The DAV Class 7 Science Solutions and DAV Class 7 Science Chapter 11 Question Answer – Electric Charges at Rest are essential study tools for DAV public school students in Class 7.
DAV Class 7 Science Ch 11 Question Answer – Electric Charges at Rest
DAV Class 7 Science Ch 1 Solutions – Electric Charges at Rest
Something To Know
A. Fill in the blanks.
Question 1.
There are ________ types of charges in nature.
Answer:
Two
Question 2.
________ charges repel each other and unlike charges ________ each other.
Answer:
Like, attract
Question 3.
The materials, that normally do not get ________ on rubbing, and allow electric current to flow through them with ease, are known as ________
Answer:
Charged, charged
Question 4.
Lightning and ________ occur together.
Answer:
thunder
Question 5.
The scientist, who proved that lightning is electrical in nature, was ________
Answer:
benjamin franklin
B. Write true or false for the following statements.
1. The charges acquired by a glass rod, and the silk cloth with which it is rubbed, are equal and of the same sign.
2. During charging by conduction, the charged object shares its charge with the uncharged object.
3. A charged body loses its charge if we touch it with our hand.
4. Lightning is a natural electrical discharge phenomenon.
5. We do not need any special device to protect tall buildings from lightning.
Answer:
1. The charges acquired by a glass rod, and the silk cloth with which it is rubbed, are equal and of the same sign. (False)
2. During charging by conduction, the charged object shares its charge with the uncharged object. (True)
3. A charged body loses its charge if we touch it with our hand. (True)
4. Lightning is a natural electrical discharge phenomenon. (True)
5. We do not need any special device to protect tall buildings from lightning. (False)
C. Tick (✓) the correct option.
1. A charged plastic comb (comb B) is brought near another similarly charged, and suspended plastic comb (comb A). Then comb A would
(a) Remain unaffected
(b) Move to warns comb B
(c) Move away from comb B
(d) Start oscillating
Answer:
(c) Move away from comb B
2. A student is provided with four objects:
(a) A plastic comb
(b) A copper rod
(c) A rubber balloon
(d) A woollen cloth
Answer:
The one that cannot be easily charged by rubbing, is the object labelled as
(a) A
(b) B
(c) C
(d) D
Answer:
(b) B
3. Two charged objects, are brought close to each other. The two objects would
(a) Always repel each other
(b) Always attract each other
(c) Either attract or repel each other
(d) Neither attract nor repel each other
Answer:
(c) Either attract or repel each other
4. During charging, by induction, it is
(a) The charged object, as well as the earth, that share their charges with the object to be charged.
(b) Only the charged object that shares its charges, with the object to be charged.
(c) Only the earth that ‘shares its charges’, with the object to be charged.
(d) The uncharged object is able to produce charges on itself.
Answer:
(b) Only the charged object that shares its charges with the object to be charged.
5. During charging (i) by conduction, and (ii) by induction, the charge on the charging object
(a) Decreases in both the cases.
(b) Decreases in the first case but remains unchanged in the second case.
(c) Decreases in the second case but remains unchanged in the first case.
(d) Remains unchanged in both the cases.
Answer:
(a) decreases in both the cases.
6. During lightning, we can have an electric discharge
(a) Only between two opposite charged clouds.
(b) Only between a charged cloud and the ground.
(c) Either between two opposite charged clouds or between a charged cloud and the ground.
(d) Neither between two opposite charged clouds nor between a charged cloud and the ground.
Answer:
(b) Only between a charged cloud and the ground
D. Answer the following questions in brief.
Question 1.
A glass rod is rubbed against a silk cloth. Which object/s gets/get charged in this process?
Answer:
When a glass rod is rubbed against silk cloth, both of them get charged in this process.
Question 2.
Name two objects (a) that can be charged by rubbing (b) that do not get charged on rubbing.
Answer:
Objects that can be charged by rubbing: Plastic comb, glass rod Objects that do not get charged on rubbing: Iron rod, metal knife
Question 3.
State the main difference between a conductor and an insulator.
Answer:
A conductor allows electric current to flow through it, while an insulator does not allow it.
Question 4.
Name the device used for protecting tall buildings from lightning strikes.
Answer:
Lightning conductor
Question 5.
Give the meaning of the term ‘earthing’.
Answer:
The process of transfer of charge from a charged body to the earth is called earthing.
Question 6.
List down two precautions that a person can take if he is caught outside in the open during a lightning strike.
Answer:
Safety precautions during lightning for a person caught outside in the open:
- At the first signs of lightning, immediately go to a safe place, such as a house or a building.
- Inside of car and bus is also safe, if the windows and doors are closed.
- Avoid open areas and sheds.
- Stay away from trees.
E. Answer the following questions.
Question 1.
A charged drinking straw is brought near another suspended plastic straw, that is (a) similarly charged (b) uncharged. Explain what we would observed in each case and why.
Answer:
When a charged drinking plastic straw is brought near another similarly charged straw, both of them repel each other. This happens because similar charges repel each other. When a charged drinking plastic straw is brought near an uncharged plastic straw, the charged straw attracts the uncharged one. This happens because a charged object attracts uncharged light weight objects.
Question 2.
Why do we say that ‘charges are always produced in pairs’?
Answer:
When an object is rubbed against another object both of them get charged up. Due to this, it is said that charges are always produced in pairs.
Question 3.
Describe a simple experiment to show that the total charge on a pair of objects, that have been rubbed together is zero.
Answer:
For this, we need an electroscope, a glass rod and a silk cloth. Rub the glass rod on silk cloth; so that both of them get charged up. Now, touch the metallic rod of electroscope with glass rod. Note the degree of separation of gold leaves. Wait for some time, till the gold leaves get discharged.
After that, touch the metallic rod of electroscope with silk cloth. Note the degree of separation of gold leaves. You will notice that in both cases the degree of separation between gold leaves is similar. This shows that the total charge on a pair of objects rubbed together is zero because charges on both of them are equal but opposite.
Question 4.
Why does a charged object, lose its charge when touched?
Answer:
When a charged object is touched, our body provides a route for earthing of the charge. Due to this, when a charged object is touched, it loses its charge.
Question 5.
Can we say that a charged object attracts an uncharged object by first charging it through induction? Explain your answer.
Answer:
When an uncharged object is brought near a charged object, the uncharged object is charged by induction. The uncharged object acquires a charge which is opposite to the charge on the charged object. Thus, the uncharged object is first charged and then attracted towards the charged object.
Question 6.
State the sign of the charge acquired by an object when it is charged, by a positively charged rod, through fa) conduction (b) induction. Give the reason for your statement.
Answer:
In case of charging by conduction, the object would be positively charged. In case of charging by induction, the object would be negatively charged.
Question 7.
Is it important to have a very good contact between the lower end of a lightning conductor and the earth? If so, why?
Answer:
It is indeed important to have a very good contact between the lower end of a lightning conductor and the earth, because the total charge coming from lightning needs to be passed to the earth. This is necessary to save the building from damage. In case of faulty contact, some of the charge may get transferred to the building, which can result in loss of life and property.
Value Based Question
Tavish was driving back home from his office when it started raining heavily. The rain poured along with a thundering sound and lightning. On the way he saw an old man walking along the street. Tavish stopped the car and requested him to immediately get inside the car. Later on, he dropped the old man at his place.
Question 1.
State two values displayed by Tavish.
Answer:
Tavish is showing concern and empathy for other.
Question 2.
Did Tavish do the right thing by asking the old man to get inside his car? Give reason for your answer.
Answer:
Tavish did the right thing because a car is much safer than an open space during lightning.
Something To Do
Question 1.
To design a simple metal leaf electroscope.
We can design a simple version of a gold leaf electroscope by using
(i) An aluminium leaf
(ii) A wooden stand
(iii) A metal wire and
(iv) A box made from thermocole sheets with transparent paper walls.
We can use it for applications similar to that of a gold leaf electroscope.
Answer:
Do it yourself.
Question 2.
Solve the crossword with the help of given clues.
Across:
1. Charging by rubbing.
2. A natural electric discharge phenomenon.
3. A positive charge gets attracted by a charge.
4. Loud sound.
Down:
5. Transfer of charge from one object to another.
6. Charging an object without touching it.
7. Charging by making contact.
8. Transfer of charge to earth.
Answer:
DAV Class 7 Science Chapter 1 Solutions – Electric Charges at Rest
I. Fill in the blanks.
Question 1.
Two positively charged objects would ________ each other.
Answer:
Repel
Question 2.
A positively charged object would ________ negatively charged object.
Answer:
Attract
Question 3.
Contact between two objects is necessary for charging by ________
Answer:
Conduction
Question 4.
Air is a ________ of electric current.
Answer:
Bad conductor
Question 5.
Lightning is always accompanied by ________
Answer:
Bad conductor
II. State whether the following statements are True or False.
1. A metallic rod can be charged by rubbing.
2. A plastic comb cannot be charged by rubbing.
3. Clouds get charged up because of friction.
4. Lightning conductor can be seen on top of skyscraper.
5. Lightning damages the ozone layer.
6. Lightning helps in nitrogen fixation in soil.
Answer:
1. A metallic rod can be charged by rubbing. (False)
2. A plastic comb cannot be charged by rubbing. (False)
3. Clouds get charged up because of friction. (True)
4. Lightning conductor can be seen on top of skyscraper. (True)
5. Lightning damages the ozone layer. (False)
6. Lightning helps in nitrogen fixation in soil. (True)
III. Answer the following questions in brief.
Question 1.
Name some objects that can be charged by rubbing.
Answer:
Plastic comb, drinking straw (plastic), glass rod, balloon, etc.
Question 2.
Name four objects that do not get charged on rubbing.
Answer:
Iron rod, copper rod, metal knife, etc.
IV. Answer the following questions.
Question 1.
What do you understand by conductors?
Answer:
Metallic materials allow electric charges to flow through them and hence are called conductors. As electric charge can flow through them, so they do not get charged up on rubbing.
Question 2.
What do you understand by insulators?
Answer:
Insulators: Non-metallic materials do not allow electric charges to flow through them and hence are called insulators. As electric charge cannot flow through them, so they get charged up on rubbing
Question 3.
Which are the various methods of charging up objects?
Answer:
Methods of Charging Up:
- Charging by rubbing or friction.
- Charging by contact or conduction.
- Charging by induction.
Question 4.
Explain electrical induction.
Answer:
When a charged object is brought near an uncharged object, the charges get transferred to the uncharged object. The opposite charges move to the nearer end and the like charges move to the farther end of the uncharged object. This process of separation of charges is called electrical induction.
Question 5.
Write a brief note on lightning conductor.
Answer:
This is a device which is installed on buildings to prevent the building from harmful effects of lightning. It is made up of a long flat strip of metal. A conducting wire is attached to it which goes deep into the ground. When lightning strikes, the electric charge is transferred via the metallic wire to the earth. Thus, building is saved from harmful effects of lightning. Lightning conductor can be seen on tall buildings, telephone towers, temples and churches.
Question 6.
A glass rod is rubbed against silk cloth and an ebonite rod is rubbed against a piece of flannel. List the pairs which have
(a) similar
(b) dissimilar charges on them.
Answer:
Glass rod and ebonite rod have similar charges on them. Silk cloth and flannel have similar charged on them. Glass rod and silk cloth have opposite charges on them. Ebonite rod and flannel have opposite charges on them.
Question 7.
State at least two advantages of lightning.
Answer:
Lightning converts nitrogen into its oxide which then comes down along with rains. This helps in nitrogen fixation in soil. Lightning also helps in conversion of oxygen into ozone and thus helps in formation and maintenance of ozone layer in the atmosphere.
V. Projects
Question 1.
Make a model of lighting conductor and display it in your classroom.
Answer:
Do it yourself.
Question 2.
With the help of an inflated balloon, show electrical charges and their interaction.
Answer:
Do it yourself.