Static Electricity

Objective Questions

Question 1

Write true or false for each statement:

(a) The number of electrons and protons in an atom are same.

(b) If the charge is not in motion, we call it static electricity.

(c) Human body is a conductor of electricity.

(d) When an ebonite rod is rubbed with fur, the electrons move from ebonite to fur.

(e) When a glass rod is rubbed with dry silk cloth, the electrons move from glass to silk.

(f) The cap of gold leaf electroscope is made of copper.

(g) If a glass rod rubbed with silk is brought near the cap of a negatively charged electroscope, the divergence of leaves will decrease.

(h) In induction, a positively charged body can make an uncharged body positively charged.

(i) A lightning conductor saves the building from lightning.

(j) When a comb is rubbed with dry hair both comb and paper get similarly charged.

(k) A glass rod rubbed with silk repels an ebonite rod rubbed with fur.

(l) When an ebonite rod is rubbed with fur, the protons move from the ebonite rod to the fur.

(m) A conductor has a large number of free electrons.

(n) An ebonite rod can be charged by touching it with a charged copper rod.

(o) To find whether a body is charged or not, an uncharged electroscope is used.

(p) To find whether the charge on a body is positive or negative, an uncharged electroscope is used.

(q) If a negatively charged rod is brought near a negatively charged pith ball electroscope, the pith ball will be stuck with the rod.

Answer

(a) True

(b) True

(c) True

(d) False
Correct Statement — When an ebonite rod is rubbed with fur, the electrons move from fur to ebonite.

(e) True

(f) False
Correct Statement — The cap of gold leaf electroscope is made of brass.

(g) True

(h) False
Correct Statement — In induction, a positively charged body can make an uncharged body negatively charged.

(i) True

(j) False
Correct Statement — When a comb is rubbed with dry hair both comb and paper get oppositely charged.

(k) False
Correct Statement — A glass rod rubbed with silk attracts an ebonite rod rubbed with fur.

(l) False
Correct Statement — When an ebonite rod is rubbed with fur, the free electrons move from fur to ebonite rod.

(m) True

(n) False
Correct Statement — An ebonite rod can be charged by rubbing it with fur.

(o) True

(p) False
Correct Statement — To find whether the charge on a body is positive or negative, a charged electroscope is used.

(q) False
Correct Statement — If a negatively charged rod is brought near a negatively charged pith ball electroscope, the pith ball will move away from the rod.

Question 2

Fill in the blanks:

(a) Like charges ............... while unlike charges ............... .

(b) Mercury is a ............... of electricity while pure water is ............... of electricity.

(c) An ebonite rod when rubbed with fur acquires the ............... charge.

(d) When an uncharged conductor is brought in contact with the disc of a gold leaf electroscope, its leaves ............... .

(e) Charge is shared in charging a conductor by the method of ............... .

Answer

(a) repel, attract

(b) conductor, insulator

(c) negative

(d) will remain unchanged

(e) conduction

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

When a glass rod is rubbed with dry silk cloth, the charge acquired by the silk cloth is:

1. Positive
2. Negative
3. Both positive and negative
4. None of the above

Answer

Negative

Reason — When a glass rod is rubbed with silk, the free electrons move from glass rod to silk. The silk gains some electrons and becomes negatively charged.

Question 4b

Select the correct alternative:

When an ebonite rod is rubbed with fur, the rod acquires:

1. positive charge
2. negative charge
3. no charge
4. none of the above

Answer

negative charge

Reason — When an ebonite rod is rubbed with fur, the free electrons from fur are transferred to ebonite rod. The ebonite rod gains electrons and becomes negatively charged.

Question 4c

Select the correct alternative:

When a negatively charged body is brought closer to another negatively charged body, then they will show:

1. attraction
2. no effect
3. repulsion
4. none of the above

Answer

repulsion

Reason — Similar kinds of charges repel each other. So when both the negatively charged bodies are brought closer they repel each other.

Question 4d

Select the correct alternative:

Charging a conductor by bringing another charged conductor close to it without touching is called:

1. induction
2. conduction
3. convection
4. radiation

Answer

induction

Reason — Induction is the process by which a conductor is charged by bringing another charged conductor close to it without touching.

Question 4e

Select the correct alternative:

The factor responsible for charging a conductor is:

1. transfer of protons
2. transfer of neutrons
3. transfer of electrons
4. transfer of both protons and electrons

Answer

transfer of electrons

Reason — Conductors are charged due to transfer of electrons because the body which loses electrons becomes positively charged and the body which gains electrons becomes negatively charged.

Question 4f

Select the correct alternative:

Two objects when rubbed together get charged. The charges on them are:

1. equal and opposite
2. equal and similar
3. unequal and similar
4. unequal and opposite

Answer

equal and opposite

Reason — When two objects are rubbed there is transfer of free electrons. One body loses electrons and becomes positively charged while the other body gains equal number of electrons and becomes negatively charged. So the charges are equal and opposite.

Question 4g

Select the correct alternative:

When a glass rod is rubbed with silk, the glass rod and the silk get charged because:

1. electrons are transferred from the silk to the glass rod
2. electrons are transferred from the glass rod to the silk
3. protons are transferred from the silk to the glass rod
4. protons are transferred from the glass rod to the silk

Answer

electrons are transferred from the glass rod to the silk

Reason — When glass rod is rubbed with silk, the free electrons are transferred from glass rod to silk.

Question 4h

Select the correct alternative:

The conductor of electricity is:

1. wood
2. glass
3. ebonite
4. human body

Answer

human body

Reason — Human body is a conductor of electricity because if we touch a charged body the charge of the body will pass to earth through our body.

Question 4i

Select the correct alternative:

A gold leaf electroscope is to be charged positively by conduction. For this:

1. a positively charged rod is held close to the disc of electroscope
2. a positively charged rod is placed in contact with the disc of electroscope
3. a negatively charged rod is held close to the disc of electroscope
4. a negatively charged rod is touched with the disc of electroscope

Answer

a positively charged rod is placed in contact with the disc of electroscope

Reason — A positively charged rod when touched with the disc of electroscope, it will be positively charged because by conduction a conductor gets charged by similar kind of charge as on charging body.

Question 4j

Select the correct alternative:

A glass rod rubbed with silk is touched with the disc of a negatively charged gold leaf electroscope. The divergence of leaves will:

1. decrease
2. increase
3. remain unchanged
4. first decreases and then increases

Answer

decrease

Reason — A glass rod when rubbed with silk gets positively charged. When it is touched with the disc of a negatively charged gold leaf electroscope, the divergence of leaves decreases because opposite charges attract each other.

Question 4k

Select the correct alternative:

The rod in a gold leaf electroscope is made up of:

1. wood
2. brass
3. glass
4. ebonite

Answer

brass

Reason — The rod in a gold leaf electroscope is made up of brass.

Question 4l

Select the correct alternative:

Lightning conductor is made up of:

1. copper
2. glass
3. ebonite
4. wood

Answer

copper

Reason — A lightning conductor is a long copper rod whose lower end is connected to a copper plate buried deep into the ground and its upper end has sharp spikes projecting above the top of the building.

Short/Long Answer Questions

Question 1

What do you understand by electricity at rest?

Answer

Electricity at rest refers to electric charge that is not moving or flowing. When electricity is at rest, there is no current flowing and no electrical energy is being transferred.

Question 2

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

Answer

When a plastic comb is rubbed with dry hair it acquires an electric charge due to friction. As the comb is charged it acquires the property to attract bits of paper.

Question 3

Who discovered the way of producing electricity by friction?

Answer

Greek philosopher Thales discovered the way of producing electricity by friction.

Question 4

Name two substances which can be charged by friction.

Answer

Glass, ebonite are the two substances which can be charged by friction.

Question 5

What are the two kinds of charges?

Answer

The two kinds of charges are positive charge and negative charge.

Question 6

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

Answer

When a glass rod is rubbed with silk, the glass rod 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

When an ebonite rod is rubbed with fur, the rod acquires negative charge and the fur acquires positive charge.

Question 8

Describe an experiment to demonstrate that there are two kinds of charges.

Answer

The following experiment demonstrates that there are two kinds of charges:

1. Take a glass rod A1. Rub it with silk and suspend it with a thread. Take another glass rod B1. Rub it with silk and bring it near one end of the suspended rod A1. It is observed that the suspended glass rod A1 gets repelled.
2. Take an ebonite rod A2. Rub it with fur and suspend it with thread. Take another ebonite rod B2. Rub it with fur and bring it near one end of the suspended rod A2. It is observed that the suspended ebonite rod A2 gets repelled.
3. Suspend glass rod A1 rubbed with silk and bring ebonite rod B2 near one end of the suspended rod A1. It is observed that rod A1 gets attracted towards rod B2.

In all the above steps, if the rods have not been rubbed with silk or fur, no repulsion or attraction between the rods would have been observed.

Conclusion — From this experiment we conclude that:

1. There are two kind of charges.
2. Like charges repel and unlike charges attract each other.

Question 9

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

Answer

The following experiment demonstrates that like charges repel and unlike charges attract each other:

1. Take a glass rod A1. Rub it with silk and suspend it with a thread. Take another glass rod B1. Rub it with silk and bring it near one end of the suspended rod A1. It is observed that the suspended glass rod A1 gets repelled.
2. Take an ebonite rod A2. Rub it with fur and suspend it with thread. Take another ebonite rod B2. Rub it with fur and bring it near one end of the suspended rod A2. It is observed that the suspended ebonite rod A2 gets repelled.
3. Suspend glass rod A1 rubbed with silk and bring ebonite rod B2 near one end of the suspended rod A1. It is observed that rod A1 gets attracted towards rod B2.

In step 1, both the rods A1 and B1 were rubbed with silk, so, they must have similar kind of charges. Similarly, in step 2 also rods A2 and B2 must have similar kind of charges. As A1 and A2 got repelled by B1 and B2, respectively hence, we conclude that like charges repel each other. In step 3, A1 got attracted to B2 hence, we conclude that A1 and B2 are oppositely charged and 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

When a glass rod rubbed with silk is suspended near an ebonite rod rubbed with fur, both the rods attract each other. The glass rod when rubbed with silk acquires positive charge as free electrons move from glass to silk. Similarly when an ebonite rod is rubbed with fur, it acquires negative charge as free electrons move from fur to rod. As the charges on the rods are opposite, hence they attract each other.

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

When an ebonite rod rubbed with fur is suspended near another ebonite rod rubbed with fur, they will repel each other. Ebonite rod acquires a negative charge when rubbed with fur as free electrons move from fur to rod. As the charges on both the rods are the same, hence they repel each other.

Question 12

What do you mean by conservation of charges?

Answer

When two objects 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. This is called conservation of charges.

Question 13

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

Answer

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

Question 14

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

Answer

The three constituents of an atom are electrons, protons and neutrons. The electrons are negatively charged, protons are positively charged and neutrons have no charge (i.e., they are neutral).

Question 15

What is the net charge on an atom?

Answer

The net charge on an atom is zero i.e. it is electrically neutral as the number of protons is equal to the number of electrons.

Question 16

Briefly describe the structure of an atom.

Answer

An atom has protons (positively charged) and neutrons (no charge) inside its nucleus at the centre around which the electrons (negatively charged) revolve in different orbits. The nucleus is positively charged. The number of electrons in an atom is equal to the number of protons. Hence, the atom is electrically neutral.

Question 17

What are free electrons?

Answer

The electrons in the orbits which are far from the nucleus are free to move and can easily leave their atoms. These electrons are called free electrons.

Question 18

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

Answer

When two objects are rubbed together the free electrons are transferred from one object to other. The object which gains free electrons becomes negatively charged, while the object which loses free electrons, becomes positively charged.

Question 19

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

(a) A glass rod when rubbed with silk.

(b) An ebonite rod when rubbed with fur.

Answer

(a) Glass rod loses electrons.

(b) 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

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

Question 21

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

Answer

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 it becomes positively charged. The ebonite rod gains the same number of electrons and it becomes negatively charged by an equal amount.

Question 22

Distinguish between conductors and insulators of electricity.

Answer

Question 23

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

Answer

Copper is an example of conductor and wood is an example of insulator of electricity.

Question 24

State two ways of charging a conductor.

Answer

The two ways of charging a conductor are:

1. By Conduction
2. By Induction.

Question 25

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

Answer

In charging by conduction, the charge is shared.

Question 26

Describe the method of charging a conductor by conduction.

Answer

In charging by conduction method, the conductor to be charged is touched or brought in contact with a charged body. The conductor gets charged by the similar kind of charge as on the charging body.

Method of charging

Suppose we want to charge a metal rod A with a positive charge.

1. Take the metal rod A and place it on an insulating stand.
2. Take a positively charged glass rod B provided with an insulating handle and touch it with the metal rod A as shown in the figure below. Some positive charge of the glass rod B is shared by the metal rod A.
3. Remove the glass rod B.
4. The rod A gets positively charged.

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

Rod B should be positively charged.

Question 28

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

Answer

Suppose we want to positively charge a metal rod A. Place it on an insulating stand and touch it with a positively charged glass rod B. The rod A gets positively charged.

Explanation in terms of movement of electrons — The glass rod B is positively charged so it has a deficiency of electrons. When it is touched with the metal rod A, the free electrons of the rod A move to the glass rod B. Due to loss of electrons in the rod A, it becomes positively charged.

Question 29

Describe the method of charging a conductor by induction.

Answer

Suppose we want to charge a metal rod A negatively, proceed as follows:

1. Place the rod A on insulating stand.
2. Take a positively charged glass rod B provided with an insulating handle.
3. Bring it near the rod A.
4. The near end P of the metal rod A will acquire negative charge while the far end Q acquires positive charge as shown in below figure.
5. Now touch the end Q of rod A by hand. Then remove hand and glass rod B simultaneously.
6. The rod A becomes negatively charged.

Question 30

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

Answer

Suppose we want to charge a metal rod A negatively by induction. Bring a positively charged glass rod B near one end of rod A.

The near end P of the rod A acquires negative charge and the far end Q acquires positive charge. The positively charged glass rod B attracts the free electrons from the end Q towards the end P of rod A. Thus, end P becomes negatively charged as it gains the electrons and end Q becomes positively charged as it loses the electrons.

Now touch the end Q of rod A by hand and then remove hand and rod B simultaneously. The rod A becomes negatively charged.

On touching the end Q of rod A, the free electrons move from earth to neutralize the positive charge at end Q. On removal of glass rod B and hand simultaneously, the negative charge at the end P gets distributed throughout the rod A making it negatively charged.

Question 31

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

Answer

In Fig.(a), rod AB gets charged by conduction. There is negative charge at both ends A and B of the metal rod AB.

In Fig.(b), rod AB gets charged by induction. It's end A which is near to the rod C becomes positively charged and the end B which is far from the rod C becomes negatively charged.

Question 32

Can you charge an insulator by the method of conduction?

Answer

No, an insulator cannot be charged by the method of conduction.

Reason — An insulator does not contain any free electrons so it does not allow free electrons from the charged body to flow towards itself.

Question 33

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

Answer

An electroscope is a device which is used to detect the presence and nature of charge on a body. It detects whether the body is charged or uncharged and if the body is charged whether it is positively charged or negatively charged.

The two types of electroscopes are:

1. Pith ball electroscope.
2. Gold leaf electroscope.

Question 34

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

Answer

Pith ball electroscope consists of a small pith ball suspended by a dry silk thread from an insulating stand.

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

Question 35

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

Answer

Using Pith ball electroscope, we can determine the type of charge on the body by following the below steps:.

1. Take a charged pith ball electroscope. Let it be positively charged.
2. Bring the charged body near the pith ball without touching it.
3. If the pith ball moves away, the body has positive charge.
4. If the pith ball moves towards the body, the body has negative charge.

Question 36

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

Answer

Construction of gold leaf electroscope:

1. A gold leaf electroscope consists of two gold or aluminium leaves hanging from a brass rod, having a brass disc at its upper end.
2. The rod passes through an ebonite cork fitted in the mouth of a glass bottle.
3. The glass bottle has tin foils on its sides near its bottom which are earthed.

Figure of gold leaf electroscope:

Question 37

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

Answer

When the positively charged glass rod is touched with the disc of an uncharged gold leaf electroscope, the leaves diverges. The leaves acquires positive charge due to conduction and as like charges repel, they diverge.

Question 38

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

Answer

Bring the body and touch it with the brass disc of the gold leaf electroscope. If the leaves diverge, the body is charged. But if the leaves do not diverge, the body is uncharged.

Question 39

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

Answer

Gold leaf electroscope is used to detect the type of charge present in a charged body by the following method:

1. Take a charged gold leaf electroscope (Suppose the electroscope is positively charged).
2. Bring the charged body to be tested in contact with the brass disc of the electroscope.
3. If divergence of leaves increases, the body has positive charge.
4. If divergence of leaves decreases, the body has negative charge.

For the case when electroscope is negatively charged, if divergence of leaves increases, then body is negatively charged and if it decreases then body is positively charged.

Question 40

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

Answer

The divergence of leaves will increase.

Question 41

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

Answer

The rod is negatively charged.

Question 42

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

Answer

Franklin's experiment:

1. Benjamin Franklin took a kite made of silk.
2. At the top corner of the kite, he fixed a metal wire about 30 cm long.
3. The other end of the wire was joined to a string.
4. At the lower end of the string, he tied a metal key and then a silk strip. The silk strip acts as an insulating handle. He flew the kite in thunderstorm.

Observation — He obtained a number of sparks between his knuckle and key when the string got wet.

Conclusion — He concluded that during thunderstorm the clouds acquire an electric charge. Hence, with his kite experiment, he proved that lightning is a form of electricity.

Question 43

What causes lightning?

Answer

During a thunderstorm, when a charged cloud passes over the earth (or over the other cloud), it acquires an opposite charge by induction. A spark may occur between the two oppositely charged clouds (or a charged cloud and the earth). This spark is called lightning.

Question 44

What are the effects of lightning?

Answer

The effect of lightning on objects is very dangerous and serious. It can kill humans and animals. It can cause a fire and destroy buildings.

Question 45

What is a lightning conductor? How does it work?

Answer

A lightning conductor is a device which is used to protect buildings from being damaged due to lightning.

The lightning conductor works on the principle of induction.

When a charged cloud passes over the tall building, an opposite charge is induced on the spikes of lightning conductor. This charge passes to the earth through the copper rod. Thus, the lightning spark is prevented and the building is protected from being damaged.

Question 46

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

Answer

A lightning conductor is used to protect buildings from being damaged due to lightning. It consists of a long copper rod with sharp points or spikes projecting above the top of the building. The lower end of the rod is connected to a copper plate buried deep into the ground. When a charged cloud passes over the tall building, the spikes of lightning conductor get oppositely charged. This charge passes to the earth through the copper rod. Thus, the lightning spark is prevented and the building is protected from being damaged.

Question 47

State three safety measures that you will observe in a thunderstorm.

Answer

The three safety measures that are observed during thunderstorm are:

1. Do not stand under a tree or near a tall building.
2. Do not carry an umbrella over your head particularly if walking in an open ground.
3. Take out the plugs of TV sets, computers, etc. Do not use the wired phone.