Electroscope: Definition, Types, Working, Functions

Electroscope is one of the most important instruments used by scientists to investigate electricity for many years. It has been characterized as a conducting material device used to detect and test the presence of ionizing radiation or electric charge on a body for millennia. 

Read More: Wheatstone Bridge

Key Terms: Electroscope, Electrostatic Force, Electrometer, Electrical Charge, Electricity, Capacitance, Ionizing radiation, Voltmeter, Coulomb

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What Is an Electroscope?

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Electroscope is a scientific instrument that detects the presence of an electric charge on a person's body. The first electroscope with a pivoting needle, termed Versorium, was invented by British physician William Gilbert.

The amount of charge on the test item is proportional to voltage, and electroscopes detect the movement of the test object due to the Coulomb Eectrostatic Force on it. Electroscopes are regarded as primitive voltmeters when the electric charge of the object is equal to its capacitance.

Coulomb Electrostatic Force, which causes the motion of the test charge, is used by the electroscope to detect the charge. Because the electric charge of an object is equal to its capacitance, an electroscope can be used as a primitive voltmeter. The term "electrometer" refers to a device that measures charge quantitatively.

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Working of an Electroscope

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The operational basis of an electroscope is based on atomic structure, charge induction, the internal structure of metal elements, and the premise that like charges repel one another while unlike charges attract.

• Step 1: A pair of metal leaves dangling from the connecting rod's bottom are attached to the metal detector knob on the top of an electroscope.
• Step 2: The metal leaves dangle loosely downward when there is no charge present. When a charged object is brought close to an electroscope, one of two things can happen.
• Step 3: Electrons in the metal of the electroscope are attracted to the charge and travel upward out of the leaves when the charge is positive.
• Step 4: As a result of this, the leaves get a transient positive charge, and because like charges repel each other, the leaves separate. When the charge is released, the electrons return to their original positions, and the leaves relax.
• Step 5: When the charge is negative, the electrons in the electroscope's metal reject each other and migrate toward the bottom leaves. The leaves get a momentary negative charge as a result of this, and because like charges repel each other, the leaves separate once more. 
• Step 6: When the charge is removed, the electrons return to their original place, and the leaves relax.
• Step 7: The passage of electrons into or away from the leaves is how an electroscope responds to the presence of a charge.
• Step 8: The leaves split in both circumstances. It's vital to remember that the electroscope can't determine if the charged object is positive or negative since it only looks for electrical charge.

Read More: Electromagnetic Field

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Types of Electroscopes 

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Electroscopes can be grouped into three categories in general:

Pith-ball Electroscope

An electroscope is a device that consists of one or two little balls made of pith, a lightweight non-conductive substance. The object is brought close to the uncharged pith-ball to determine whether it is charged or not using this electroscope. Between the two of them, there is a force of attraction.

Working of Pith-ball Electroscope:

• Step 1: The Pith-ball electroscope detects static electric charges and determines the polarity of unknown charges by detecting static electric charges.
• Step​ 2: The pith-ball A is formed up of molecules with positive and negative charges near together.
• Step 3: When a charged object B is brought close to pith-ball A, the charges inside it begin to disperse slightly. It's worth noting that if an object is positively charged then the electrons, i.e., negative charges in the molecules will be attracted to it. 
• Step 4: They will gravitate toward molecules that are near to the item. Positive charges, such as nuclei, will repel each other and shift to the side of molecules away from the object.
• Step 5: The force of attraction between electrons and external charges is larger than the force of repulsion between nuclei because electrons are closer to external charges than nuclei.
• Step 6: Despite the tiny separation of charges, the cumulative force (due to the high number of atoms in the pith-ball) is sufficient to draw the pith-ball towards the external charge.
• Step 7: The pith-ball electroscope is used in this fashion to detect the existence of a static charge on an object as if the object is drawn to it when brought close to it. Furthermore, the pith-ball can assist you in determining the polarity of a charge on an object by being attracted to one with the opposite charge and repelling one with the same charge.

Gold-leaf Electroscope

It's made up of a vertical conductive rod with a metal ball on top and two thin gold leaf strips joined at the bottom. This electroscope, invented by Abraham Bennet in 1787, is more sensitive than a pith-ball electroscope.

The gold leaf is maintained in a glass bottle to protect it from air draughts. When a charged object is brought close to the gold leaves, which are stored in a glass flask to prevent them from being affected by air, they spread apart creating an inverted "V."

Gold-Leaf Electroscope

Working of Gold-leaf Electroscope:

• Step 1: A gold-leaf electroscope is used to detect and determine the polarity of electric charge in a body. The principle of electrostatic induction and similar charge repulsion underpins its operation.
• Step 2: Two thin leaves and an electrically conductive substance are strung nearby and in contact with this electroscope. Because the leaves are so thin, they have little firmness and droop limply.
• Step 3: These leaves become separated when they gain charge.
• Step 4: The angle that occurs between them is determined by the amount of charge applied to them. The angle can be precisely matched to static voltage if the instrument is insulated and the capacitance is fixed. It's worth noting that this electroscope displays potential rather than charge and that the voltage is determined by measuring the separation angle.

Read More: Electric Charges and Fields Important Questions

Needle Electroscope

It comprises a plate attached to a support stand and a free-swinging needle pivoted on either side of the stand. When a charged object comes close to the plate, the needle gains the same charge and swivels away.

Needle Electroscope

Working of Needle Electroscope: 

• Step 1: The plate, support stand, and needle of this electroscope are all made of a conducting substance that allows free passage of electrons as well as the distribution of excess charge throughout the instrument.
• Step 2: By examining the deflection of the needle, it is possible to identify the existence of charge in both the electroscope and the neighboring item.
• Step 3: This electroscope's operation frequently emphasizes the charging process using induction. In this technique, the presence of a charged object brought close to the electroscope's plate causes electrons within the electroscope to migrate in the desired direction.
• Step 4: When a charged object is held close to the plate where the electroscope is contacted, electrons begin to flow between the electroscope and the ground, giving the electroscope a charge. When the charged object is pushed away from the electroscope, the needle deflects, indicating the overall charge.

Also read: Electric field formula

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Uses of Electroscope

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An electroscope can be used to:

• Detect ionizing radiation or an electric charge on a body
• Determine the magnitude of charges and compare them.
• Determine the force that exists between two charges.
• Determine the nature of the charges and their relative magnitude.

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Previous Year Questions

1. The system will be in equilibrium if the value of q is…. [WBJEE 2016]
2. A hollow metal sphere of radius R  is uniformly charged. The electric field due to the sphere... [NEET 2019]
3. In the given circuit, what will be the equivalent resistance between the points…. [JIPMER 2006]
4. The energy required to rotate the dipole by 90^∘ is….[NEET 2013]
5. When the  Gaussian spherical surface is doubled, then then the outward electric flux will be… [NEET 2011]
6. A solid sphere of radius R has a charge Q distributed in its volume… [BITSAT 2015]
7. The equilibrium separation between the balls... [NEET 2013]
8. In the given circuit, what will be the equivalent resistance between the points…. [JIPMER 2006]
9. two charges of equal amount +Q are placed on a line...[WEBJEE 2016]
10. The total energy of a system is...[JEE Main 2018]
11. Which of the following does not show electrical conductance? 
12.  Consider a sphere of radius R which carries a….[JEE Main 2020]
13.  Charge density of long wire λ… [ BITSAT 2009]
14. How to adjust a system of three identical capacitors to get high electrostatic energy….. [ UPSEE 2016]
15. The number of excess electrons on the drop is… [VITEEE 2011]
16. The surface charge density (in Cm⁻²  ) of the earth is.. [ DUET 2009 ]
17. The kinetic energy attained by the particle after moving a distance y is…. [KEAM]
18. In the given circuit, what will be the equivalent resistance between the points A  and B? [ JIPMER 2006]
19.  One kilowatt-hour is equal to….[NEET 1997]
20. If the ammeter has a coil of resistance 480ohm and a shunt of 20ohm , the reading in the ammeter will be….[NEET 2015]

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Things to Remember 

• The electroscope, invented by the British physician William Gilbert circa 1600, is one of the most important instruments used by scientists to investigate electricity for many years.
• Electroscopes are regarded as primitive voltmeters when the electric charge of the object is equal to its capacitance.
• An electroscope is a scientific instrument that detects the presence of an electric charge on a person's body.
• The operational basis of an electroscope is based on atomic structure, charge induction, the internal structure of metal elements, and the premise that like charges repel one another while unlike charges attract.
• The different types of Electroscopes are Pith Ball Electroscope, Gold Leaf Electroscope and Needle Electroscope. 
• Electroscope functions to detect ionizing radiation or an electric charge on a body. It also determines the force that exists between two charges.