Electrostatics
14 Grade 10 Physics questions on Electrostatics, with worked marking schemes. 14 questions are available on this topic in full.
Question 1
1 markA dry plastic comb is run through a learner's hair during the dry season in Kajiado and then held near small pieces of paper on the desk. The paper pieces jump up to the comb. The best explanation is that the
A. comb has lost all its electrons and become a magnet for paper
B. comb has become heated by friction and the warm air lifts the paper
C. comb has gained charge and induces opposite charge on the near side of each paper piece
D. paper pieces are naturally charged and are simply released by the comb
(1 mark)
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2 marksState the law of electrostatic charges.
(2 marks)
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3 marksExplain how a polythene rod becomes negatively charged when it is rubbed with a woollen cloth.
(3 marks)
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1 markLightning conductors fitted on tall buildings in Nairobi end in a set of sharp spikes rather than a smooth ball because
A. charge concentrates at sharp points, making discharge into the air easier
B. sharp points repel the lightning away from the building
C. sharp points reduce the weight of the conductor on the roof
D. sharp points store more charge so the building stays safe
(1 mark)
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4 marksDescribe how you would use a leaf electroscope to test whether a given rod is charged, and state how you would know.
(4 marks)
Marking scheme
1 mark: Start with an uncharged electroscope, that is, one whose leaf is closed. 1 mark: Bring the rod slowly near the cap of the electroscope without touching it. 1 mark: Observe the leaf; if the leaf diverges (rises) the rod is charged. 1 mark: If the leaf does not move at all as the rod approaches, the rod is uncharged. (Accept mention that the divergence here is by induction, so the test shows the presence of charge but not its sign.)
Question 6
3 marksOutline three practical applications of electrostatics, stating in each case how charge is used.
(3 marks)
Marking scheme
1 mark each for any three, each needing both the application and the role of charge. (i) Electrostatic spray painting: the paint droplets are charged and the metal object is oppositely charged/earthed, so the paint is attracted and spread evenly with little waste. (ii) Electrostatic precipitator in a cement or sugar factory chimney: smoke particles are charged and then attracted to oppositely charged plates, removing dust from the flue gases. (iii) Photocopier/laser printer: a charged drum attracts toner powder onto the image areas, which is then pressed onto the paper. (iv) Lightning conductor: sharp points discharge the cloud's charge safely and carry the strike to earth. (Do not credit an application named without an explanation.)
Question 7
2 marksSuggest a reason why workers refuelling farm machinery at a maize plantation in Trans Nzoia are advised not to wear nylon overalls, and state one safer alternative.
(2 marks)
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3 marksA learner charges a metal sphere and leaves it on an insulating stand in the laboratory. During the long rains the sphere loses its charge much faster than it does in the dry season. Account for this observation.
(3 marks)
Marking scheme
1 mark: During the rains the air is humid, so it contains much more water vapour. 1 mark: A thin film of moisture forms on the sphere and on the insulating stand, and moisture (with dissolved impurities) conducts. 1 mark: Charge therefore leaks away through this conducting film to the surroundings/earth, so the sphere discharges quickly; in dry air there is no such path.
Question 9
1 markA rod is brought slowly near the cap of a leaf electroscope that already carries a positive charge. The divergence of the leaf increases. The rod is
A. positively charged
B. negatively charged
C. uncharged but made of metal
D. uncharged but made of an insulator
(1 mark)
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1 markA metal sphere on an insulating stand is charged by induction using a negatively charged rod. While the rod is still held near the sphere, the sphere is earthed by touch, the finger is removed and then the rod is taken away. The sphere ends up
A. positively charged
B. charged negatively on one side and positively on the other
C. uncharged
D. negatively charged
(1 mark)
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1 markA tanker delivering petrol to a station in Nakuru is connected to the ground by a metal chain before offloading begins. The purpose of this chain is to
A. hold the tanker firmly in place while the pump is working
B. cool the fuel so that it does not evaporate during offloading
C. conduct away charge that builds up as the fuel flows, preventing a spark
D. measure how much fuel has left the tanker
(1 mark)
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1 markA glass rod is rubbed with a piece of silk cloth and becomes positively charged. This is because
A. electrons moved from the glass rod onto the silk
B. protons moved from the glass rod onto the silk
C. the glass rod created new positive charge by friction
D. protons moved from the silk onto the glass rod
(1 mark)
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1 markA hollow metal sphere mounted on an insulating stand is given a large negative charge. Where does the excess charge finally settle?
A. On the outer surface of the sphere only
B. Spread evenly throughout the metal of the sphere
C. Concentrated at the centre of the hollow space
D. On the inner surface of the sphere only
(1 mark)
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2 marksDistinguish between charging by contact and charging by induction.
(2 marks)
Marking scheme
1 mark: In charging by contact the charged body touches the conductor, and the conductor acquires the SAME sign of charge as the charging body. 1 mark: In charging by induction the charging body is only brought near (it does not touch) and the conductor is earthed, so the conductor acquires the OPPOSITE sign of charge. (Award each mark only where the candidate names both the contact/no-contact difference and the resulting sign.)
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