Samacheer Kalvi 8th Science Solutions Chapter 5 Pdf

Samacheer Kalvi 8th Science Solutions Chapter 5 Pdf

Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 5: Tamilnadu State Board Solution Class 8 Science Chapter 5 – Electricity.

Samacheer Kalvi 8th Science Solutions Chapter 5: Overview

Board

Samacheer Kalvi
Class

8

Subject

Science
Chapter

5

Chapter Name

Electricity


Samacheer Kalvi 8th Science Solutions Chapter 5 Pdf

( I ) Choose the best answer.

1 ) When an ebonite rod is rubbed with fur, the charge acquired by the fur is

a ) negative

b) positive

c) partly positive and partly negative

d) None of these

2 ) The electrification of two different bodies on rubbing is because of the transfer of

a) neutrons

b) protons

c) electrons

d) protons and neutrons

3) Which of the following a simple circuit must have?

a) Energy source, Battery, Load

b) Energy source, Wire, Load

c) Energy source, Wire, Switch

d) Battery , Wire , Switch

4) An electroscope has been charged by induction with the help of charged glassrod.The charge on the electroscope is

a) negative

b) positive

c) both positive and negative

d) None of the above

5) Fuse is

a) a switch

b) a wire with low resistance

c) a wire with high resistance

d) a protective device for breaking an electric circuit

(II ). Fill in the blanks .

1 ) Transfer of electron takes place by rubbing objects together.

2 ) The body which has lost electrons becomes positive.

3) Lightning arrestor is a device that protects building form lightning strike.

4) Electric fuse has a thin metallic filament that melts and breaks the connection when the circuit is overheated.

5) Three bulbs are connected end to end from the battery. This connection is called series circuit.

(III.) State true or false. If false, correct the statement.

1 ) The charge  acquired by an ebonite rod rubbed with a piece of flannel is negative.

This statement is true.

2) A charged body induces an opposite charge on an uncharged body when they are brought near.

This statement is true.

3) Electroscope is a device used to charge a body by induction.

This statement is true

4) Water can conduct electricity.

This statement is true .

5) In parallel circuit, current remains the same in all components.

This statement is false.

Explanation :

In parallel circuit, current does not remain the same in all components but rather it is voltage that remains the same in all components.

(IV ) Match the following .

Two similar charges

acquires a positive charge
Two dissimilar charges

prevents a circuit from overheating

When glass rod is rubbed with silk

repel each other
When ebonite rod is rubbed with fur

attract each other

Fuse

acquires a negative charge

Answer :

Two similar charges repel each other
Two dissimilar charges attract each other
When glass rod is rubbed with silk acquires a positive charge
When ebonite rod is rubbed with fur acquires a negative charge
Fuse prevents a circuit from overheating

(V) Give reason for the following.

1 ) When a glass rod is rubbed with silk cloth both get charged.

There is transmission of free electrons from the glass rod to the silk cloth ,when a glass rod is rubbed with a silk cloth. This happens because ,in the glass rod, the free electrons are not so tightly bound when compared to the silk cloth. Therefore, the glass rod looses electrons ,as it does not have sufficient of electrons and hence positive charge is acquired. But, there are plenty of electrons in the silk cloth. As a result of which it acquires negative charge.

2 ) When a comb is rubbed with dry hair it attracts small bits of paper.

A comb becomes negatively charged ,when it is rubbed with hair because it gains electrons from the hair. There is  accumulation of these electrons on the surface of the comb. There is presence of both positive and negative charges at the edges of the bits, when a piece of paper is teared into bits. The positive charges in the bits get’s attracted by the negative charges in the comb. For this reason ,there is movement of paper bits in the direction of the comb.

3) When you touch the metal disc of an electroscope with a charged glass rod the metal leaves gets diverged.

When a charged object touches the brass disc of the electroscope ,there is transmission of electric charges to the gold leaf through the rod. This makes the gold leaves more apart from one another . This occurs because both the leaves possess similar charges.

4) In an electroscope the connecting rod and the leaves are all metals.

These are composed of metals because this will enable the electrons to move freely.

5) One should not use an umbrella while crossing an open field during thunderstorm.

During thunderstorm, it is not advisable to carry an umbrella. As an umbrella is made in such a manner, that the rod and it’s supporting wires are composed of metals. So, for this reason, one should avoid carrying such conducting object during thunderstorm.

(VI) Consider the statements given below and choose the correct option.

1 ) Assertion :  People struck by lightning receive a severe electrical shock.

Reason : Lightning carries very high voltage.

Answer : a ) Both assertion and reason are true and reason is the correct explanation of assertion.

2 )  Assertion : It is safer to stand under a tall tree during lightning.

Reason : It will make you the target for lightning.

Answer : d) Assertion is false but reason is true.

Explanation:

During lightning, one should avoid standing under a tall tree.

(VII) Answer briefly .

 1 ) How charges are produced by friction?

When combing the hair, there is transmission of charges from the hair to the comb because of friction. There is reduction in the occurrence of friction between the hair and the comb, when the hair is wet and this ultimately decrease the number of electrons that are transferred from hair to comb. Therefore, when certain materials are rubbed with each other it can result in the expansion of electrical charges on the surfaces. This clearly depicts that, there is transmission of charges by friction.

2) What is earthing ?

If the insulation inside electrical devices fails, a safety measure is mainly designed for the purpose of preventing people from getting shocked and this is known as earthing. Electrical earthing refers to the process in which ,the discharge of electrical energy are transferred directly to the earth with the support of poor- resistance wire. We acquire electrical energy from distinct sources. Battery falls under the category of such source. Wall clocks and cell phones are few examples. We use domestic power supply in order to operate refrigerators, air conditioners ,washing machines ,televisions, laptops and water heaters. Live ,neutral and earth are namely the three wires, where a heater and iron box are usually fitted. There is connection of the earth wire to the metallic body of the appliance. This is executed for the purpose of avoiding accidental shock.

3) What is electric circuit?

Electric circuit refers to the path in which there is flow of electrons from one end to another end of the source. There are four elements in a simple circuit . They are : an electrical source ( battery) , a path or conductor in which there is flow of electricity (wire) , a switch in order to control the circuit and an electrical ressistor (lamp ) which needs electricity in order to operate.

4) What is electroplating?

One of the most common applications of chemical effects of electric current is electroplating. Electroplating refers to the process, where a layer of one metal are deposited over the surface of another metal by passing electric current. There are applications of electroplating in several fields. In order to provide strength, we use iron in bridges and automobiles. But iron shows the tendency of rusting and corrosion. For this reason, there is deposition of zinc coating on iron, for the purpose of protecting it from rusting and corrosion. Chromium possess a shiny appearance. It is not subjected to corrosion. It possess a capacity to withstand scratches. The whole object could not be made of chromium because it is quite expensive and is not budget friendly. For this reason, car parts ,bath taps ,kitchen gas burners and wheel rims are composed of inexpensive metal and there is deposition of only a layer of chromium over it.

5) Give some uses of electroplating.

There are applications of electroplating in several fields. In order to provide strength, we use iron in bridges and automobiles. But iron shows the tendency of rusting and corrosion. For this reason, there is deposition of zinc coating on iron, for the purpose of protecting it from rusting and corrosion. Chromium possess a shiny appearance. It is not subjected to corrosion. It possess a capacity to withstand scratches. The whole object could not be made of chromium because it is quite expensive and is not budget friendly. For this reason, car parts ,bath taps ,kitchen gas burners and wheel rims are composed of inexpensive metal and there is deposition of only a layer of chromium over it.

 

(VIII) Answer in detail.

1) Explain three ways of charge transfer.

Transfer of Charges

There is transmission of electrons from one substance to another substance. The substance becomes negatively charged when they gain electrons and the substance becomes positively charged when they loose these electrons. The three ways of charge transfer are as follows:

  • Transfer by Friction
  • Transfer by Conduction
  • Transfer by Induction

Transfer by Friction

A comb becomes negatively charged, when it is rubbed with hair because it gains electrons from the hair. There is accumulation of these electrons on the surface of the comb. There is presence of both positive and negative charges at the edges of the bits, when a piece of paper is teared into bits. The positive charges in the bits get’s attracted by the negative charges in the comb. For this reason, there is movement of paper bits in the direction of the comb.

When combing the hair, there is transmission of charges from the hair to the comb because of friction. There is reduction in the occurrence of friction between the hair and the comb, when the hair is wet and this ultimately decrease the number of electrons that are transferred from hair to comb. Therefore, when certain materials are rubbed with each other it can result in the expansion of electrical charges on the surfaces. This clearly depicts that, there is transmission of charges by friction.

Transfer by Conduction

There is transmission of electrons from the woollen cloth to the eboniterod, when the ebonite rod is rubbed with woollen cloth. Now, ebonite rod will acquire negative charge. The positive charges in the cylinder attracts the negative charges in the rod when it is brought near the paper cylinder. When the rod touches the cylinder, there is transmission of some negative charges to the paper. Therefore, there is a repulsion of the negative charges in the rod by the negative charges in the cylinder. Here , we can affirm that there is transmission of charges to an object ,if it is brought in connection with a charged body. This procedure of charge transmission from one body to another body is known as transfer by conduction.

Transfer by Induction

We know that, when we touch an uncharged object by a charged object ,we can charge it. However, a body can also acquire charges even without having any contact with other charged body. Induction may be defined as the process in which an uncharged body can obtain charges by bringing a charged body near to it, instead of touching it. At the near end, the uncharged body obtains an opposite charge and at the furtherend it acquires similar charge.

2) What is electroscope?  Explain how it works.

An electroscope is a scientific device that is mainly designed for the purpose of detecting the presence of electric charge on a body. Electroscope was first invented by British physician William Gilbert. It is considered as the first electrical device. Pith -ball electroscope and gold leaf electroscope are mainly the two types of electroscope. An electroscope is composed of metal, which is a conducting material. It operates on the principle that similar charges exhibits the tendency to repel one another. In a simple electroscope, there are two metal sheets that are hanged in contact with one another. There is connection of such metal sheets to a metal rod that expands in an upward direction and there is a knob at the end where it ends. If a charged object is brought close to the knob , electrons will try to come out of it or will try to go within it. This will lead to accumulation of charges on the metal leaves within the electroscope. The free electrons in the electroscope will tend to move downward into the leaves, when a negatively charged object is brought near the top knob of the  electroscope, rendering the top positive. As both the leaves are negatively charged, so they exhibit the tendency to repel each other and draw apart . The free electrons in the electroscope will tend to move upward in the direction of the knob , when a positively charged object is brought near the top  knob of the electroscope. This clearly indicates that, there is a net positive charge at the bottom. Again, the leaves will spread apart.

3) Explain series and parallel circuit.

Series Circuit

There is more than one resistor in a series circuit but the electrons can travel only through one path. The electrons move along one path from one end of the battery in the absence of any branches through the resistor to another end of the cell. There are end-to-end connection of all the components in a series circuit. So, current remains constant throughout the circuit. But, there is splitting of the voltage across the bulb in the circuit. Two bulbs are used as resistors in the following series circuit. Charges from the battery have only one path to travel in this series circuit. There are connection of battery, key and two bulbs in series. One at a time, there is flow of charges from the battery to each bulb, in the sequence they are wired to the circuit. There would be a disruption in the flow of current to another bulb , if one bulb in the circuit is screwed out. During festivals, we put serial lights. One burned-out bulb can disable all the other lights, if they are in a series circuit. The radiance of the light will fade to a considerable extent, if  the number of bulbs in a circuit with a battery increase. This is because numerous resistors are functioning on the similar power from the battery.

Parallel Circuit

There is more than one resistor in a parallel circuit and they are organized on several paths. This clearly depicts that, electrons can move from one end of the cell through several branches to another end of the cell. Here, voltage remains constant across the resistors but there is splitting of the current that is flowing through the circuit across each resistor.ABEFA and ABCDEFA are the two paths in which current can flow in parallel circuit. In order to return to the cell, the electricity from the cell can choose either path ABEFA or path ABCDEFA . So, it will be noticed that, the other bulbs will continue to operate even if one bulb is burned- out . This is because the   electricity is flowing through two or more paths. In our homes, all the light bulbs are connected in parallel circuit. The burning- out of one bulb will not bring about any effect on the working of other bulbs in the room. The bulbs in parallel circuit will not be dimmer as compared to the lights in serial circuits . This is due to the fact that, voltage across all other branches is same as the voltage across one branch.

4) How lightning takes place?

When there is discharge of electricity from cloud to cloud or from cloud to ground ,then there is formation of lightning. Air moves rapidly in an upward direction during thunderstorm. Small air crystals are carried by this air in an upward direction. Simultaneously, there is movement of small water drops in a downward direction. Their collision enables the ice crystals to become positively charged and move in an upward direction and the water drops move in a downward direction as they are negatively charged.Therefore, the upper part of the cloud have acquired positive charge and the lower part of the cloud have acquired negative charge. The positive charges in the ice crystals attracts the electrons in the water drops, the moment they come into contact. As a result of which, there is generation of electricity and lightning is perceived. Sometimes, the positive charges assembled near the mountains, trees and people on the earth, get’s contacted with the lower part of the cloud which is negatively charged. This discharge generates tremendous heat and sparks which ultimately give rise to lightning.

5) What is electroplating? Explain how it is done.

One of the most common applications of chemical effects of electric current is electroplating. Electroplating refers to the process, where a layer of one metal are deposited over the surface of another metal by passing electric current.

A glass jar should be filled with copper sulphate solution. A copper metal plate should be connected to the positive terminal of battery. Then ,an iron spoon should be connected to the negative terminal of the battery. Now, they should be dipped into the copper sulphate solution. By passing an electric current through the copper sulphate solution, we will notice a deposition of a thin layer of copper metal on the iron spoon and a disappearance of an equal quantity of copper by the copper plate.

Updated: November 19, 2021 — 10:55 am

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