Dec 1989 P2 Q5
In the figure below, L is a large conducting sphere connected through a switch to the positive terminal of a supply of high voltage. A light conducting sphere S hangs from an insulating thread as shown. Both spheres are initially uncharged.
(a) Describe the movement of electrons which takes place when the switch is closed and sphere L becomes charged.
(b) Describe and explain the effect produced by the charge on sphere L on the conducting sphere S.
(c) Sphere S is now connected to earth. Describe the effect this has on (i) the charge on S, (ii) the position of S.
(d) Describe an experiment you could carry out to identify the sign of any charge remaining on S after the earth connection has been removed.
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Answer:
(a) Electrons from the sphere L moves to the positive of the high voltage supply and the sphere becomes positively charged.
(b) Some electrons from the farther end of S moves to the nearer end, causing negative charge at the nearer end. The sphere S is attracted towards L and comes to the position shown above.
(c)
(i) The charge on S is negative as some electrons from the earth drifts to S.
(ii) S is at the nearest position possible from L.
(d) One way is to use a negatively charged gold leaf electroscope. The negatively charged gold leaf electroscope is brought near the sphere S. The divergence of the gold leaf will increase, hence showing that the remaining charge on S is negative.
