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Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
Which metal leaves are used in gold leaf electroscope?
Electroscope is an instrument used for detecting the presence of charge and its nature. A gold-leaf electroscope is a widely used instrument.
Description: A metal rod P passes through an insulating stopper into a vessel fitted with glass panes [Fig. 1.3]. Two thin foils of gold (L, L) are hung at the lower end of the metal rod. The upper end of the rod is capped with a metal disc D. The glass vessel protects the leaves from air current. Some fused cal-cium chloride is placed inside the vessel as a hygroscopic sub-stance. Two tin-foils (t, t) are attached to the inner face of the glass walls. These foils which are in contact with the metallic base of the vessel are earthed.
Charging the electroscope by conduction: To charge the gold-leaf electroscope by conduction, a charged body is touched on the disc of the electroscope. A glass rod rubbed with silk becomes positively charged. If this glass rod is made to touch the disc of the electroscope, a part of the positive charge spreads in the instrument. As a result, the leaves, being positively charged, diverge due to repulsion [Fig. (a)], and remain so even after the removal of the glass rod [Fig. (b)].
If the disc is now touched, all the charges of the electroscope will flow to the earth through the body of the experimenter and the leaves will collapse. This is called earthing of a gold-leaf electro-scope [Fig.(c)].
Charging by conduction is however not a good process at all. If the charged body carries a large amount of charge, then as soon as it is brought in contact with the disc, the divergence of the leaves will be so great that they may get detached from the rod.
Uses:
i. Detection of charge: The body to be tested is brought slowly near or in contact with the disc of an uncharged electroscope. If the leaves are deflected, the body is charged; otherwise, uncharged.
ii. Determination of the nature of charge: To test the nature of charge on a body, we have to start with a charged electroscope.
Suppose, the electroscope is positively charged with its leaves divergent. The body to be tested is brought slowly to touch the disc. If the divergence of the leaves increases, the body is positively charged. If the divergence decreases, the body is negatively charged.
The nature of the charges would be just opposite if we started initially with a negatively charged electroscope.
An uncharged body may also diminish the divergence to some extent, so it cannot definitely indicate that the charge of the test body is opposite in nature to that on the electro-scope. Thus increase in divergence of the leaves of the electroscope provides the surer test for the nature of charge on a body.
The experimental results are given in the following table:
Obviously, an insulator will have no effect on the diver-gence of the leaves.
iii. Identification of conductor and insulator: The body to be tested is brought slowly to touch the disk of a charged gold- leaf electroscope. In this case, the body must be grounded with a conductive wire. If the test body is a conductor then all the charges of the electroscope will flow to the earth through the conductive wire and the leaves will collapse. If the body is an insulator then the charge of the electroscope will not be able to flow to the earth. So there will be no change in divergence of the leaves of the electroscope. This way, the identification of conductor and insulator can be done using a gold-leaf electroscope.
Proof Plane
It is often difficult to bring up a strongly charged test body near the electroscope. A portion of the charge from the body can be picked and taken to an electroscope for testing by using what is called a proof plane. A proof plane consists of a small metallic disc mounted with an insulating handle [Fig.].
It is held by the insulating handle and the metal disc is momentarily placed in contact with the charged test body. The disc gets charged by conduction. It is now brought to a charged electroscope for testing the nature of the charge. A proof plane can also be used to detect the presence of charge or the nature of distribution of charge on a body.