Static Electricity

Both A and R are true and R is the correct explanation of A

Explanation

A fuse is a short length of wire made from a material with a low melting point. It is inserted in the live wire of a circuit so that, if the current exceeds a predetermined value because of over-loading or short-circuit, the fuse wire heats up and melts. This creates an open gap in the circuit, immediately stopping the current and thereby protecting the connected appliance and wiring from damage or fire. Because its action (melting and opening the circuit) is precisely what protects the appliance from excessive current, the Reason correctly explains the Assertion.

Both A and R are true and R is the correct explanation of A

Explanation

An MCB responds very quickly (in a few-tens of milliseconds) to over-current or short-circuit conditions, automatically opening the circuit and preventing damage. Once the fault is cleared the same device can be reset by simply flipping its switch, so it is reusable and convenient.
A fuse, on the other hand, melts when excessive current flows; it then has to be replaced with a new one. Because of this ease of resetting (together with its rapid, reliable action), an MCB is considered more effective than a fuse.

Assertion is true but reason is false

Explanation

Lightning indeed represents a large-scale electrostatic discharge, so the Assertion is correct.
However, it is produced by the build-up of opposite charges—typically negative charges in the lower part of a cloud and positive charges on Earth’s surface or in another cloud—not by the build-up of like charges. Therefore the Reason is incorrect.

Assertion is false but reason is true

Explanation

Frictional charging is effective only for insulators. When a conductor is rubbed, any charge produced immediately moves through the material and leaks away to the surroundings (usually to the Earth) unless the conductor is completely insulated. Therefore the Assertion is false.
The Reason correctly states that charges in a conductor are free to move, so a conductor does not easily hold static charge; hence the Reason is true and correctly explains why the Assertion is false.

(a) Like charges repel while unlike charges attract.

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

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

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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:

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.

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.

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.

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.

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.

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.

repulsion

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

induction

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

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.

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.

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.

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.

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.

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.

brass

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

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.

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.

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.

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

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

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

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.

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).

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.

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.

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.

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

The two ways of charging a conductor are:

1. By Conduction
2. By Induction.

In charging by conduction, the charge is shared.

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.

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.

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.

The divergence of leaves will increase.

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.

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.

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.

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.

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

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

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.

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.

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.

(a) Glass rod loses electrons.

(b) Fur loses electrons.

Rod B should be positively charged.

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.

The rod is negatively charged.

(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.