Observe the given figure and answer the following questions:
Arrange the given steps for purification of air, in proper sequence.
A. Attachment of electrons to dust particles giving them a net negative charge.
B. Passage of dirty air with particulate matter at low velocity.
C. Attraction of charged dust particles to grounded collection plate.
D. Release of electrons from electrode wires at several thousand volts.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B, D, A, C
Dirty air with particulate matter passes slowly between the plates (B); the electrode wires at several thousand volts release electrons (D); the electrons attach to dust particles and give them a negative charge (A); the grounded plates attract the charged dust (C).
Note: This question is from Chapter 16, Environmental Issues, of the pre-rationalised NCERT book. This chapter is not in the present NCERT book (2026-27). The extract below is from the pre-rationalised edition.
NCERT Extract (Pre-rationalised edition, Chapter 16, Environmental Issues):
Air Pollution and Its Control – “… There are several ways of removing particulate matter; the most widely used of which is the electrostatic precipitator (Figure 16.1), which can remove over 99 per cent particulate matter present in the exhaust from a thermal power plant. It has electrode wires that are maintained at several thousand volts, which produce a corona that releases electrons. These electrons attach to dust particles giving them a net negative charge. The collecting plates are grounded and attract the charged dust particles. The velocity of air between the plates must be low enough to allow the dust to fall. A scrubber (Figure 16.1) can remove gases like sulphur dioxide. In a scrubber, the exhaust is passed through a spray of water or lime.”
