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# Samacheer Kalvi 8th Science Books Electricity

## Science : Chapter 5 : Electricity

TEXTBOOK EXERCISES

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. electronsâ€ƒ

d. protons and neutrons

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

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

[Answer: (d) a protective device for breaking an electric circuit]

II.â€‚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 from 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.â€‚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. [Answer: True]

2. A charged body induces an opposite charge on an uncharged body when they are brought near. [Answer: True]

3. Electroscope is a device used to charge a body by induction. [Answer: True]

4. Water can conduct electricity. [Answer: True]

5. In parallel circuit, current remains the same in all components. [Answer: False]

Correct statement: In parallel circuit, voltage remains the same in all components.

IV.â€‚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

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.â€‚Give reason for the following.

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

Reason : When a glass rod is rubbed with a silk cloth the free electrons in the glass rod are transferred to silk cloth. It is because the free electrons in the glass rod are less tightly bound as compared to that in silk cloth. Since the glass rod looses electrons, it has a deficiency of electrons and hence acquires positive charge. But, the silk cloth has excess of electrons. So, it becomes negatively charged.

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

Reason: Comb rubbed with hair gains electrons from the hair and becomes negatively charged. These electrons are accumulated on the surface of the comb. When a piece of paper is teared into bits, positive and negative charges are present at the edges of the bits. Negative charges in the comb attract positive charges in the bits. So, the paper bits are moving towards the comb.

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

Reason : The leaves of an electroscope diverge because when a glass rod is touched on the metal disc, the charge travels through the metal rod to the leaves. Since, like charge repel, the charge travels till the leaves and then open up as both the leaves have like charges.

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

Reason: These are made of metals so that the electrons become free to move.

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

Reason:

(i) Carrying an umbrella is not advisable during thunderstorm.

(ii) As the rod of umbrella and its supporting wires are made up of metals.

(iii) Thus during thunderstorm, the conducting object should be avoided.

VI.â€‚Consider the statements given below and choose the correct option.

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

b. Both assertion and reason are true and reason is not the correct explanation of assertion.

c. Assertion is true but reason is false.

d. Assertion is false but reason is true.

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: The assertion is false but reason is true]

1. How charges are produced by friction?

Answer: Rubbing certain materials with one another can cause the built-up of electrical charges on the surfaces. So charges are produced by friction. Eg.: Combing hair charges are transferred from the hair to comb due to friction.

2. What is earthing?

Answer: Earthing is the process of connecting the exposed metal parts of an electrical circuit to the ground.

3. What is electric circuit?

Answer: The path through which electrons flow from one terminal to another terminal of the source, is called electric circuit.

4. What is electroplating?

Answer: The process of depositing a layer of one metal over the surface of another metal by passing electric current is called electroplating.

5. Give some uses of electroplating.

Answer: Electro plating is applied in many fields :

(i) We use iron in bridges and automobiles to provide strength.

(ii) A coating of zinc is deposited on iron to protect it from corrosion and formation of rust.

(iii) Chromium has a shiny appearance. It does not corrode.

(iv) It resists scratches.

1. Explain three ways of charge transfer.

Transfer of charges takes place in the following three ways.

Transfer by Friction

Transfer by Conduction

Transfer by Induction

(a) Transfer by Friction : This method of charging an uncharged body by rubbing it against another body is called charging by friction. Eg.: While combing hair charges are transferred from the hair to comb due to friction.

(b) 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 called transfer by conduction. Eg. : 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.

(c) 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. Eg.: 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.

2. What is electroscope? Explain how it works.

(i) An electroscope is a scientific instrument used to detect the presence of electric charge on a body.

(ii) An electroscope is made out of conducting materials, generally metal.

(iii) It works on the principle that like charges repel each other.

(iv) In a simple electroscope two metal sheets are hung in contact with each other.

(v) They are connected to a metal rod that extends upwards, and ends in a knob at the end.

(vi) If you bring a charged object near the knob, electrons will either move out of it or into it.

(vii) This will result in charges on the metal leaves inside the electroscope.

(viii) 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.

(ix) Since both the leaves have negative charge, they repel each other and move apart.

(x) 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.

(xi) This means that the bottom has a net positive charge. The leaves will spread apart again.

3. Explain series and parallel circuit.

Series Circuit

(i) A series circuit is one that has more than one resistor (bulb) but only one path through which the electrons can travel.

(ii) From one end of the battery the electrons move along one path with no branches through the resistors (bulbs) to the other end  of the cell.

(iii) All the components in a series circuit are connected end to end.

(iv) So, current through the circuit remains same throughout the circuit.

(v) But, the voltage gets divided across the bulbs in the circuit.

(vi)  In the following series circuit two bulbs are used as resistors.

(vii) Let I be the current through the circuit and V1,V2, V3 be the voltage across each bulb.

(viii) The supply voltage V is the total of the individual voltage drops across the resistances. V= v1 + v2 + v3

Parallel Circuit:

(i) In a parallel circuit, there is more than one resistor (bulb) and they are arranged on many paths.

(ii) This means charges (electrons) can travel from one end of the cell through many branches to the other end of the cell.

(iii) Here, voltage across the resistors (bulbs) remains the same but the current flowing through the circuit gets divided across each resistor.

(iv) Let us consider three bulbs connected in series.

(v) Let V be the voltage across the bulbs and I1,I2,Ibe the current across each bulb.

(vi) The current I from the battery is the total of the individual current flowing through the resistances. I =  I1+I2+I3

4. How lightning takes place?

(i) Lightning is produced by discharge,of electricity from cloud to cloud or from cloud to ground.

(ii) During thunderstorm air is moving upward rapidly.

(iii) This air which moves rapidly, carries small ice crystals upward.

(iv) At the same time, small water drops move downward.

(v) When they collide, ice crystals become positively charged and move upward and the water drops become negatively charged and move downward.

(vi) So the upper part of the cloud is positively charged and the lower part of the cloud is negatively charged.

(vii) 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.

(viii) 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.

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

Electroplating

(i) Electroplating is one of the most common applications of chemical effects of electric current.

(ii) The process of depositing a layer of one metal over the surface of another metal by passing  electric current is called electroplating.

(iii) Take a glass jar and fill it with copper sulphate solution.

(iv) Take a copper metal plate and connect it to the positive terminal of battery.

(v) Connect an iron spoon to the negative terminal of the battery.

(vi) Now, dip them in the copper sulphate solution.

(vii) 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.

## Student Activities

Activity 1

Take a comb and place it near some pieces of paper. Are they attracted by the comb? No. Now comb your dry hair and place it near them. What do you see? You can see that the paper pieces are attracted by the comb now. How is it possible?

Answer: Comb attracts a bits of paper because Comb is charged object and paper is in neutral so, to be charged one must be in neutral and Comb is negative charged as it gains electrons from hair. Since ,when Comb is placed near to a bits of paper then papers get charged due to induction.

Comb rubbed with hair gains electrons from the hair and becomes negatively charged. These electrons are accumulated on the surface of the comb. When a piece of paper is teared into bits, positive and negative charges are present at the edges of the bits. Negative charges in the comb attract positive charges in the bits. So, the paper bits are moving towards the comb. While combing hair, charges are transferred from the hair to comb due to friction. If the hair is wet, the friction between the hair and the comb reduces which will reduce the number of electrons transferring from hair to comb. Hence, rubbing certain materials with one another can cause the build-up of electrical charges on the surfaces. From this it is clear that charges are transferred by friction.

A neutral object can become positively charged when electrons get transferred to another object; not by receiving extra positive charges.

Activity 2

Take a sheet of paper. Turn it into a hollow cylinder. Tie one end of the cylinder with a silk thread and hang it from a stand. Now take an ebonite rod and charge it by rubbing it with a woollen cloth. Bring this charged ebonite rod near the paper cylinder. The cylinder will be attracted by the rod. If you touch the paper cylinder by the charged rod, you will see the paper cylinder repelling the rod. Can you say the reason?

Answer: 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.

Activity 3

Bring a negatively charged plastic rod near a neutral rod. When the negatively charged plastic rod is brought close to the neutral rod, the free electrons move away due to repulsion and start piling up at the farther end. The near end becomes positively charged due to deficit of electrons. When the neutural rod is grounded, the negative charges flow to the ground. The positive charges at the near end remain held due to attractive forces and the electrons inside the metal becomes zero. When the rod is removed from the ground, the positive charges continue to be held at the near end. This makes the neutral rod a positively charged rod.

Similarly, when a positively charged rod is brought near an uncharged rod, negatively charged electrons are attracted towards it. As a result there is excess of electrons at nearer end and deficiency of electrons at the farther end. The nearer end of the uncharged rod becomes negatively charged and far end is positively charged.

Activity 4

Rub your foot on a carpet floor and touch a door knob. What do you feel? Do you feel the shock in your hand? Why does this happen?

Answer: Getting a shock from a doorknob after rubbing your foot on a carpet floor, results from discharge. Discharge occurs when electrons onthe hand are quickly pulled to the positively charged doorknob. This movement of electrons,which is felt as a shock, causes the body to lose negative charge.

Activity 5

Take two pieces of wire, an LED light and a battery, and make a simple electric circuit. Take some water in a glass and put the wires in the water as shown in the figure. Does the LED bulb glow? What do you understand from this?

Answer: Yes, the LED bulb glows. From this activity we understood that liquids also conduct electricity.

Activity 6

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.

Activity 7

Take a battery, a bulb, a switch and few connecting wires. Make an electric circuit as shown in the figure. Keep the switch in the â€˜OFFâ€™ position.

Does the bulb glow?

Answer: No, the bulb does not glow.

Now move the electric switch to the â€˜ONâ€™ position and let the bulb glow for a minute or so. Touch the bulb now. Do you feel the heat?

Answer: Yes, the bulb is hot because electrical energy is transformed to heat energy. This is known as heating effect of electric current.

If a positively charged glass rod is brought near another glass rod, the rods will move apart as they repel each other. If a positively chargedglass rod is brought close to a negatively charged ebonite rod, the rods will move toward each other as they attract. The force of attraction or repulsion is greater when the charged objects are closer.

The materials which allow electric charges to pass through them easily are called conductors of electricity. For example,metals like aluminium, copper are good conductors of electricity. Materials which do not allow electric charges to pass through them easily are called insulators. Rubber, wood and plastic are insulators.

The first electroscope developed in 1600 by William Gilbert was called versorium. The versorium was simply a metal needle allowed to pivot freely on a pedestal. The metal would be attracted to charged bodies brought near.

Lightning’s extreme heat will vaporize the water inside a tree, creating steam that may burn out the tree.

During lightning and thunder, we should avoid standing in ground or open spaces. You should make yourself as small as possible bysquating. It is however safe to stay inside a car because the car acts as a shield and protects us from the electric field generated by the storm.

The electric eel is a species of fish which can give electric shocks of upto six hundred fifty watts of electricity. But if the eel repeatedlyshocks, its electric organs become completely discharged. Then a person can touch it without being shocked.