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Electroscope

  • Last Updated : 30 Jun, 2021

Thales of Miletus, Greece, is credited with discovering the fact that amber rubbed with wool or silk cloth attracts light things approximately 600 BC. The word Electricity comes from the Greek word Electron, which means Amber. Many similar pairs of materials were known to attract light things such as straw, pith balls, and fragments of paper when rubbed together create an attractive force between them. Humans get a similar effect at home by doing the following activities. Long, thin strips of white paper should be cut out and lightly ironed. Bring them close to a television or computer monitor. It is observed that the strips are got attracted to the screen. In fact, it stays glued to the television for quite some time.

Electric Charge

Electric charge is the fundamental physical characteristic of a matter that makes it experience a force when placed in an electric or magnetic field.

When two glass rods rubbed with a piece of wool or silk cloth and brought close together repulsive force is observed between them. The two strands of wool or two pieces of silk cloth piece used to rub the rods repel each other as well. The glass rod and the wool, on the other hand, were attracted to one other. Two plastic rods brushed with cat fur repelled each other but attracted the fur. The plastic rod, on the other hand, attracts the glass rod and repels the silk or wool used to rub the glass rod. The fur is repelled by the glass rod.

After many rigorous examinations by several experts, it was determined that there were only two types of an entity known as the electric charge. The process of electrification takes place at the bodies of glass or plastic rods, silk, fur, and pith balls. Rubbing induces an electric charge. According to the research on pith balls, there are two types of electrification that are like charges repel each other, and unlike charges attract each other. The investigations also showed that when the rods and the pith balls come into touch, the charges are transferred from the rods to the pith balls. The pith balls are supposed to be electrified or charged by contact. The polarity of charge is the feature that distinguishes the two types of charges. 

When a glass rod is rubbed with silk, the rod receives one type of charge while the silk receives the other. This is true for any pair of electrified items that are rubbed together. When the electrified glass rod comes into contact with the silk it was rubbed with, it no longer attracts it. They don’t attract or repel other light things like they used to when they were electrified. As a result, when the charged bodies come into contact, the charges acquired during rubbing are gone.



The effect of the opposite charges accumulated by the items is neutralized or nullified. As a result, American scientist Benjamin Franklin labeled the charges positive and negative.

Electroscope

An electroscope is a scientific apparatus that helps in the detection of an electric charge on a body. The earliest electroscope, known as Vesorium, was a pivoting needle electroscope invented in 1600 by British physicist William Gilbert.

The movement of the test charge is triggered by the Coulomb electrostatic force, which is measured by an electroscope. Electroscopes are frequently used as basic voltmeters since an object’s electrostatic charge is proportional to its capacitance. A large enough concentration of charge to be observed by an electroscope necessitates a large number of volts. 

As a result, electroscopes are frequently used with high-voltage sources like static electricity and electrostatic devices. An electrometer is an instrument used to measure the quantitative electrostatic charge.

Working of an Electroscope

An electroscope’s operating principle is based on the atomic structure of atoms, charge induction, the internal structure of metal elements, and the premise that like charges repel each other while unlike charges attract.

An electroscope is made up of two thin gold leaves joined to the bottom end of a vertical metal rod contained in a box. When a charged gadget comes into contact with the metal knob at the top of the rod, the charge is transferred to the leaves, causing them to diverge. The amount of charge is indicated by the degree of divergence.

  1. Electrons in the metal of the electroscope have an attractive force between the charge and travel upward out of the leaves when the charge is positive. The leaves gain a brief positive charge as a result of this, and because like charges repel each other, the leaves separate. The electrons return to their normal locations when the charge is released, and the leaves relax.
  2. 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. The electrons then return to their normal location when the charge is withdrawn, and the leaves relax.

As soon as the charged body is removed, the electrons return to their original state in both circumstances. Furthermore, the electroscope cannot determine whether a charge is positive or negative; it can only detect its presence in a body.

Types of an Electroscope

The following are the two most common varieties of electroscopes, that are:



  • Pith-ball electroscope: John Canton, a British physicist, and schoolmaster, devised it in 1754. It is made up of one or two small non-conductive balls that are suspended from the hook of an enclosed stand using silk or linen thread. Hold the item closer to the uncharged pith-ball to determine the presence of a charge on it. If the charge of the test material is positive, electrons, or negative charges in the molecules, will be pulled to it. They shift to the side of the material’s molecules that are closest to it. The positively charged molecule, i.e. the nucleus, on the other hand, repels and diverts away from the substance. With the use of a pith-ball electroscope, this approach can be used to investigate the existence of electrostatic charge on a material.

Pith-ball electroscope

  • Gold-leaf electroscope: It was developed in 1787 by British scientist Abraham Bennet as more sensitive equipment than the Pith-ball electroscope. The gold-leaf electroscope is used to detect and classify the electrical charge existing in the body. It is based on the electrostatic induction and repulsion theory. It is made out of a single brass rod with two slender gold leaves on one end and a metal disc on the other. The two thin leaves, as well as an electrically conductive material, are hung parallel to one another and in close proximity to one another. The rod is inserted into the cylindrical glass jar, allowing the gold leaves to remain within while the disc remains outside. A little amount of CaCl2 is inserted within the gold-leaf electroscope to keep the air inside the jar dry, and the lower portion is made up of tin foil. The leaves have no firmness and hang timidly since they are weak and sensitive. When they charge, they divide and form an angle, the charge voltage of which is determined by the amount of impact on the leaves. The potential, not the charge, is displayed on this electroscope. The voltage can also be measured by looking at the angle of separation.

The gold-leaf electroscope

Uses of Electroscope

The electroscope can be used for the following purposes,

  • Its purpose is to detect static charges.
  • An electroscope can be used to determine the nature of electric charges.
  • An electroscope can be used to compare the magnitudes of two distinct charges.

Conductors and Insulators

Some materials readily enable electricity to pass through them, whereas others do not. Conductors are materials that allow electricity to flow freely through them. They have comparatively free-moving electric charges (electrons) inside the material. Conductors include metals, human and animal bodies, and the ground itself. Most non-metals, such as glass, porcelain, plastic, nylon, and wood, have a high resistance to electricity passing through them termed as Insulators.

Differences Between Conductor and Insulators are:

Conductor

Insulator

  • Materials that allow electricity or heat to move through it.
  • Materials that do not allow heat and electricity to move through it.
  • Some examples of a conductor are silver, aluminum, and iron.
  • Some examples of an insulator are plastic, dry paper, dry wood, and rubber.
  • Electron particles freely within the conductor.
  • Electron particles do not move freely within the insulator.

Charging By Induction

Induction charging is a charging method in which an object is charged without actually touching another charged object. The charged particle is held near an uncharged conductive material that is grounded on a neutrally charged material during the charging by an induction procedure. When a charge flows between two objects, the uncharged conductive material develops a charge with the polarity opposite that of the charged object.

Sample problems

Problem 1: Two identical metallic spheres of exactly equal masses are taken. One is given positive charge q coulombs and the other an equal negative charge. Are their masses after charging equal? Explain.

Solution:

No, the body’s positive charge is caused by an absence of electrons, whereas the negative charge is caused by an abundance of electrons. As a result, the negatively charged sphere will have a somewhat higher mass than the positively charged spheres.



Problem 2: A positive charge of 6×10-6 C is 0.040 m from the second positive charge of 4×10-6 C. Calculate the force between the charges.

Solution:

Given,

A positive charge q1 is 6×10-6 C.

The second positive charge q2 is 4×10-6 C.

The distance between the charges r  is 0.040 m.

F_e=k\frac{q_1q_2}{r^2}

Substitute the values in the above expression,

F_e=9\times10^9\times\frac{6\times10^{-6}\times4\times10^{-6}}{0.04^2}\\F_e=9\times10^9\times\frac{24\times10^{-12}}{0.04^2} \\F_e=134.85\text{ N}

Problem 3: What is an electroscope?



Solution:

An electroscope is a scientific apparatus that helps in the detection of an electric charge on a body. The earliest electroscope, known as Vesorium, was a pivoting needle electroscope invented in 1600 by British physicist William Gilbert.

The movement of the test charge is triggered by the Coulomb electrostatic force, which is measured by an electroscope. Electroscopes are frequently used as basic voltmeters since an object’s electrostatic charge is proportional to its capacitance. A large enough concentration of charge to be observed by an electroscope necessitates numerous volts. As a result, electroscopes are frequently used with high-voltage sources like static electricity and electrostatic devices. An electrometer is an instrument used to measure the quantitative electrostatic charge.

Problem 4: Explain the working of an electroscope?

Solution:

An electroscope’s operating principle is based on the atomic structure of atoms, charge induction, the internal structure of metal elements, and the premise that like charges repel each other while unlike charges attract. An electroscope is made up of two thin gold leaves joined to the bottom end of a vertical metal rod contained in a box. When a charged gadget comes into contact with the metal knob at the top of the rod, the charge is transferred to the leaves, causing them to diverge. The amount of charge is indicated by the degree of divergence.

  • Electrons in the metal of the electroscope have an attractive force between the charge and travel upward out of the leaves when the charge is positive. The leaves gain a brief positive charge as a result of this, and because like charges repel each other, the leaves separate. The electrons return to their normal locations when the charge is released, and the leaves relax.
  • 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. The electrons then return to their normal location when the charge is withdrawn, and the leaves relax.

As soon as the charged body is removed, the electrons return to their original state in both circumstances. Furthermore, the electroscope cannot determine whether a charge is positive or negative; it can only detect its presence in a body.

Problem 5: A uniform line charge with linear density λ lies along the y-axis. What flux crosses a spherical surface centered at the origin with r = R.

Solution:

Given,



Line charge density is λ.

The radius of the surface is R.

The length of wire inside the sphere is 2R.

Now,  λdl=dθ

For total charge enclosed by radius R is 

\int_{0}^{Q}Q_{inc}=\int_{0}^{R}\lambda{dl}\\ Q_{inc}=2\lambda{R}

Now, ϕ = Qinco

or

ϕ = 2λR/εo

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