NAEST 2020: Electroscope Charging and Photoelectric Effect D

Question: In the video, the performer charges a rod by rubbing and brings it close to an electroscope. He sometimes touches the aluminium lid on the top of the electroscope. The leaves spread out. The aluminium lid is connected to the top aluminium plate of the electroscope. Choose the correct option(s).

1. At the end of this process, the two leaves get the same kind of charge (positive/negative) as that was on the rod.
2. At the end of this process, the two leaves get the charge (positive/negative) opposite to what was on the rod.
3. Just before the person touches the aluminium lid at the top, the charge on the aluminium plate and the charges on the leaves are of the same nature (positive/negative).
4. Just before the person touches the aluminium lid at the top, the charge on the aluminium plate and the charges on the leaves are of opposite nature (positive/negative)

Question 2: The person brings a normal torch above the top aluminium plate and then a bigger UV lamp above this aluminum plate. In the latter case, the spread between the leaves decreases. Choose the correct option(s).

1. The UV light makes photoelectric effect to eject some electrons from the aluminium plate
2. The leaves of the electroscope had positive charge when the UV lamp was brought.
3. The leaves of the electroscope had negative charge when the UV lamp was brought.
4. The rod rubbed in the initial part of the video had negative charge
5. The rod rubbed in the initial part of the video had positive charge

Solution 1: To begin with, the leaves and aluminium plate are neutral. Let the rod is positively charged. When a positively charged rod is brought close to the aluminium plate, it attracts a negative charge (free electrons in aluminium) towards it. These free electrons get accumulated in the aluminium plate and it becomes negatively charged. A few electrons move from the leaves to the aluminium plate. The leaves become positively charged and they repel each other. Thus, just before the person touches the aluminium lid at the top, the charge on the aluminium plate and the charges on the leaves are of opposite nature. In this case (when the rod is positively charged), the plate is negative and leaves are positive. Thus, option (D) is correct.

When the person touches the lid while maintaining the rod close to the plate, leaves converge. The positive charge on the leaves is neutralized by the flow of negative charge from the fingers to the leaves. In this situation, the leaves become neutral but the plate is still negatively charged. Why? Because the negative charges on the plate are bound by the positive charges on the rod. These are bound charges and they will not move till the rod is kept at its place. When the rod is moved away, these negative bound charges become free to move. The charge gets redistributed. Some of the negative charges move to the leaves and the leaves diverge again. Thus, at the end of this process, the two leaves get the negative charge which is opposite to the positive charge on the rod. Hence, option (B) is also correct.

As an exercise, repeat the above analysis for the negatively charged rod and convince yourself that the answers remain the same.

Solution 2: The leaves converge when UV lamp is brought above the aluminium plate. The high-energy UV rays eject electrons from a metal surface. This is called the photoelectric effect. Thus, option (A) is correct.

For this demonstration to work, the electroscope shall be negatively charged. The leaves and the aluminium plate shall be negatively charged. Thus, option (C) is correct.

For the leaves to be negatively charged, the rod in the initial part shall be positively charged as explained above. Thus, option (E) is correct.

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