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Versorium, The Pith Ball Electroscope, The Gold Leaf Electroscope, Practice Problems, FAQs

Electroscopes have been in use for a long time. It is used to observe or measure the static net charge present on an object. Electroscopes work on the principle of electrostatic force. This is the same force that a charged particle exerts on another charged particle. Electroscope was invented around . It is one of the earliest electrical measuring devices. The main drawback it has is that it needs large voltage to operate in a proper manner. This device can detect charges which are quite large. 

Table of content:

  • Versorium
  • The Pith Ball Electroscope
  • The Gold Leaf Electroscope
  • Practice Problems
  • FAQs

Versorium

The first electroscope was invented by a British physician, William Gilbert in the seventeenth century. It was a needle which was free to turn around on a pivot. It is similar to a compass needle but it is unmagnetized. It was called versorium. It is able to distinguish between charged and uncharged objects. Gilbert was a physician of the Queen of England. 

Versorium works on the electrostatic principle of Coulomb’s law of attraction. When a test charged object is brought nearer to the freely rotating needle, the electric field of the test object polarises the needle. The end of the needle, which is closer to the charged test object, acquires an opposite charge. So, it gets attracted to it. The end which is farther away from the test object, acquires the same charge as in the test object and gets repelled by it. Thus, the needle swings on its pivot. It confirms the presence of charge on the test object. Since the time of Gilbert’s invention, better electroscopes have been invented.

Versorium is similar to the magnetic compass, but is influenced by electrostatic forces. At this time people didn’t differentiate or understand the difference between magnetic force field and static electricity force field. Gilbert was the first to draw a clear distinction between magnetism and static electricity and is credited with establishing the term electricity. Gilbert was the first to make a distinction between magnetism and static electricity and is known for establishing the term electricity.

The versorium needle responds identically regardless of the polarity of the test body charge. It can’t distinguish between a positive and a negative charge.


The Pith Ball Electroscope

In 1731, Stephen Gray used a simple hanging thread In 1731, Stephen Gray made use of a simple hanging thread that got attracted to charged objects placed nearby. This was the first improvement on versorium. This was the first time when an improvement on versorium was made. The pith ball electroscope was invented by John Canton in 1754. Canton was a schoolmaster. 

The pith ball electroscope consists of a pith ball which hangs by a thread. Here, one or two small balls either one or two small sized balls made of a light weight non conductive substance, is used. Pith is originally a spongy plant material originally a plant material which is rather spongy. The balls are suspended by silk or linen thread from the hook of an insulated stand. Tiberius Cavallo made an electroscope in 1770 with suspended by linen or silk thread from the hook connected to an insulated stand. Tiberius Cavallo invented an electroscope in 1770 with similar pith balls but silver wires at the end. Modern electroscopes use balls made of plastic use balls which are made of plastic. When we bring a charged object closer to the pith ball, the electric field created by the charged object polarizes the molecules inside the pith ball. The face which is closer to the charged object acquires an opposite polarity as that of the charge of the object. The face farther away from the object acquires the same polarity as that of the charged object. Then, the pith ball gets attracted towards the object due to electrostatic force. Pith ball electroscope can’t determine the polarity of the net charge on the object. The pith ball behaves the same way irrespective of the polarity of the charge in the object.

Because of the induced polarization of the atoms inside the pith ball, the attraction occurs. Everything consists of electrically charged particles. Each atom consists of a positively charged nucleus with a Each atom consists of a nucleus having positive charge with a negatively charged cloud of electrons surrounding it. The pith is an insulator. That is why the electrons in the ball are tightly bound to atoms of the pith. They are not free to leave. the electrons inside the ball are bound to atoms of the pith. Since they are bound tightly, they can not freely leave. If we bring a positively charged object nearer to the pith ball, the negative electrons in each atom will get attracted. They will move slightly towards the space nearer to the object. Because the negative charges in the pith ball are nearer the object than the positive charges, the attraction will be greater than the repulsion of the positive charges. It results in a net attractive force. This separation of charge is microscopic in scale. Since there are so many atoms, these tiny forces will add up to a large force which moves the light weight pith ball. the negative charges present in the pith ball are now even more near to the object than the positive charges, so the attraction will be more than the repulsion of the positive charges. Now this results in a net attractive force. This separation of charge is very small and is microscopic in scale. Since there are so many atoms, these tiny forces will add up to a big force which moves the light weight pith ball.

Pith balls in an electroscope can be charged even by touching it by a charged object. Thus, some of the charges on the surface of the charged object move to the surface of the pith ball. These balls can be used to distinguish the polarity of charge on other objects. They will be repelled by objects charged with the same polarity but attracted to charges of opposite polarity. in any electroscope can also be charged by touching it by a charged body. So, some charges on the surface of the charged body move to the surface of the pith ball. These balls can be used to distinguish the polarity of charge on other bodies. They will be repelled by objects charged with the same polarity but attracted by charges of opposite polarity.

Usually, the electroscopes have a pair of suspended pith balls. This helps to infer whether the pith balls are charged. If one of them is touched to a charged object, the pith ball will get charged. The second one will be attracted and touch the first ball. Now both balls have the same charge and they will repel each other. The balls hanging by the strings will form an inverted 'V' shape. The distance between the balls gives a rough idea of the magnitude of the charge. The distance will depend on the wires, pith balls, charges on the balls, permittivity of the space and the gravitational acceleration of that space. 


The Gold Leaf Electroscope

Abraham Bennet, a British clergyman and physicist, invented the gold leaf electroscope in 1787.

The gold leaf electroscope consists of a conducting metal rod with a circular knob at its head. This is inserted in a glass chamber. The knob end sticks out of the chamber by several centimetres. Two thin gold leaves are suspended from the other end of the rod. The chamber is sealed and evacuated in order to prevent polarisation if any due to the air molecules in the chamber.

If a charged object is brought close to the electroscope or even in contact with it, the electric field due to the charged object polarizes the metal rod. The end of the metal rod which is closer to the charged object gets the opposite polarity as that on the object. The end of the metal rod further away from the object which also includes the two gold leaves gets the opposite polarity as that on the charged object. 

Thus, the charge on the gold leaves is similar. So, they repel each other. The leaves diverge and form an inverted ‘V’ shape. It confirms the presence of a net charge on the object. The gold leaf electroscope is more sensitive than the pith ball electroscope. It can detect smaller charges which are not detectable by the pith ball electroscope. 

The gold leaf electroscope cannot distinguish between positive and negative charges. It behaves the same in both situations, just the leaves deflect.


Practice Problems

Q 1. How many types of electroscopes are there?

A. There are two types of electroscopes and they are as follows:

a. Pith-ball electroscope: 

It was invented by John Canton in . It consists of one or two small light balls made of non-conductive substances (like pith). In order to find if an object is charged or not, it is brought near to the uncharged ball. If the ball gets attracted towards the object, it means the object is charged.

b. Gold-leaf electroscope: 

It was developed by Abraham Bennet in 1787. It is more sensitive than a pith-ball electroscope. It consists of a vertical metal rod which has two parallel strips of thin flexible gold leaf hanging. In order to prevent the gold leaf from the disturbances of air or polarization due to air, the whole system is kept inside a glass bottle. The gold leaves spread apart into an inverted “V” shape when a charged object is brought nearer to it.

Q 2. What are the parts of a gold leaf electroscope?

A. These are the parts of a gold leaf electroscope:

a. circular metal top
b. metal rod
c. gold leaves
d. glass enclosure

Q 3. What are the uses of electroscopes?

A. Following are the typical uses of electroscopes:

a. It is used in detecting static charges.
b. It is used in detecting the nature of electric charges. 
c. It is used in detecting the magnitudes of charge in a body by the measurement of the deflection in the gold leaves.

Q 4. What is the working principle of electroscope?

A. The working of an electroscope is based on Coulomb's law. According to the Coulomb’s law, the force between two charged bodies in a vacuum can be given by the equation,

 

where, is the force between the charges, is the permittivity in vacuum, and are the charges in the bodies and is the distance between the charges. It is to be noted that the charges can be either positive or negative.

When the leaves inside the glass enclosure get similar types of charges, they repel each other. The repulsive force is calculated by Coulomb's law. More is the charges on the leaves more will be the deflection as

FAQs

Q 1. How is an electroscope used to detect charge?
A. When a charged object is brought near the electroscope or even touched, the electrostatic force generated causes the molecules in the electroscope to get polarised. When the object is positively charged, the electrons in the metal knob of the electroscope get attracted to the test object. When the object is negatively charged, the electrons in the metal knob get repelled. The electroscope also demonstrates electrostatic induction. 

Q 2. Why are gold leaves used in electroscopes?
A. Gold is such a material/metal which does not corrode easily and lose its property. Gold is also a malleable metal so it can easily be made into thin sheets which make the gold leaves have less mass. Less mass is necessary in order to easily deflect the gold leaves. This makes it more sensitive to smaller electric charges. Thus, gold is the ideal material for an electroscope.

Q 3. Why is the electroscope earthed?
A. There are grounded metal plates or foil strips in the bottle just beside the gold leaves on either side. These are for safety measures. If an excessive charge is applied to the delicate and sensitive gold leaves, the gold leaves will touch the grounding plates and they will discharge. The earthing system also captures charge leaking through the air which accumulates on the glass walls. This increases the sensitivity of the instrument.

Q 4. How can gold leaves be made to deflect without even touching them with a charged object?
A. The gold leaves can be made to deflect without even touching them. This is achieved by electrostatic induction. A charged object is brought nearer to the terminal. The electric field from the object induces an opposite charge in the plate and same charge at the electroscope leaves. We know, the opposite type charges attract each other while the same-sign charges repel each other. Thus, the charged leaves repel each other. 

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