Electroscope: Invention, Types, Applications

The electroscope was invented by the British physician William Gilbert around \(1600.\) Have you touched the leaf of the touch-me-not plant and watched it curl? There is an instrument that responds to the presence of charges just like that plant. The leaves of a Touch Me Not plant curl up when someone touches them. Let us learn about an instrument whose Leaves converge or diverge depending upon the nature of a charge on a material.

We have already studied the presence of static electric charges. But, how can we detect the presence of these static electric charges? Let us study a device, which is used to detect a charge. It is called an electroscope. An electroscope is thus a charge detecting device, which detects the presence of even weak electric charges and determines whether that charge is positive or negative.

Methods of Charging

The following methods can charge a body-

Charging by Friction: By rubbing two bodies together, both positive and negative charges in equal amounts appear simultaneously due to the transfer of electrons from one body to the other.
1. A glass rod rubbed with silk makes the rod positively charged and silk negatively charged. The decrease in the mass of the glass rod is equal to the total mass of electrons lost by it.
2. Ebonite(a type of plastic) on rubbing with wool becomes negatively charged, and the wool becomes positively charged.
3. William Gilbert observed that when two glass rods, each rubbed against silk, are brought closer, they repel each other. Similarly, two ebonite rods rubbed against the cat’s skin repel each other. On the other hand, a glass rod rubbed against silk, and an ebonite rod rubbed against a cat’s skin on being brought closer attracts each other.
4. Clouds also get charged by friction.
5. A comb moving through dry hair gets electrically charged. It starts attracting small bits of paper. 
6. During landing or take-off, the tyres of an aircraft get electrified; therefore, special material is used to manufacture them.
7. A truck carrying explosives has a metal chain touching the ground to conduct the charge produced by friction.
8. Following is the list of some objects placed in an order such that if two objects among them are rubbed, the object that comes first in the list acquires a positive charge and the object that appears later acquires a negative charge.

Charging by Conduction: Bring two conductors, one charged and the other uncharged, in contact with each other. The charge (whether \(-ve\) or \(+ve\)) under its repulsion will spread over both the conductors. This method is called charging by conduction (through contact).

Charging by Electrostatic Induction: If a charged body is brought near an uncharged body, one side of the neutral body (closer to charged body) becomes oppositely charged while the other side becomes similarly charged. 

Note:
1. An induced charge can be lesser or equal to an inducing charge (but never greater). Its maximum value is given by \(Q’ = – Q\left[ {1 – \frac{1}{K}} \right]\) where \(Q\) is the inducing charge and \(K\) is the dielectric constant of the material of the uncharged body.

What is Electroscope?

An electroscope is used to ascertain whether a substance is charged or not. The electroscope works on the principle, “Like charges repel”. A metal rod is passed through a non-conducting cork. A thin leaf-like strip of metal is attached to it and kept in a glass case at the lower end of the metal rod. At the upper end of the rod, a disc is attached. Now, rub a glass rod with a silk cloth so that the rod gets positively charged. When a charged substance touches the disc, some portion of the charge from the substance is transferred to the remote ends of the rod and some portion to the metallic strip. In this manner, both the disc and the metal strip have the same type of charge when the leaf-like thin strip gets repelled and moves away from the rod.

There are three types of electroscopes:
1. The Pith Ball Electroscope
2. The Gold-Leaf electroscope
3. The Needle Electroscope

Gold Leaf Electroscope

The gold-leaf electroscope was developed in the year \(1787\) by British clergyman and physicist Abraham Bennet as a more sensitive instrument than pith ball or straw blade electroscopes then in use. 

Construction

It consists of a vertical conducting metal rod to which a brass ball is fixed at the upper end, and a pair of rectangular pieces of gold foil is attached to the lower end. The electroscope is known as the gold-leaf electroscope. This arrangement is placed inside a glass jar to prevent any disturbance due to air. An insulating material like cork is fitted on the mouth of the glass vessel to hold the metal rod in its place. 

As shown in Figure, a gold-leaf electroscope consists of a glass jar placed on a non-conducting surface like wood. The mouth of the glass jar is sealed with shellac material. A brass rod passes through the seal. Inside the jar, the lower end of the brass rod is flattened like a strip, and a small gold foil is fixed at the lower end of the brass rod parallel to the brass strip.

At the bottom and lower lateral sides of the jar, tin foils are fixed, which help the charge to stay on the gold foil for a longer time. A brass cap is provided at the upper end of the brass rod.

Working

Now, let us see how this electroscope can be charged. Initially, when the electroscope is neutral, the gold leaves hang vertically downwards due to gravity. Take a glass rod and rub it with a silk cloth. The rod becomes positively charged. This charged rod can be used to charge the electroscope by two methods: charging by conduction and induction.

Fig – Charging of a neutral electroscope

In the case of charging by conduction, a positively charged glass rod is brought in contact with the metal ball on the top. The electrons of the electroscope get attracted towards the positively charged rod and enter the rod. Thus, the electroscope acquires an overall positive charge, which spreads over the entire electroscope. Since, like charges repel, both the positively charged gold leaves repel each other and move apart. Thus, the electroscope gets positively charged. Next, if either a perspex or ebonite rod is rubbed with fur, the rod becomes negatively charged. Bring it in contact with the neutral electroscope. You can see that both the gold leaves repel each other and move apart. This phenomenon is because the negative charges of the rod repel the electrons of the electroscope.

The positive charges, which are near the rod, get neutralized by the negative charges of the rod. The negative charge spreads over the entire electroscope, and the gold leaves repel each other. Thus, the electroscope gets negatively charged. In the case of charging by conduction, a positively charged glass rod makes the electroscope positively charged, and a negatively charged ebonite rod makes the electroscope negatively charged. Thus, both the electroscopes receive the same charge as that possessed by the charging body.

The electroscope can also be charged by placing the rod near the electroscope but not touching it. This process is called charging by induction. Take a negatively charged ebonite rod and bring it near a neutral electroscope. But, do not bring the rod in contact with the electroscope. The negatively charged rod repels the electrons within the electroscope, which move towards the gold leaves. Thus, a positive charge is induced on the ball, and a negative charge is induced on the leaves. Hence, the leaves repel each other and move apart. Now, touch the ball with your finger so that it gets grounded. The electrons of the electroscope flow through the finger to the ground, and leaves stop repelling each other and collapse.

When you remove your finger, the earthing is removed, but still, the leaves remain collapsed. Lastly, when we remove the ebonite rod from the vicinity of the electroscope, the positive charge gets distributed all over the electroscope. The leaves repel each other and get diverged. Thus, the electroscope becomes positively charged. Next, bring a positively charged glass rod near a neutral electroscope so that the rod does not touch the electroscope. The electrons of the electroscope are attracted towards the positively charged rod, and the positive charges are repelled. Both the leaves acquire the positive charge and repel each other. Now, touch the electroscope with your finger without removing the positively charged glass rod, thereby earthing it.

The electrons from the ground are attracted towards the positive charges of the electroscope. The positive charges of the electroscope combine with the negative charges and neutralize them, because of which the leaves collapse. Now remove the earthing without removing the positively charged glass rod. Thus, there is an excess of negative charge in the electroscope. Finally, on removing the glass rod, the negative charge gets evenly distributed to the leaves. Hence, the leaves again repel each other and move apart, and the electroscope is negatively charged. This process is called charging by induction.

Such a charged electroscope can be used to determine whether a particular body is positively or negatively charged. Take a positively charged electroscope. The diverged gold leaves indicate the presence of a charge on the electroscope. The type of the charge on a body can be determined either by conduction or by induction.

Take any charged body and bring it in contact with this electroscope. If the leaves get further diverged, then we can say that the charge on the body is positive or the body is positively charged. But, if the divergence decreases, then the concerned body is negatively charged. To find the charge on this body, replace this electroscope with a negatively charged electroscope. Its leaves are diverged, indicating the presence of the charge. Touch the body to this electroscope. If the leaves show no change in divergence, then the body does not have any charge or is neutral. But, if the leaves show further divergence, the body can be identified as having a negative charge.

The charge on the body can also be identified by the method of induction. If a positively charged body is brought near a positively charged electroscope, then the leaves show further divergence; but, if a negatively charged body is brought near a positively charged electroscope, then the divergence of the leaves decreases. Hence, when any charged body is brought near a charged electroscope and the divergence of leaves increases, then the body has the same charge as that of the electroscope. On the other hand, if the divergence of the leaves decreases, then the body has a charge opposite to that of the electroscope.

Pith Ball Electroscope

Construction

This electroscope consists of a small pith ball, which is suspended from a stand with the help of a silk thread. 

Working

Take a glass rod and rub it with silk. Bring the rod near the pith ball. You can see that the pith ball gets attracted towards the glass rod. It touches the rod for a while and then gets repelled by the rod. Similarly, bring this glass rod near another pith ball electroscope; you will find the same observation again. Now, bring this electroscope near the previous one. You can see that both the pith balls get repelled. Why does this happen?

When the glass rod is rubbed with silk, it gets positively charged. And, when it is brought towards the neutral pith ball, it gets attracted towards the rod and touches it. But, while in contact with the rod, the pith ball also acquires a positive charge. Like charges repel each other, hence the positively charged pith ball gets repelled by this rod and moves apart. Similarly, when the other pith ball is touched by the positively charged glass rod, the pith ball also acquires the positive charge, and when both the positively charged pith balls are brought near each other, they get repelled.

Next, take perspex or a vulcanite rod and rub it with fur. In doing so, the rod gets negatively charged, and when this negatively charged rod is made to touch another neutral pith ball, the pith ball acquires a negative charge. Now, bring one of the original pith balls in the vicinity of this pith ball electroscope. As both the pith balls are oppositely charged, you can see that both the pith balls get attracted to each other. Hence, we have seen how an electroscope can be charged.

Now, let us see how we can determine the nature of the charge of a body with the help of this electroscope. If a charged body is brought near this positively charged electroscope and if the pith ball moves away from the body, the body can be identified as positively charged because we know that charges always repel each other. If the positively charged pith ball is attracted towards the body, we can easily determine that it is negatively charged because, unlike charges, they always attract each other. Hence, we have seen how a pith ball electroscope helps us identify the nature of the charge of different bodies.

Needle Electroscope

Construction

A plate is connected to a support stand, and a pivoted-swinging needle is fixed on either side of the stand. If a charged object is brought near the plate, the needle will gain the same charge and swivel away.

Working

In this electroscope, support stand, the plate, and needle are all made of a conducting material that allows the flow of electrons and the distribution of excess charge throughout the electroscope. Here, the presence of charge in the electroscope and the nearby object is determined by observing the deflection of the needle. The working of the Needle Electroscope often emphasizes the induction process of charging. In this process, the charged object brought near the plate of the electroscope influences electrons within the electroscope to move accordingly. When the charged object is held close to the plate to which the electroscope is touched, the electrons will start flowing between the electroscope and the ground, providing the electroscope with an overall charge. If the charged object is pulled away, the needle deflects, thus indicating the overall charge on the electroscope. 

Applications of Electroscope

Following are the applications of electroscope:
1. To detect the nature and relative amount of charges.
2. To detect the presence of electric charge or ionizing radiation on a body.
3. To identify and compare the magnitude of charges.
4. To calculate the force between two charges.

Summary

1. An electroscope is a charge-detecting device, which detects the presence of even weak electric charges and determines whether the charge is positive or negative.
2. An electroscope has to be charged to detect the charge on a body.
3. The charging of a gold-leaf electroscope can be done by two methods: conduction and induction. In both methods, a previously charged body is used to charge the electroscope.
4. Once the electroscope is charged, it can detect the charge on anybody, again, by two methods. That is, by conduction and induction. In both methods, based on the behaviour of gold leaves, we can detect the presence and the type of the charge on anybody.

FAQs on Electroscope

Q.1. What is the importance of an electroscope?
Ans: An electroscope is used to detect the presence of charge on a body, determine the nature of a charge on a body, and measure the exact amount of charge on a body.

Q.2. Why is gold leaf used in electroscopes?
Ans: Gold leaves are used in electroscopes because:
1. The leaves of the gold are thin and light.
2. Gold is a malleable metal that can be hammered to produce very thin leaves.
3. Gold does not react with air.

Q.3. Which metal is used in gold leaf electroscope?
Ans: The metal used in the gold leaf electroscope is brass.

Q.4. What is the working principle of an Electroscope?
Ans: Electroscope works on the principle of ‘like charges repel and opposite charges attract.’

Q.5. What are the different types of electroscopes?
Ans: There are three types of electroscopes:
Pith ball electroscope, Gold leaf electroscope and Needle electroscope.