Samacheer Class 8 Science - Electricity

Two similar charges repel each other. When two objects carry the same type of charge, whether both positive or both negative, they experience a repulsive force that pushes them apart. Two dissimilar charges attract each other. When one object carries a positive charge and another carries a negative charge, they experience an attractive force that pulls them together. When a glass rod is rubbed with silk, the glass rod acquires a positive charge. Silk has a greater tendency to accept electrons, so electrons are transferred from the glass rod to the silk, leaving the glass rod positively charged. When an ebonite rod is rubbed with fur, the ebonite rod acquires a negative charge. Ebonite has a greater tendency to accept electrons, so electrons are transferred from the fur to the ebonite rod, giving it a negative charge. A fuse is a safety device that prevents a circuit from overheating. It consists of a thin wire made of a metal alloy with a low melting point. When excessive current flows through the circuit due to a short circuit or overload, the fuse wire heats up rapidly and melts, breaking the circuit and stopping the flow of current. This protects the circuit and connected devices from damage due to excessive heat.

When a glass rod is rubbed with a silk cloth, both materials become charged due to the transfer of electrons between them. The reason for this charge transfer lies in the difference in how tightly electrons are bound in each material. In a glass rod, the free electrons are less tightly bound compared to those in silk cloth. During rubbing, these loosely bound electrons from the glass rod are transferred to the silk cloth through friction. Since the glass rod loses electrons, it develops a deficiency of electrons and acquires a positive charge. Conversely, the silk cloth gains these excess electrons and becomes negatively charged. This process demonstrates that charging by friction occurs when electrons move from one material to another based on their relative ability to hold electrons, a property related to the electronegativity of the materials involved.

When a comb is rubbed with dry hair, it becomes charged through the transfer of electrons. The comb gains electrons from the hair during the rubbing process and becomes negatively charged. These electrons accumulate on the surface of the comb, creating a region of negative charge. When a piece of paper is torn into small bits, both positive and negative charges are distributed throughout the paper, but they become separated at the edges of the bits. The negatively charged comb exerts an attractive force on the positive charges present at the edges of the paper bits, since opposite charges attract each other. This electrostatic attraction causes the paper bits to move toward the comb and stick to it. This phenomenon demonstrates how a charged object can attract uncharged or weakly charged objects through the redistribution of charges within those objects.

The leaves of an electroscope diverge when a charged glass rod touches the metal disc because of the transfer and distribution of charge. When the positively charged glass rod makes contact with the metal disc, the charge is transferred from the rod to the metal disc. Since the connecting rod and leaves are made of metal, which is a good conductor of electricity, the charge travels freely through the metal rod to reach the metal leaves. Once the charge reaches both leaves, they both acquire the same type of charge as the rod, which is positive charge. Since like charges repel each other, the two leaves experience a repulsive force and move away from each other, causing them to diverge or spread apart. The degree of divergence indicates the amount of charge transferred to the electroscope, making it a useful device for detecting and comparing electric charges.

Using an umbrella during a thunderstorm is not advisable because of the risk of lightning strike. The rod of an umbrella and its supporting wires are typically made of metals such as iron or steel. Metals are excellent conductors of electricity, meaning they allow electric current to flow through them very easily. During a thunderstorm, lightning is essentially a massive electrical discharge from clouds to the ground. If a person carries a metal umbrella in an open field, the metal parts act as a conducting path that can attract lightning. When lightning strikes the umbrella, the electrical current travels through the metal rod and wires directly to the person holding it, causing severe injury or death. Therefore, it is safer to avoid carrying any conducting objects like umbrellas during thunderstorms and to seek shelter in a building or vehicle instead. The general rule during thunderstorms is to avoid being in open spaces and to stay away from tall conducting objects.

Charges are produced by friction when two different materials are rubbed together. During the rubbing process, electrons are transferred from one material to another due to differences in how tightly the electrons are bound in each material. The material that loses electrons becomes positively charged because it has a deficiency of electrons, while the material that gains electrons becomes negatively charged because it has an excess of electrons. This process of charge production through friction is called charging by friction or triboelectric effect. A common example is combing dry hair with a comb. When the comb is rubbed through the hair, electrons are transferred from the hair to the comb due to friction. The hair loses electrons and becomes positively charged, while the comb gains electrons and becomes negatively charged. This is why a charged comb can attract small pieces of paper or cause hair to stand on end. The amount of charge produced depends on the nature of the materials, the force applied during rubbing, and the duration of the rubbing process.

Electroplating is a process with numerous practical applications in various industries. One major use of electroplating is to protect metals from corrosion and rust. For example, iron is widely used in bridges, automobiles, and other structures because of its strength and durability. However, iron is susceptible to corrosion and rust when exposed to moisture and oxygen. To protect iron from these harmful effects, a coating of zinc is deposited on the iron surface through electroplating. This zinc coating acts as a barrier, preventing direct contact between iron and corrosive agents, thereby extending the life of the iron structure. Another important use of electroplating is to improve the appearance and durability of objects. Chromium is often used for electroplating because it has a shiny, attractive appearance and does not corrode easily. Chromium coating also resists scratches and wear, making it ideal for decorative and functional purposes. Electroplating is also used to deposit other metals like nickel, copper, and silver on various objects to enhance their properties, improve their appearance, or provide specific functional characteristics required for particular applications.

Transfer of charges takes place in the following three ways:
Transfer by Friction
Transfer by Conduction
Transfer by Induction
1. Transfer by Friction:
This method of charging an uncharged body by rubbing it ; against another body is called charging by friction.
Example:
While combing hair charges are transferred from the hair to comb due to friction.
2. Transfer by conduction:
Charges can be transferred to on object by bringing it in contact with a charged body. This method of transferring charges from one body to other body is calledtransfer by conduction.
Example:
When the ebonite rod is rubbed with woollen cloth, electrons from the woollen cloth are transferred to the ebonite rod. Now ebonite rod will be negatively charged.
(i) When it is brought near the paper cylinder, negative charges in the rod are attracted by the positive charges in the cylinder.
(ii) When the cylinder is touched by the rod, some negative charges are transferred to the paper. Hence, the negative charges in the rod are repelled by the negative charges in the cylinder.
3. Transfer by Induction:
The process of charging an uncharged body by bringing a charged body near to it but without touching it is called induction.
Example:
we can charge an uncharged object when we touch it by a charged object. But, it is also possible to obtain charges in a body without any contact with other charges.

1. An electroscope is a scientific instrument used to detect the presence of electric charge on a body.
2. An electroscope is made out of conducting materials, generally metal.
3. It works on the principle that like charges repel each other.
4. In a simple electroscope two metal sheets are hung in contact with each other.
5. They are connected to a metal rod that extends upwards, and ends in a knob at the end.6. If you bring a charged object near the knob, electrons will either move out of it or into it.
7. This will result in charges on the metal leaves inside the electroscope.
8. If a negatively charged object is brought near the top knob of the electroscope, it causes free electrons in the electroscope to move down into the leaves, leaving the top positive..
9. Since both the leaves have negative charge, they repel each other and move apart.
10. If a positive object is brought near the top knob of the electroscope, the free electrons in the electroscope start to move up towards the knob.
11. This means that the bottom has a net positive charge. The leaves will spread apart again. Explain series and parallel circuit.

Series Circuit:A series circuit is one that has more than one resistor (bulb) but only one path through which the electrons can travel.
From one end of the battery the electrons move along one pathwith no branches through the resistors (bulbs) to the other end of the cell.
All the components in a series circuit are connected end to end.
So, current through the circuit remains same throughout the circuit.
But, the voltage gets divided across the bulbs in the circuit.
In the following series circuit two bulbs are used as resistors.
Let I be the current through the circuit and V 1 V 2 , V 3 be the voltage across each bulb.
The supply voltage V is the total of the individual voltage drops across the resistances. V = V 1 + V 2 + V 3
Parallel Circuit:In a parallel circuit, there is more than one resistor (bulb) and they are arranged on many paths.
This means charges (electrons) can travel from one end of the cell through many branches to the other end of the cell.
Here, voltage across the resistors (bulbs) remains the same but the current flowing through the circuit gets divided across each resistor.
Let us consider three bulbs connected in series.
Let V be the voltage across the bulbs and I 1 , I 2 , I 3 be the current across each bulb.
The current I from the battery is the total of the individual current flowing through the resistances. I = I 1 + I 2 + I 3

1. Lightning is produced by discharge of electricity from cloud to cloud or from cloud to ground.
2. During thunderstorm air is moving upward rapidly.
3. This air which moves rapidly, carries small ice crystals upward.
4. At the same time, small water drops move downward.
5. When they collide, ice crystals become positively charged and move upward and the water drops become negatively charged and move downward.6. So the upper part of the cloud is positively charged and the lower part of the cloud is negatively charged.
7. When they come into contact, electrons in the water drops are attracted by the positive charges in the ice crystals. Thus, electricity is generated and lightning is seen.
8. Sometimes the lower part of the cloud which is negatively charged comes into contact with the positive charges accumulated near the mountains, trees and even people on the earth. This discharge produces lot of heat and sparks that results in what we see as lightning.

Electroplating:Electroplating is one of the most common applications of chemical effects of electric current.
The process of depositing a layer of one metal over the surface of another metal by passing electric current is called electroplating.
Take a glass jar and fill it with copper sulphate solution.
Take a copper metal plate and connect it to the positive terminal of battery.
Connect an iron spoon to the negative terminal of the battery.
Now, dip them in the copper sulphate solution.
When electric current is passed through the copper sulphate solution, you will find that a thin layer of copper metal is deposited on the iron spoon and an equivalent amount of copper is lost by the copper plate.

When a comb is placed near pieces of paper without any prior treatment, no attraction occurs because the comb is electrically neutral and the paper pieces are also uncharged. However, when the comb is rubbed through dry hair, the situation changes dramatically. During the rubbing process, electrons are transferred from the hair to the comb due to friction. The comb gains electrons and becomes negatively charged, while the hair loses electrons and becomes positively charged. These electrons accumulate on the surface of the comb, creating a region of concentrated negative charge. When the charged comb is brought near the paper pieces, the electric field around the comb influences the distribution of charges within the paper. Although the paper pieces are initially neutral, the presence of the negatively charged comb causes the positive charges in the paper to be attracted toward the comb while the negative charges are repelled away. This redistribution of charges within the paper creates an attractive force between the negatively charged comb and the positive charges in the paper. As a result, the paper pieces are attracted to the comb and stick to it. This demonstrates the principle of electrostatic attraction between oppositely charged objects.

In this experiment, the paper cylinder initially shows attraction to the charged ebonite rod because the rod is negatively charged and the cylinder is electrically neutral. The electric field around the negatively charged rod causes the positive charges in the paper to move toward the rod while the negative charges move away, resulting in a net attractive force. However, when the charged ebonite rod actually touches the paper cylinder, the situation changes significantly. During contact, electrons from the negatively charged rod are transferred to the paper cylinder. The paper cylinder now acquires a negative charge from the rod. Once both the rod and the cylinder have the same type of charge, which is negative, they experience a repulsive force according to the principle that like charges repel each other. This repulsion causes the paper cylinder to move away from the rod. The transition from attraction to repulsion clearly demonstrates the difference between the electrostatic forces acting on a neutral object near a charged object and the forces acting when both objects carry the same type of charge.

When you rub your foot on a carpet floor and then touch a doorknob, you experience an electric shock. This phenomenon occurs due to the process of charge discharge. Rubbing your foot on the carpet causes electrons to be transferred from the carpet to your body through friction. Your body accumulates a large number of excess electrons and becomes negatively charged. When you touch the doorknob, which is typically made of metal and is connected to the ground, a sudden discharge occurs. The excess electrons on your body are rapidly transferred to the doorknob and then to the ground through the metal. This rapid movement of a large number of electrons in a short time constitutes an electric current, which is felt as a shock or tingling sensation in your hand and fingers. The shock is the result of this sudden discharge of accumulated charge. The intensity of the shock depends on the amount of charge accumulated on your body and the speed at which the discharge occurs. This is why the shock is more noticeable on dry days when the air has low humidity, as moisture in the air would allow the charge to dissipate gradually rather than accumulating.

Yes, when you touch a bulb that has been glowing for a minute or so, you will feel that it is hot. This occurs because of the heating effect of electric current. When electric current flows through the filament of the bulb, which is made of a material with high electrical resistance, the electrical energy is converted into heat energy. The resistance of the filament causes the moving electrons to collide with the atoms of the filament material, transferring their kinetic energy to the atoms. This energy transfer causes the atoms to vibrate more vigorously, which manifests as an increase in temperature. As the temperature of the filament increases, it becomes hot and also emits light, which is why the bulb glows. The heating effect of electric current is not limited to light bulbs but occurs in all electrical devices where current flows through a resistive material. This principle is utilized in various heating appliances such as electric heaters, toasters, and electric irons. The amount of heat produced depends on the resistance of the material, the magnitude of the current flowing through it, and the duration for which the current flows.