Solution ICSE class 8 Electricity

class-8 Electricity 

Ch – Electricity ( part B static electricity)

Solutions

Question-1:

What do you understand by electricity at rest?

Answer-1:

Static electricity or Electricity at rest is produced by friction between two insulating materials. When the materials are rubbed together, electrons are transferred from one material to another , giving rise to a static electricity ( Electricity at rest)

Question-2:

Why does a plastic comb rubbed with dry hair attract bits of paper?

Answer-2:

Plastic comb rubbed with dry hair gets electrically charged and therefore it attracts the bits of paper.

Question-3:- 

Who discovered the way of producing electricity by friction?

Answer-3 :

The ancient Greek philosopher Thales  discovered the electricity by friction.

Question-4:

Name two substances which can be charged by friction.

Answer-4 :

Two substances which can be charged by friction are

1. Glass.

2. Ebonite.

Question-5:

What are the two kinds of charges?

Answer-5:

The two kinds of charges are

1. Negative charge

2. Positive charge

Question-6:

A glass rod is rubbed with silk. State the kind of charge acquired by each.

Answer-6:

glass rod is rubbed with silk, a glass rod has  acquires positive charge and the silk acquires negative charge.

Question-7:- 

An ebonite rod is rubbed with fur. State the kind of charge acquired by each.

Answer-7 :

When an ebonite rod is rubbed with fur, the ebonite rod acquires negative charged. The fur acquires a positive .

Question-9 :

How will you show that like charges repel and unlike charges attract each other?

Answer-9 :

To show - Like charges repel and unlike charges attract each other

Experiment - Take a glass rod rubbed with a silk piece that is positively charged. Suspend it in a stand with the help of a thread and bring a negatively charged  ebonite rod rubbed with fur near the glass rod.  We see that two rods are attracted. Now suspend the ebonite rod and bring the glass rod near, we observe attraction between them again.

Observation - This shows that unlike charges attract each other.

Question-10:

A glass rod rubbed with silk is suspended near an ebonite rod rubbed with fur. What will be your observation? Give a reason to your answer.

Answer-10:

A glass rod rubbed with a silk piece that is positively charged and a negatively charged ebonite rod rubbed with fur , two rods will be attracted because they have opposite charges.

Question-11 :

An ebonite rod rubbed with fur is suspended near another ebonite rod rubbed with fur. State your observation and give a reason to support your answer.

Answer-11 :

Two ebonite rods rubbed with fur will have similar negative charges and like charges repel each other. Hence, the two rods will repel each other.

Question-12:

What do you mean by conservation of charges?

Answer-12 :

Conservation of Charge is the principle states when two bodies are rubbed together, both are charged equally, but the charges on them are of the opposite kinds .Thus, the total charge of the objects before and after rubbing remains same. 

Question-13:- 

An ebonite rod is rubbed with fur. Compare the charges acquired by them.

Answer-13:

When an ebonite rod is rubbed with fur, the electrons are transferred from fur to ebonite rod. This makes  number of charges on ebonite rod equal to charges on fur but opposite  

Question-14:

Name three constituents of an atom and state the kind of charge on each of them.

Answer-14:

The three constituents of an atom are: electrons, protons and neutrons 

When an ebonite rod is rubbed with fur, fur loses electrons and ebonite rod gains electrons. fur becomes positively charged and rod becomes negatively charged.

Question-15

What is the net charge on an atom? 

Answer 15:- 

The net charge on an atom is zero.

Question 16:-

Briefly describe the structure of an atom?

Answer 16:-

The central part of an atom called the nucleus contains protons and neutrons, Orbit around the nucleus contains electrons. The number of electrons in an atom is equal to number of protons. Thus an atom is electrical neutral. 

Question 17 :- 

What are the free electrons? 

Answer17:-

The electrons in the outer shells (valence electrons) are loosely bound to the atoms, hence they are free to move , are called free electrons.

Question 18:-

What causes charging of two objects when they are rubbed together ?

Answer:-

When two objects are rubbed, there is transfer of electrons from one object to another. The object which has excess electrons is negatively charged and the object which has deficit electrons is positively charged.

Question-19:-

In each of the following case , state which body loses electrons:- 

a) A glass rod when rubbed with silk.

b) An ebonite rod when rubbed with fur

Answer 19:-

a) When a glass rod when rubbed with silk – glass rod loses electrons 

b) When an  ebonite rod when rubbed with fur,  fur loses electrons. 

Question-20:-

A glass rod is rubbed with silk. Explain the charging of the glass rod and the silk on the basis of electron movement. 

Answer 20:-

 When a glass rod is rubbed with silk, the electrons from the glass rod are transferred to the silk. The glass rod loses some electrons, so it becomes positively charged, and the silk gains the same number of electrons, so it becomes negatively charged by an equal amount.  

Question-21:-

An ebonite rod is rubbed with fur . Explain the changing of the ebonite rod and the fur on the basis of electron movement. 

Answer21:-

When an ebonite rod is rubbed with fur, the free electrons from the fur are transferred to the ebonite rod. The fur loses some electrons and so it becomes positively charged and the ebonite rod gives the same number of electrons, so it becomes negatively charged by the same amount.

Question-22:- 

Distinguish between conductors and insulators of electricity.

Answer-22:

Conductors

They allow the current of electricity to flow through them. They have large numbers of free electrons.

Metals like silver, copper, iron, gold and wet wood are conductors. They cannot be charged by rubbing.

Insulators-They do not allow the current of electricity to flow through them.

They have a small number of free electrons. Cotton, dry wool rubber, glass are insulators. They can be charged by rubbing.

Question-23:

Give one example each of a conductor and an insulator of electricity.

Answer-23:

Conductor - Iron

Insulator - Mica

Question-24:

State two ways of charging a conductor.

Answer-24:

The two ways of charging a conductor are:

(i) Conduction: The process of charging the uncharged object by bringing it in contact with another charged object is called charging by conduction.

(ii) Induction: The process of charging the uncharged object by bringing another charged object near to it, but not touching it, is called charging by induction.

Question-25:

Name the way of charging a conductor in which the charge is shared.

Answer-25:

By conduction method 

Question-26 :

Describe the method of charging a conductor by conduction.

Answer-26 :

The process of charging the uncharged object by bringing it in contact with another charged object is called charging by conduction.

A charged object has an unequal number of negative (electrons) and positive charges (protons). Hence, when a charged object is brought in contact with the uncharged conductor, the electrons get transferred from the charged object or from conductor to charged body .

Question-27:

A metal rod A is to be charged positively by using another charged rod B. What should be the kind of charge on the rod B if charging is to be done by conduction?

Answer-27:

 Rod A is uncharged and rod B is charged. Rod B should have a similar charge as required by Rod B. Rod A is to be charged positively, so rod B should be positively charged by the process conduction.

Question-28:

Explain the charging by conduction in terms of movement of electrons?

Answer-28 :

Place rod A on an insulating stand and bring positively charged conductor rod B near the rod A and touch it. The charge from rod B is transferred on Rod A till they both have a similar charge.

Question-29:- 

Describe the method of charging a conductor by induction.

Answer-29:

The process of charging the uncharged object by bringing another charged object near to it, but not touching it, is called charging by induction.

Consider an uncharged metal sphere and negatively charged plastic rod as shown in below . If we bring the negatively charged plastic rod near to the uncharged sphere , charge separation occurs.

The process of charging the uncharged object by bringing another charged object near to it, but not touching it, is called charging by induction.

The positive charges in the sphere get attracted towards the plastic rod and move to one end of the sphere that is closer to the plastic rod. Similarly, negative charges get repelled from the plastic rod and move to another end of the sphere that is farther away from the plastic rod. Thus, the charges in the sphere rearrange themselves in a way that all the positive charges are nearer to the plastic rod and all the negative charges are farther away from it.

Q30 Explain the charging by induction in terms of movement of electrons. 

Answer:-

Solution: Let uncharged conductor B mounted on insulated stand is placed n positively charged conductor A mounted on an insulated stand.

Electrons and protons are equal in uncharged conductor B, and the electrons of B are attracted towards protons of A making the end P of conductor B negatively charged called bound charge and protons remain at the end Q of conductor B called free charge. There is an induced charge in B because conductor A is near B.

Question-31:-

Figure below shows a metal rod AB placed on an insulating stand. In figure (a) a negatively charged ebonite rod C is touched with the metal rod AB, while in figure (b), the negatively charged ebonite rod C is held near the rod Ab. State the kind of charged at the ends A and B of the rod in each case.

Answer -

At the end of A- positive charges

At the end of B - negative charges

32 Can you changes an insulator by the method of  conduction?

Answer - No, we cannot change an insulator by the method of connection. 

33. What is an electroscope? Name the two types electroscopes.

Answer- An Electroscope is a device which is used to detect the presence and nature of the charge on a body.  

Two types of electroscope- 1 pith ball electroscope. 2. Gold leaf electroscope 

34. Describe a pith ball electroscope. How can yo use it to test whether a body is charged or uncharged?

Answer- Pith ball Electroscope- It consists of a small pith ball suspended by a dry thread from an insulating stand. 

To test whether a body is charged or uncharged: Bring the body near the pith ball (without touching it). If the pith ball moves towards the body, the body is charged. But if the pith ball remains stationary, the body is uncharged.

35. How will you use a pith ball electroscope to fin out whether the charge on a charged body is positive or negative ?

Answer- 

To test whether the charged body has a positive charge or a negative charge :

Take a charged pith ball electroscope (i.e., the pith ball is provided with some charge). Suppose the pith ball is positively charged. Bring the charged body near the pith ball (without touching it). If the path ball moves away, the body has a positive charge and if the pith ball moves towards the body, the body has a negative charge.

36. Draw a labelled diagram of a gold leaf electroscope and describe its construction.

Answer-

ii) Gold leaf electroscope

Construction -. It consists of two gold (or aluminum) leaves hanging from a brass rod having a brass disc at its upper end. The rod passes through an ebonite cork fitted in the

mouth of a glass bottle. The glass bottle has tin foils on its sides near its bottom which ar earthed. The glass bottle protects the gold leaves from the effect of air , wind etc.

37 A positively charged glass rod is touched with the disc of an uncharged gold leaf electroscope What will be your observation ?

Answer

A positively charged glass rod is touched with the disc of an uncharged gold leaf electroscope , leaves diverge. 

38. How will you use a gold leaf electroscope to find out whether a body is charged or uncharged?

Answer-

Bring the body and touched with electroscope, if leaves diverge means body is charged body but if leaves do not diverge, uncharged body.

39. How will you use a gold leaf electroscope to find out whether the charge on a charged body is positive or negative ?

Answer- 

Take a positively charged gold leaf electroscope. Bring the charged body to be tested in the contact with the brass disc of electroscope ,if the divergence of leaves increases the body has the positive charge but if the divergence of the leaves decreases the body has a negative charge.

40. A negatively charged ebonite rod is touched with the disc of a negatively charged gold leaf electroscope. What will be your observation?

Answer 

A negatively charged ebonite rod is touched with the disc of a negatively charged gold leaf electroscope, divergence of leaves increases 

41 . When a charged rod is touched with the disc of A positively charged gold leaf electroscope, it is observed that the divergence of leave decreases what is the kind of charge on the rod ?

Answer-

When a charged rod is touched with the disc of A positively charged gold leaf electroscope, it is observed that the divergence of leave decreases, rod is negatively charged. 

Question-42:

Describe Franklin's experiment. What did he conclude from his experiment?

Answer-42:

Franklin's experiment led to the invention of the lightning rod and the understanding of positive and negative charges. The connection between electricity and lightning was known but not fully understood. By conducting the kite experiment Franklin proved that lighting was an electrical discharge and realized that it can be charged over a conductor into the ground providing a safe alternative path and eliminating the risk of deadly fires.

Question-43:

What causes lightning?

Answer-43:

Lighter, positively charged particles form the top of the cloud. Heavier, negatively charged particles sink to the bottom of the cloud. When the positive and negative charges grow large enough, a giant spark - lightning - occurs between the two charges in the cloud.

Question-44:

What are the effects of lightning?

Answer-44:

Lightning can cause widespread destruction of buildings and loss of lives of humans and animals.

Question-45:

What is a lightning conductor? How does it work?

Answer-45:

A lightning conductor is a device used to protect buildings from the damaging effect of the lightning. It runs from the top to the bottom of the building which is to be protected. If lightning strikes the building, the lightning conductor provides an easy and harmless path for the lightning bolt pass through the ground without damaging the building.

The lightning conductor works on the principle of induction, such that when a charged cloud passes by the building, the conductor gets charge X opposite to that of the loud through the process of induction. 

Question-46:- 

How is a tall building protected from damage due to lightning?

Answer-46 :

By using lightning conductors, tall buildings can be protected from lightning. when a charged cloud passes by the building, the conductor gets charge X opposite to that of the cloud through the process of induction. The spikes are pointed and have a small area which cannot hold the charge and it leaks into air, such that the opposite charge in air and cloud is cancelled. This acquired charge moves to the earth through the earthing system.

Question-47 :

State three safety measures that you will observe in thunderstorms.

Answer-47:

Safety measure in thunderstorms:

(i) Do not take baths during thunderstorms, and avoid contact with running water and metallic taps.

(ii) Do not lie on the ground, squat low on the ground with your hands on the knees and head in between.

Here's a video representation of Rainbow Mind Map of electric current: