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Page No 185:
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 185:
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 185:
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 186:
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 186:
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 186:
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 186:
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 186:
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 186:
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 186:
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 186:
Question 3:
A negatively-charged object will __________ a positively-charged object.
Answer:
A negatively charged object will attract a positively charged object.
Page No 186:
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 186:
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 186:
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 186:
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 186:
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 186:
Question 9:
Name the instrument used for detecting and measuring charge.
Answer:
Electroscope is used for detecting and measuring charge.
Page No 186:
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 187:
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 187:
Question 2:
Explain how lightning occurs between two clouds during a storm.
Answer:
Page No 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
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 187:
Question 8:
What is 'earthing'?
Answer:
The process of transferring charges from a charged object to the earth is called earthing.
Page No 187:
Question 1:
Explain why, when two bodies, charged by rubbing them together, acquire equal and opposite charge.
Answer:
Page No 187:
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 187:
Question 3:
What is an electric discharge? Under what conditions does it occur?
Answer:
Page No 187:
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 187:
Question 5:
How does a lightning conductor work?
Answer:
Page No 187:
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 187:
Question 7:
What is the Richter scale? Why do we say that it is not a linear scale?
Answer:
Page No 187:
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 188:
Question 3:
Explain the process of lightning striking a building or tree.
Answer:
Page No 188:
Question 4:
In what ways is lightning useful?
Answer:
(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 188:
Question 5:
Explain how an earthquake occurs.
Answer:
Page No 188:
Question 6:
List three things you will do during an earthquake.
Answer:
We should do the following things during an earthquake:
- 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.
- 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.
- If we are in a car or bus, we should not come out until the tremors stop.
Page No 188:
Question 1:
Why are taller buildings in greater danger of being struck by lightning?
Answer:
Page No 188:
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:
Page No 188:
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:
Page No 188:
Question 4:
Why do you think charging by rubbing happens best in dry weather?
Answer:
Page No 188:
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:
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