Question

Q3 Scenario: Discharge the electroscope and charge it again with the rubber rod. Rub the perspex rod with silk and bring it close to the metal disk on top of the electroscope, but do not touch the disk! Go back and forth with the perspex rod several times without touching the metal disk. (Pretend you have a magic wand and you’re casting a spell on the electroscope).

a) What happens when you rub the perspex rod with silk and bring it close to the metal disk on top of the electroscope, but without touching the disk? Explain.

b) When observing the oscillations of the needle, what direction does the electroscope needle go as you bring the rod closer to the metal disk? Explain.

c) What causes the electroscope needle to deflect in that manner? Explain.

d) Does the perspex rod have the same or opposite charge as the needle? Explain.

Answer 1

An Electroscope has a plate on top and 2 or more needles inside a bottle like container, connected via conducting material. When a charged object is brought close to the plate, the charged object attracts the oppositely charged particles present in the plate and the like charged particles and repeled away. These like charges that were repeled from the plate gets collected in the needles, and since all the needles have same charge they repel each other.

Now discharge of electroscope is when you collected the charges from the plate, the charge in the needles still remain there.

so we end up with a neutral plate but charged needles (i.e. the needles will be away from each other).

a) so now when you rub the perspex rod with silk and bring it close to the metal disk on top of the electroscope it attracts the oppositely charged particles present in the plate and the like charged particles and repeled away. These repeled charges go to the needles and the needles move further away. they were already away from each other initially be they were charged even if the plate wasn't.

b) As you bring the rod closer to the metal disk the needles point in opposite direction so that they can be as further from each other as possible, since like charges repel.

c) The force between like charges make they go in opposite direction.

d)No the rod has the same charge as the needles. lets say the rod has +ve charges then it will attract the -ve charges of the plate and repel the +ve charges of the plate. These +ve charges which were repeled ends up being in the needles. so needles and rod has like charges.

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Also if you have some doubts or confusions feel free to ask in comments.

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