# Chapter 11 | Electric Charges at Rest | Class-7 DAV Science Solutions

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## Something To Know

### A. Fill in the blanks.

1. There are ___________ types of charges in nature.

2. ___________ charges repel each other and unlike charges ___________ each other.

3. The materials, that normally do not get ___________ on rubbing, and allow electric current to flow through them with ease, are known as ___________.

4. Lightning and ___________ occur together.

5. The scientist, who proved that lightning is electrical in nature, was ___________.

Answer: (1) two (2) Like, attract (3) charged, conductors (4) thunder (5) Benjamin Franklin

### B. Write True or False for the following statements.

Answer: (1) False (2) True (3) True (4) True (5) 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—

Ans 1. 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
The object, that cannot be easily charged by rubbing, is the object labelled as—

Ans 2. B: A copper rod

3. Two charged objects, are brought close to each other. The two objects would—

Ans 3. either attract or repel each other.

4. During charging, by induction, it is—

Ans 4. only the earth 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—

Ans 5. decreases in the first case but remains unchanged in the second case.

6. During lightning, we can have an electric discharge—

Ans 6. either between two opposite charged clouds or between a charged cloud and the ground.

### D. Answer the following questions in brief.

1. A glass rod is rubbed against a silk cloth. Which object/s gets/get charged in this process?

Ans 1: When a glass rod is rubbed against a silk cloth then both the objects get charged. The glass rod becomes positively charged and the Silk cloth becomes negatively charged.

2. Name two objects (a) that can be charged by rubbing (b) that do not get charged on rubbing.

Ans 2: (a) Two objects that can be charged by rubbing are balloons and plastic scales.

(b) Two objects that do not get charged on rubbing are iron nails and steel spoons.

3. State the main difference between a conductor and an insulator.

Ans 3: the main difference between a conductor and an insulator are:

Conductor – substances that allow electric charges to pass through them are called conductors.

Insulators – substances that do not allow electric charges to pass through them are called insulators.

4. Name the device used for protecting tall buildings from lightning strikes.

Ans 4: Lightning conductor

5. Give the meaning of the term ‘earthing’.

Ans 5: The process of transfer of charge to the earth.

6. List down two precautions that a person can take if he is caught outside in the open during a lightning strike.

Ans 6: Two precautions that a person can take if he is caught outside in the open during a lightning strike are as follows:-

(1) The person should go to a safe place immediately like a house or a building.

(2) The person should go inside a car or a bus provided the windows and doors are shut.

### E. Answer the following questions.

1. A charged drinking plastic straw is brought near another suspended plastic straw, that is (a) similarly charged (b) uncharged. Explain what we would observe in each case and why.

Ans 1: (a) repulsion; because like charges repel each other.

(b) attraction; because a charged object attracts another unchanged object.

2. Why do we say that ‘charges are always produced in pairs’?

Ans 2: We say that charges are always produced in pairs because when two objects are rubbed against each other, they both get charged up with equal and opposite amounts of charges.

3. Describe a simple experiment to show that the total charge on a pair of objects, that have been rubbed together, is zero.

Ans 3: Activity:- When we rub. Ebonite rod against flannel, both get charged up. If we brought these two charged objects simultaneously near pieces of paper, the combination does not attract them. Hence, we conclude that the total charge on a pair of objects that have been rubbed together is zero.

4. Why does a charged object lose its charge when touched?

Ans 4: A charged object lose its charge when touched because when we touch a charged object, the extra charges present on the object get shared with the earth through our body.

5. Can we say that a charged object attracts an uncharged object by first charging it through induction? Explain your answer.

Ans 5: No, when we bring a charged object near an unchanged object, the two kinds of charges (Already present) in the uncharged object get separated out and opposite kind of charges tend to move towards charged object. Hence unchanged object gets attracted.

6. State the sign of the charge acquired by an object when it is charged, by a positively charged rod, through (a) conduction (b) induction. Give the reason for your statement.

Ans 6: (a) Positive charged.

Reason: The charge (on the original charged object) gets shared between the two objects.

(b) Negative charged.

Reason: When we bring a charged object near an unchanged one, the negative and positive charges get separated out. The positive charges get shared with the earth and negative charges remain near the positively charged object and thus uncharged object gets negatively charged.

7. Is it important to have a very good contact between the lower end of a lightning conductor and the earth? If so, why?

Ans 7: Yes, it is important to have a very good contact between the lower end of a lightning conductor and the earth because, the lightning charges flow down through it, deep down into the earth and not through the structure. The structure is therefore saved from the damaging effects of lightning.