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Page No 185:

Question 1:

There are two kinds of charges. How do we know this?

Answer:

When charges are brought close to each other, sometimes they attract each other and sometimes they repel each other. Hence, we can conclude that there are two types of charge present in nature. 

Page No 185:

Question 2:

What effect do the two kinds of charges have on each other?

Answer:

The two kinds of charges are basically positive charges and negative charges. They always attract each other.

Page No 185:

Question 3:

Where do the two kinds of charges come from?

Answer:

There are two kinds of charges present in nature, positive charges and negative charges. The negative charges come from an excess of negatively charged particles and the positive charges come from an excess of positively charged particles. 

Page No 185:

Question 4:

Two objects rubbed against each other acquire charge.
a. What is the relationship between the charges acquired by the two objects?
b. What action does rubbing cause, because of which the objects acquire charge?

Answer:

(a) The charges acquired by the two objects are opposite in nature but are the same in magnitude.

(b) While rubbing, some of the negatively charged particles move from one object to another object. Due to that one object is left with an excess of negatively charged particles and another with an excess of positively charged particles. That's why both the objects acquire charge. 
 



Page No 190:

Question 1:

An electroscope is used to detect and measure charges. What happens in an electroscope when its disc is touched with a charged object?

Answer:

When the disc is touched with a charged object, the charge distributes on both the strips of the electroscope. As both strips get similar charges, so they repel and it creates a divergence between the two strips.

Page No 190:

Question 2:

To find the nature of charge on an object, will you use a charged electroscope or an uncharged one?

Answer:

I will use a charged electroscope so that I can observe the change in the divergence of the strips of the electroscope. If the nature of the unknown charge would be similar to the known charge of the electroscope, the divergence of the strips would increase and if the nature of the charge would be opposite to the known charge on the strips of electroscope then the divergence would decrease. That's how we can find the nature of an unknown charge. 

Page No 190:

Question 3:

Should the rod of the electroscope connecting the disc with the strips be made of a conductor or an insulator? Why?

Answer:

The rod of the electroscope connecting the disc with the strips should be made of a conductor so that the charge given to the disc can distribute itself on both the strips of the electroscope. If it would be an insulator then the charge given to the disc would not reach to the strips and strips will never diverge. 

Page No 190:

Question 4:

Air is an insulator, but can conduct electricity under certain conditions. What are these conditions? What is conduction of electricity through air called?

Answer:

Air allows electricity to pass through it if the quantity of the charge is large and the distance is small. In such a situation a very heavy charge passes through the air in a very short span of time. The conduction of electricity through the air is known as Electrical discharge. 

Page No 190:

Question 5:

If lightning strikes a building, it flows harmlessly through the lightning conductor. What property should the material of which the lightning conductor is made have, to make this possible?

Answer:

The lightning conductor is made of a metal rod ending in spikes at the top which is connected to a copper plate buried deep inside the ground. If lightning strikes a building the metal rod attracts the excess of charge and pass it to the ground. 



Page No 193:

Question 1:

By how many times is an earthquake of magnitude 5 on the Richter scale more powerful than one of magnitude 4?

Answer:

On the Richter scale, an earthquake of magnitude 5 is 10 times more powerful than one of magnitude 4.

Page No 193:

Question 2:

Which of these lies on the earth's surface- seismic focus or epicentre?

Answer:

Epicenter lies on the surface of the earth while seismic focus lies within the crust of the earth.

Page No 193:

Question 3:

Earthquakes are most likely to occur in fault zones. Where are these fault zones normally situated?

Answer:

Earthquakes are most likely to occur in fault zones. These fault zones lie at the boundaries of the tectonic plates.

Page No 193:

Question 4:

During an earthquake, you should take shelter in high-rise buildings. Do you agree? Give reasons.

Answer:

High-rise buildings are most likely to fall during an earthquake. So, we should not take shelter in a high-rise building. Instead, we should leave the building and come out in the open ground.



Page No 195:

Question 1:

Like charges
(a) always repel each other.
(b) always attract each other.
(c) can repel or attract each other depending on conditions.
(d) can repel or attract each other depending on the quantity of charge.

Answer:

(a) always repel each other

Two like charges always repel each other.

Page No 195:

Question 2:

A body can be charged by
(a) rubbing it against another body.
(b) touching it to a charged body.
(c) bringing a charged body near it.
(d) all of the above methods.

Answer:

(d) all of the above methods

An uncharged body can be charged when it is rubbed against another body, touched with a charged body or brought near a charged body.

Page No 195:

Question 3:

When two bodies are rubbed against each other,
(a) they acquire equal and similar charges.
(b) they acquire equal and opposite charges.
(c) they acquire unequal and similar charges.
(d) they acquire unequal and opposite charges.

Answer:

(b) they acquire equal and opposite charges

When two bodies are rubbed against each other, they acquire equal and opposite charges because on rubbing, charges move from one body to another.

Page No 195:

Question 4:

The gold-leaf electroscope can be used to
(a) detect charge only.
(b) detect and measure charge only.
(c) detect, measure and find the nature of charge.
(d) none of the above

Answer:

(c) detect, measure and find the nature of charge

 

Page No 195:

Question 5:

Electric charge can flow through
(a) insulators.
(b) conductors.
(c) both insulators and conductors.
(d) neither insulators nor conductors.

Answer:

(b) conductors

Electric charge can flow through a conductor but not through an insulator.

Page No 195:

Question 6:

A lightning conductor is a
(a) piece of wire with spikes through which current can flow.
(b) substance that can be charged by clouds.
(c) metal rod with spikes, ending in a copper plate buried in the ground, fixed to a building to protect it from lightning.
(d) copper plate buried in the ground below a building to protect it from lightning.

Answer:

(c) metal rod, with spikes ending in a copper plate buried in the ground, fixed to a building to protect it from lightning

 

Page No 195:

Question 7:

To test if a body is charged or not you will use
(a) another uncharged body.
(b) a positively charged body.
(c) a negatively charged body.
(d) a positively and a negatively charged body.

Answer:

(d) positively and negatively charged bodies

To test whether a body is charged or not, we will bring it close to positively and negatively charged bodies because an uncharged body is attracted by both positively and negatively charged bodies.

Page No 195:

Question 8:

You are given an uncharged electroscope and a charged body. No other apparatus is allowed. You can use the electroscope to
(a) only detect the charge.
(b) detect and measure the charge.
(c) detect, measure and find the nature of the charge.
(d) neither of these since the electroscope is uncharged.

Answer:

(b) detect and measure the charge

We can only detect and measure charge using an uncharged electroscope and a charged body. To test the nature of the charge, we need another body with a known charge.

Page No 195:

Question 1:

When charges of static electricity flow through conductors we get current electricity. True or false?

Answer:

True.

The flow of charge in a conductor is called an electric current.



Page No 196:

Question 2:

Two combs made of the same material are rubbed against your hair and brought near each other. Will they attract or repel each other?

Answer:

When two combs made of the same material are rubbed against hair, they acquire the same nature of charge. So, when they are brought near, they repel each other.

Page No 196:

Question 3:

A negatively-charged object will __________ a positively-charged object.

Answer:

A negatively charged object will attract a positively charged object.

Page No 196:

Question 4:

What kind of charge is produced in a silk cloth when it is rubbed against a glass rod?

Answer:

When a silk cloth is rubbed against a glass rod, the glass rod becomes positively charged and the silk cloth becomes negatively charged.

Page No 196:

Question 5:

Under normal circumstances atoms contain equal number of positive and negative charges. True or false?

Answer:

True.

Under normal circumstances, in an atom, the number of particles with negative charge (called electrons) is equal to the number of particles with positive charge (called protons).

Page No 196:

Question 6:

Which charges in an atom are more free to move about−positive or negative?

Answer:

In an atom, the negatively charged particles (called electrons) are more free to move than the positively charged particles (called protons).

Page No 196:

Question 7:

Charging a body by bringing a charged body near it, but not touching it is called charging by __________

Answer:

Charging a body by bringing a charged body near it but not touching it is called charging by induction.

Page No 196:

Question 8:

Charging a body by touching it with a charged body is called charging by __________

Answer:

Charging a body by touching it with a charged body is called charging by conduction.

Page No 196:

Question 9:

Name the instrument used for detecting and measuring charge.

Answer:

Electroscope is used for detecting and measuring charge.

Page No 196:

Question 10:

To find the nature of charge using a gold-leaf electroscope, should the electroscope be initially charged or uncharged?

Answer:

To check whether a body has a positive or negative charge, it is necessary to charge the electroscope with a known charge.

Page No 196:

Question 11:

What is the flow of charge called? Can charge flow through an ebonite rod?

Answer:

The flow of charge in a conductor is called an electric current. Charges cannot flow through an ebonite rod because it is an insulator.

Page No 196:

Question 12:

The flow of heavy charge through air, accompanied by heat and light is called __________

Answer:

The flow of heavy charge through air accompanied by heat and light is called lightning.

Page No 196:

Question 13:

Lightning is due to flow of charge between clouds. True or false?

Answer:

False. Lightning is due to the discharge of two static unlikely charged clouds when they come close to each other.

 

Page No 196:

Question 14:

An electroscope is charged by induction using a negatively-charged body. What charge will the electroscope acquire − positive or negative?

Answer:

When an electroscope is charged by induction using a negatively charged body, it acquires the opposite kind of charge, i.e., positive.

Page No 196:

Question 15:

When charged by conduction a body acquires the __________ (same/opposite) charge as the charging body.

Answer:

When charged by conduction, a body acquires the same charge as the charging body.

Page No 196:

Question 16:

The intensity of an earthquake is measured on the __________ scale.

Answer:

The intensity of an earthquake is measured on the Richter scale.

Page No 196:

Question 17:

A __________ records the waves generated by an earthquake, known as __________ waves.

Answer:

A seismograph or seismometer records the waves generated by an earthquake. These waves are known as seismic waves.

Page No 196:

Question 18:

An earthquake is most likely to occur in the centre of the tectonic plates. True or false?

Answer:

False. The boundaries of tectonic plates are the zones where earthquakes are most likely to occur.

Page No 196:

Question 19:

Volcanic activity can also cause earthquakes. True or false?

Answer:

True.

During volcanic eruptions, very explosive violent gases move upwards; this may lead to earthquakes.

Page No 196:

Question 1:

Why does a comb rubbed on hair attract pieces of paper?

Answer:

When a comb is rubbed on hair, it gets charged. When this charged comb is brought near uncharged pieces of paper, they get attracted towards the comb.

Page No 196:

Question 2:

Under what conditions do charges attract or repel each other?

Answer:

Two like charges (positive or negative) repel each other and two unlike charges (one positive and one negative) always attract each other.

Page No 196:

Question 3:

Why do we say that only repulsion is a sure test of charge on a body?

Answer:

A charged body attracts a body with an opposite charge. It also attracts an uncharged body. That is why repulsion alone is a sure test to check whether a body is charged or not.

Page No 196:

Question 4:

What do you mean by 'charging by conduction'? What kind of charge does the body acquire?

Answer:

Conduction is the process of charging a body by touching it to a charged body. In this process, the body that is being charged acquires the same kind of charge as the charging body.

Page No 196:

Question 5:

What is 'charging by induction'? What kind of charge does the body acquire?

Answer:

The process of charging a body by bringing a charged body near it without touching it is called charging by induction. In this process, the body being charged acquires the opposite kind of charge to that of the body charging it.

Page No 196:

Question 6:

What is the difference between conductors and non-conductors? Give two examples of each.

Answer:

Conductors Non-Conductors
Charges can flow through a conductor. Charges cannot flow through an insulator.
They conduct electricity. They do not conduct electricity.
Examples: Copper and silver Examples: Plastic and ebonite

Page No 196:

Question 7:

What is an electroscope?

Answer:

An electroscope is a device that is used to detect, measure and find the nature of charge.

Page No 196:

Question 8:

What is 'earthing'?

Answer:

The process of transferring charges from a charged object to the earth is called earthing.

Page No 196:

Question 1:

Explain why, when two bodies, charged by rubbing them together, acquire equal and opposite charge.

Answer:

In an atom, the positively charged particles are firmly bounded, whereas the negatively charged particles are loosely bounded. So, when two bodies are rubbed together, some negatively charged particles move from one body to another. Thus, the body losing negative charges becomes positively charged and the body gaining negative charges becomes negatively charged.



Page No 197:

Question 2:

What is the difference between static and current electricity?

Answer:

Static Electricity Current Electricity
Static electricity is the study of charges at rest. Current electricity is the study of charges in motion.
When a body is charged by rubbing, it acquires static electricity. The flow of current in a conductor is called an electric current.

 

Page No 197:

Question 3:

What is an electric discharge? Under what conditions does it occur?

Answer:

The loss of static electricity as charges move from an object is called an electric discharge. When two objects with opposite charges come close to each other, there is a sudden flow of electricity between them. For example, when two clouds with unlike charges come close to each other, a very heavy current passes through the air; this results in lightning.

Page No 197:

Question 4:

In what situation can lightning be dangerous? How can the danger to tall buildings be reduced?

Answer:

Clouds can charge tall buildings by induction. The buildings, thus, acquire the opposite charge. If the built-up charge is very heavy, a heavy flow of electric charge can occur between the clouds and the buildings. This can result in the buildings catching fire. To protect tall buildings from damage by lightning, a lightning conductor is fixed on them. If lightning strikes a building, it flows harmlessly to the earth through the lightning conductor, causing no damage to the building.

Page No 197:

Question 5:

How does a lightning conductor work?

Answer:

To protect tall buildings from damage by lightning, lightning conductors are fixed on them. A lightning conductor consists of a metal rod ending in spikes at the top. The lower end of the rod is attached to a copper plate buried deep in the earth. If lightning strikes a building, it flows harmlessly to the earth through the metal rod, causing no damage to the building. 

Page No 197:

Question 6:

List four precautions you will take to save yourself from a lightning strike during a thunderstorm.

Answer:

We can take the following precautions to save ourselves from lightning strikes during a thunderstorm:

(1) We can take shelter in a building with lightning conductor.

(2) A car or bus with closed windows is also a safe place to take shelter.

(3) We should avoid taking shelter under a tall tree, as lightning may strike it.

(4) An open elevated place is not a safe place to be in. If in an open place, we should stay away from trees and poles.

Page No 197:

Question 7:

What is the Richter scale? Why do we say that it is not a linear scale?

Answer:

The magnitude or intensity of an earthquake is measured on the Richter scale. The number indicating the intensity of an earthquake on the Richter scale ranges from 0 to 9. The Richter scale is not a linear scale because the amplitude of vibrations of an earthquake varies nonlinearly with the number on the scale.

Page No 197:

Question 8:

List three effects of an earthquake.

Answer:

An earthquake may cause the following effects:

(1) It can result in the deformation of the surface of the ground.

(2) It can damage man-made structures such as buildings, roads, rails and bridges.

(3) It can cause landslides and human causalities in hilly areas. 

Page No 197:

Question 1:

Draw a labelled diagram of a gold-leaf electroscope. Describe how you will use it to detect a charge, measure a charge, and find the nature of the charge.

Answer:

Gold-leaf Electroscope

Process to Detect and Measure Charge

If a charged body (for example, ebonite rod) is touched to the metal disc of the gold-leaf electroscope, the metal strips of the electroscope gets charged by conduction. They repel each other and open up. In this way, we can detect whether an object is charged or not.

Process to Know the Nature of Charge

To check whether a body has a positive or negative charge, we need a charged electroscope with a known charge. We need to touch the charged body to the metal disc of the electroscope. If the divergence of the leaves increases, the body has a charge similar to that of the charged electroscope; and if the divergence of the leaves decreases, the body has a charge opposite to that of the electroscope.

Page No 197:

Question 2:

Explain how lightning occurs between two clouds during a storm.

Answer:

During a storm, air currents with small water droplets move upwards, while those with big water droplets move downwards. These vigorous movements cause the separation of charges due to friction. The positively charged light droplets get collected near the upper edges of the clouds, while the negatively charged big water droplets get accumulated near the lower edges. There is also accumulation of positive charges near the ground because of induction. When the magnitude of the accumulated charges becomes very large, the air between the two clouds (with unlike charges), which is an insulator, no longer resists these clouds. As a result, the negative and positive charges meet, producing streaks of bright light and sound due to electric discharge. This process is called lightning.  
 

Page No 197:

Question 3:

Explain the process of lightning striking a building or tree.

Answer:

Clouds can charge tall buildings or trees. The negative charge on the lower edge of the clouds leads to accumulation of positive charge on the upper portion of the buildings or trees due to induction. If the magnitude of the accumulated charges is very large, the air that acts as an insulator between the clouds and the buildings could no longer resists the charges. As a result, a very heavy charge passes through the air in short time because of the electric discharge and huge amount of energy releases in the form of heat, light and sound. This process is called lightning. When lightning strikes a building or tree, it catches fire because of the huge amount of energy released.   

Page No 197:

Question 4:

In what ways is lightning useful?

Answer:

Lightning is useful to us in the following ways:

(1) The heat and light released during lightning enable nitrogen in the air to combine with oxygen to form oxides of nitrogen. These get dissolved in water to form dilute nitric acid that comes down with rain. The acid reacts with salts in the soil to form nitrogenous compounds that are essential for the growth of plants.

(2) Lightning also enables ozone to be formed from the oxygen in the air. The ozone present in the air stops the harmful ultraviolet rays of the sun from reaching us. 

Page No 197:

Question 5:

Explain how an earthquake occurs.

Answer:

Earthquakes are caused by disturbances deep down the uppermost layer of the earth known as the crust. The tectonic plates of the earth float over the hot magma below and are in relative motion to each other. When these plates collide and when the rocks at the joints are sufficiently slippery, the plates slide over each other. However, sometimes, the rocks at the joints get interlocked and struck, thus resisting pressure from within. For years, the forces pushing the plates build up until the strain rips the plates apart. As a result, the rocks crack and shift, sending shock waves in all directions. These waves reach the surface and are felt as tremors.

Page No 197:

Question 6:

List three things you will do during an earthquake.

Answer:

We should do the following things during an earthquake:

  1. We should leave the building where we are and move to some open area. In outdoors, we should be away from high buildings, trees, electric poles, etc.
  2. If we are trapped in a building, we should take shelter under a table, protecting our head with hands, and should not move until the shaking stops. It is better to avoid lift during an earthquake.
  3. If we are in a car or bus, we should not come out until the tremors stop.

Page No 197:

Question 1:

Why are taller buildings in greater danger of being struck by lightning?

Answer:

Lightning is a fairly random occurrence. It may strike a tall building or a small house. But it causes greater danger to a tall building because a lot of material is used in its construction. If lightning strikes a tall building, the huge current involved can heat the building material to a high temperature, causing a potential for fire.
 

Page No 197:

Question 2:

When a positively-charged body is brought near an uncharged metal rod, the part of the rod near the charged body acquires a negative charge, and the far side a positive charge. Can you say why this happens? Remember that in a body, negative charges are mobile whereas positive charges are fixed.

Answer:

When a positively charged body is brought near an uncharged metal rod, the part of the rod near the charged body acquires a negative charge because of the process of induction. The positively charged body attracts the negative charges, i.e., electrons towards it. The negative charges get accumulated near the end of the metal rod. The far end of the rod becomes positively charged because of the deficiency of negative charges.

Page No 197:

Question 3:

If you hold a plastic comb in your hand and rub it in your hair, it will get charged. However, if you do the same with a metal comb it will not get charged. Why? But if the metal comb has a plastic handle and you hold it from the handle, it will also get charged. Why?

Answer:

Plastic is an insulator, whereas metals are good conductors of electricity. When a plastic comb is rubbed on our hair, it acquires static charges, which do not move. If we do the same with a metal comb, it will not hold static electricity; the charges move from the comb to our body. But if the metal comb has a plastic handle, it will hold the charges, as the charges cannot move from the comb to our body.
 

Page No 197:

Question 4:

Why do you think charging by rubbing happens best in dry weather?

Answer:

Charging by rubbing happens best in dry weather. Air is an insulator, but the presence of moisture in air makes it conductive, which, in turn, makes static charges to flow through the air to make the object electrically neutral. Because the moisture level is quite low in dry weather, charging happens best in it.
 

Page No 197:

Question 5:

The rod of a lightning  conductor accidentally broke into two from the middle. What danger will this pose if lightning strikes the building? Why?

Answer:

If the rod of a lightning conductor installed in a building accidently breaks into two from the middle and lightning strikes that building, it can cause huge damage to life and property. The heavy charges would easily find another conductive path through the building structure such as building wiring or plumbing and cause fire or other disaster.
 

Page No 197:

Question 1:

An aircraft flies just below a negatively charged thundercloud.



Answer these questions.
1. Will any electrostatic charges be included on the aircraft?
2. If so, draw the signs and positions of the induced charges on the aircraft.
3. Explain the distribution of the charges that you have shown on the basis of the movement of electrons.
4. Draw a diagram to show the charges after the aircraft has flown away from the cloud.
​5. Explain the distribution of charges you have shown in Ans. 4 on the basis of the movement of electrons. 

Answer:

(1) Yes, aircraft is made up of conducting material, so the electrostatic charge will get induced on the surface of aircraft. 

(2) Induced charges on the aircraft:

 

(3) When an aircraft passes below the negatively charged cloud, due to the force of repulsion of negative charge from the cloud, the electrons on the surface of aircraft shift away from the surface and leave the surface positively charged. As you can see in the above figure. Due to the movement of electrons, the surface of the aircraft becomes positively charged while the bottom of the aircraft becomes negatively charged.

(4) After the aircraft has flown away from the cloud:



(5) As soon as the aircraft passes the cloud, the charges redistribute themselves to their original places. The electrons which moved to the bottom of the aircraft due to the presence of highly negatively charged cloud, return to their original places and neutralize the positive charge.



Page No 198:

Question 1:

When an earthquake suddenly struck Rani's village, she shouted to tell the people that they should come out of their houses and go into the fields, since there were no trees, electric poles or buildings nearby.
Later she went about organizing help for the injured and people who had been rendered homeless.
​What values did Rani show?

Answer:

Rani showed the following values:

(i) Rani is an intelligent girl. She knows exactly what to do when an earthquake hits. 

(ii) She is also very caring, supporting and a responsible citizen of the society because she does not save only herself but also tells other people to come out in the open and save themselves. Later she participates in helping the injured people too.



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