials
 

• Daily Practice: Grab this Class 12 Physics Worksheet to sharpen your basics.
• Self-Assessment: Try this Unsolved Practice Paper to see where you stand.
• Exam Readiness: Review the Solved Practice Paper to understand the ideal way to present your answers to examiners.

HOTS EXTENSION QUESTIONS

1. If the electric field is non-uniform, how would the motion of charged particles change inside the precipitator? Predict the impact on efficiency.

2. Suggest how increasing plate area or voltage could optimize particle collection without increasing energy consumption excessively.

Need More Help?

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