Static Electricity Question

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1. The Figure below shows a method of producing sandpaper using static electricity. Glue is sprayed from spray gun onto a moving strip of paper. The glue droplets become negatively charged by friction once they leave the nozzle. The sticky paper passes between two metal plates. Sand moving on a conveyor belt also passes between the metal plates.

                                    

(a) Explain how the glue droplets become negatively charged by friction.
(b) State and explain the advantage of having all the droplets negatively charged.
(c)(i) On the Figure, draw the charge distribution on the sand.
    (ii) Explain why the sand moves towards the sticky paper.

(a) Analysis:
Method: charging by friction => insulators
Process: transfer of electrons from one object to another
Droplets become negatively charged => it gains electron.

Answer:
Transfer of electrons from nozzle to glue droplets due to charging by friction.
Since glue droplets now have excess electrons, they become negatively charged.
(b) Since the droplets are negatively charged, they repel each other hence the glue droplets are evenly spread out. Thus, evenly coated / uniform layer of coating on the paper.
(c)(i) Analysis:
Sand is on the conveyor belt which is near to the positively charged metal plate, so the electron in the sand will be attracted to the bottom and left behind the positive charged on the top as the Earthed metal plate is on the top. So you should be able to draw the charge distribution as shown below

                                    

    (ii) When sand on the conveyor belt passes between the metal plates, electrons would be attracted to the bottom of the sand and positive charges remain at the top. Once the negatively charged sticky paper pass-by, the sand will be attracted to the paper as unlike charges attract.

2. To clean the air in a special room where smoking is allowed, an air cleaner is used. The principles of operation of the cleaner are shown below.

                                    

(a) Explain how the cleaner removes particles of smoke (some of which are positively charged and some of which of which are negatively charged) from the air.
      (i) Action of positively charged grid
      (ii) Action of negatively charged grid
      (iii) Action of fan
(b) In practice, there is a battery connected between the two charged grids.
      (i) Explain why is this needed.
      (ii) State the terminals of the battery which should be connected to the grids.

(a)(i) Positively charged grid attracts the negatively charged particles.
      (ii) Negatively charged grid attracts the positively charged particles.
      (iii) Rotating fan creates a pressure difference for the air to flow so that the dust and smoke particles can be removed as it flows from a higher pressure region.
(b)(i) As the grids are constantly discharged by the smoke particles, the battery is used to charge them up.
      (ii) Positively charged grid is connected to the positive terminal.
Negative charged grid is connected to the negative terminal.

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