Electric Charges and Fields


An electroscope is a scientific tool which helps in detecting the presence of an electric charge on a body. The first electroscope was a pivoted needle electroscope invented in the year 1600 by a British physicist William Gilbert, known as vesorium.

Electroscope measures a charge that is dependent on the Coulomb electrostatic force that triggers the movement of the test charge. Electroscopes are often considered as basic voltmeter because the electrostatic charge of an object is adequate to its capacitance. The concentration of enough charge to be detected by an electroscope requires a vast amount of volts. Hence, the use of electroscopes with high voltage sources such as static electricity and electrostatic devices. The instrument for measuring the quantitative electrostatic charge is an electrometer.


Working of Electroscope

The basic theory behind a working electroscope depends on the atomic arrangements of elements, the induction of the charge, the internal structure of metal elements and the basic idea of unlike charges which attract each other while like charges repel each other.

The electroscope consists of a metal detector knob on the top, which is attached to a pair of metal leaves hanging from the bottom of the connecting rod. When there is no charge, the metal leaves loosely project downwards. But, when a charged object held close to an electroscope, one of the two things are possible.

  1. When there is a positive charge, the electrons in the electroscope metal are drawn to it and pushes upwards from the leaves. Which results in leaves obtaining a temporary positive state, and since like charges repel, the leaves become separated. As the positively charged body withdraws from the knob, the electrons return to their original location, and the leaves settle.
  2. When the charge is negative, the electrons in the electroscope metal repel and shift towards the leaves on the bottom. Thus, resulting in obtaining a transient negative charged state of the leaves, and since like charges repel, the leaves split again. As the negatively charged body withdraws, the electrons return to their original condition, and the leaves recover.

In both cases, the electrons return to their original state as soon as the charged body withdraws.

Moreover, the electroscope cannot evaluate whether the charge is positive or negative, but it only indicates the presence of the charge in a body.

Types of Electroscopes

The two types of classic electroscopes that are still in use in the field of physics, help in explaining the concept of electrostatics are the pith-ball and the gold-leaf electroscopes.

The Pith-Ball Electroscope

It was invented by the British physicist and schoolmaster John Canton in 1754. It consists of one or two mini lightweight balls of non-conductive material, hung from the hook of an enclosed stand by the help of silk or linen thread.

In order to measure the presence of a charge on a material, hold the material closer to the uncharged pith-ball. If the test material charge is positive, then the electrons, i.e. negative charges in the molecules, would be drawn to it. They shift to the side of the molecules close the material. On the other hand, the positively charged molecule, i.e. the nucleus, repel and divert away from the material. This technique is useful to study the presence of electrostatic charge on the material with the help of pith-ball electroscope.

The Gold-Leaf Electroscope

It was invented by British physicist Abraham Bennet, in the year 1787 as a bit more sensitive tool compared to the Pith-ball electroscope.

The Gold-leaf electroscope serves to detect the electrical charge present in the body and to classify its polarity. Its works on the theory of electrostatic induction and repulsion. It consists of a single brass rod to which two slender gold leaves are attached to one end and metal disc at another end.

The two thin leaves and an electrically conductive material are hanged parallel to each other and practically in contact with each other. The rod is placed inside the cylindrical glass jar so that the gold leaves stay inside and disc lies outside. The lower portion consists of a tin foil and, a small amount of CaCl2 is placed inside the gold-leaf electroscope to keep the air inside the jar dry.

Since the leaves are fragile and sensitive, they don’t have any stiffness, and they hang timidly. When they charge, they divide and form an angle which depends on the amount of the impact on the leaves that determines the charge voltage. This electroscope only shows the potential, not the charge. Also, the voltage is measurable by studying the angle of separation.

Uses of Electroscope

The various benefits of an electroscope are:

  1. They are useful to analyze the electrostatic charges and any ionizing radiation present in a body.
  2. The nature of the electrical charge is measurable using an electroscope.
  3. Also, with the help of an electroscope, we can easily compare the magnitudes of two different charges.

FAQs about Electroscope

Q.1. Which kind of electroscope is the most common and what is its application? 

Answer: Gold leaf type of electroscope is most commonly in use. It helps to detect and measure the static electricity and voltage. It has several applications such as –

  1. Detect charge:

The test material is taken and touched on a metal cap. If the leaves diverge, it represents the body as charged, and if there is no impact on the leaves, then the material is uncharged.

  1. To determine the nature of the charge:

A body charges the electroscope (say, positively charged body) and then the body is extracted. Next, the testing material makes contact with the metal cap. The body has the same charge(positive) if the leaves diverge more. If the leaves come closer, then it represents that the bodies are opposite charged(negative).

  1. Identify a body either as a conductor or an insulator :

Grab two electroscopes. Charge one of the electroscopes so that it can diverge its leaves. Then take the testing material and connect it with both the electroscopes. The body is a conductor if there is a diversion in the leaves of the other electroscope. But, if there is no change, then it is considered as an insulator or semiconductor.

Q.2. Draw a comparison between gold-leaf electroscope and pith ball electroscope.

Answer. There are several differences between these two varieties of the electroscope. These are:

  1. Pith-ball electroscope, when a charged body moves nearer to the pith-ball, some of the charges transmit from the body to the ball and make it repel from the body, detecting the changes. While the gold leaf electroscope uses two fragile leaves, hanging from a metal cylinder with a plate above it.
  2. The gold-leaf electroscope has more sensitivity compared to the pith-ball electroscope.

Q.3. What is the name of the first electroscope? 

Answer. The first-ever invented electroscope was a pivoted needle electroscope called vesorium. It was invented in the year 1600 by a British physicist William Gilbert.


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