STATIC ELECTRICITY

When you move across a carpet in a room and reach for a metal door knob, you felt an

annoying spark of electricity. As a result you may feel a shock. This shock we call it as

static electricity. Likewise on touching a metal part of car while getting down we get the

similar shock. Above mentioned phenomena are happening because of electrostatic

nature of our body and answer to this lies in the subject called electrostatic.

The word electricity comes from Greek word ‘elektron’ which means amber. Amber is a

petrified tree resin.

If you rub an amber rod with a piece of cloth then it is seen that the amber rod attracts

the small pieces of dry leaves or dry paper. Same effect is being observed by rubbing a

glass rod or plastic comb with silk or dry hair. The substances which acquire the

property of attraction when rubbed with another substance were said to be charged

with electricity or have been electrified.

The scientist William Gilbert showed that not only amber, but glass, wax and sulphur

also acquired the ability to attract light things when rubbed.

Experimental Evidences:

1) Run a comb through the dry hair, you will find that the comb attracts small bits of

dry paper.

2) Rub a glass rod with smooth clothing, you will find the glass rod also shows the

attracting property.

3) Rub an inflated balloon with wool, the balloon will stick to the wall or ceiling of a

room for hours together.

Benjamin Franklin$ in 1700 conceptualized that there are two and only two kinds of

electric charges which are named as positive and negative charges.

Supporting Experiments:

1) Rub a rubber rod with fur and a glass rod with silk.

2) Both rubber rod as well as glass rod have been electrically charged.

3) On bringing these two rods close together, it is observed that they attract each

other.

4) On bringing two charged rubber rods or two charged glass rods close together, the

effect is seen as repulsion.

Glass rod always acquires positive charge because it tends to lose the loosely bounded

electrons. Hence, it is said to be charged positive. Rubber rod always acquires negative

charge because it tends to gain electrons from other substance. Hence, it is said to be

negatively charged.

Conclusion: Like charges repel *

Unlike charges attract.

*This is because of the structure of the atoms of a substance.

$The choice of positive or negative charge is purely arbitrary.

This law is known as Law of Charge.

Simple Atomic Model:

An atom is the smallest part of an element which can take part in a chemical reaction.

1) The central core of an atom is called the nucleus.

2) The nucleus which contains the entire positive charge comprises of two particles,

proton and neutron.

3) The proton is a positively charged* particle while neutron doesn’t have any charge

4) The electrons revolve around the nucleus in certain permitted orbits. The electrons are

negatively charged* and the electrons in outermost orbit are called valence or free

electrons because they are loosely bound to the attraction of nucleus.

5) The total negative charge in the atom equals the total positive charge on the protons

and the atom on the whole is neutral.

6) A proton and a neutron are roughly of the same size and mass.

7) Mass of proton is about 1836 times the mass of the electron.

8) The mass of the neutron is about 1840 times the mass of an electron.

9) The neutron and proton are held inside the nucleus by strong nuclear forces.

* the charge of an electron is -e while the charge of a proton is +e where ‘e’ is the

smallest possible value of charge and its value is 1.6×10-19

coulomb.

• Charging by friction

� When a body is rubbed with another body then due to heat generated by the friction,

free electrons get separated from body.

� Free electrons are being separated because they are loosely bound with the nucleus. In

this process the body which gains the electron is treated as negatively charged while

the body which looses the electron is treated as positively charged.

• Conductors and insulators:

� Bodies which do not allow the charge to pass though them are called as insulators.

Insulators don’t have free electrons which are responsible for the conduction of charge.

e.g. Glass, mica, ebonite, fur, silk, rubber etc. On these substances charge stays on

them and that is the reason why they get charged on rubbing with another body.

� Bodies which allow the charges to pass through them are called conductors. A

conductor has a large number of free electrons which are free to move inside the

conductor. e.g. Iron, human body, copper etc.

� If an iron or copper is rubbed with some other material, it acquires charge which flows

through human body or into another conducting material. If we rub these metal rods

held by insulating handle then these charges can be precisely noticed. This is the

reason why we use rubber gloves for touching these conducting materials having the

movement of charged particles.

• Charging by conduction and induction:

� An object that has a stationary charge can charge another body by either conduction or

induction.

1. Charging by conduction:

� This happens only when the charged body touches another body.

� Charge a glass rod by rubbing with silk, then the glass rod acquires positive charge and

silk acquires negative charge.

� Touch the glass rod with an insulator (the body mounted on a wooden or plastic stand

since wooden or plastic considered as non-conducting material) metal body.

� The metal body acquires positive charge because of the touch of glass rod.

Conclusion:

When the positively charged glass rod touches the uncharged body then some of the

free electrons move to uncharged body. Subsequently, the positively charged rod

becomes less charged. Where as the metal uncharged body becomes more positive.

2. Charging by induction:

� This takes place when the charged body placed near to the uncharged body (body

should not be touched), consequently, the uncharged body gets charged temporarily.

� The process of charging a body temporarily with a charged body without touching each

other is called electrostatic induction.

� Bring a positively charged rod near an insulated metal body. The rod pulls electrons

from the metal body to one side near to the charged body. The opposite side of the

uncharged metal has positive charge. That means the other side can attract negative

charge.

� Now touch the metal body with your finger.

1) The electrons of the ground will travel to the metal body. (The earth is a vast

conductor. Hence, it can supply electrons to the body).

2) Remove the finger and also the positively charged rod. The observation is that the

acquired electrons from the ground are retained permanently.

� The charge developed in a body depends upon the substance rubbing it.

For example, when the glass rod is rubbed against silk, glass acquires positive but when

rubbed against fur, it acquires negative charge.

Frictional order :

Fur, flannel, shellac, sealing wax, glass, paper, silk, hand, rubber, amber, ebonite.

If any two substances in this list are rubbed against each other, the one the left hand

side acquires positve charge.

THE ELECTROSCOPE

� “An electroscope is an instrument for detecting the presence very weak charge and

sign of electric charge on a body.”

� The essential instrument used for the detection of charge on a body is Gold Leaf

Electroscope.

� Before getting into this instrument, let us check the same with the help of a simple

instrument, pith ball electroscope.

1. Pith ball Electroscope:

� This consists of a small pith ball suspended by a fine dry silk thread as shown in the

following figure.

(a) (i) If a dry glass rod which is rubbed with silk and brought near to the pith ball, the

attraction of the pith ball towards glass rod is observed because an opposite charge

is developed on pith ball due to induction.

(ii) The moment when the pith ball touches the glass rod, immediately the pith ball is

repelled because pith ball acquires similar charge of glass rod due to conduction.

(b) (i) If another pith ball is touched by the same glass rod it behaves in the same manner.

(ii) On bringing the charged pith balls near to each other, it is observed that both the

pith balls are repelled.

(c) If a pith ball is touched with a vulcanized rod (hard rubber) rubbed with fur and

then brought near to one of the pith balls, it is observed that the pith ball is

attracted by rubber. Hence, it is understood that pith ball is positively charged and

vulcanized rod is negatively charged.

2. Gold Leaf Electroscope:

Construction:

This instrument consists of a metal rod called stem with a metal disc on the top and two very

thin light metal foils (usually gold) attached at the bottom. Generally, one of the two leaves is

fixed and another one is movable. The arrangement is enclosed in a glass encloser and the

glass bottle is closed by rubber cork.

Disc

Cork

Stem

Glass case

Brass plate gold leaf

� Detection of charge by an electroscope:

� When charged object is brought close to the disc, electrons in the disc are either attracted

or repelled according to the law of charge.

� For example, if a negatively charged rod is brought near to the disc, electrons in disc get

repelled and disc becomes positively charged and the repelled electrons from the metal

disc are conducted to the gold leaves (metal foils).

� Metal foils repel because of similar charges.

� Similarly for the positively charged rod brought near the disc, the leaves also seem

diverged.

� Charging an electroscope:

An electroscope can be charged by two ways:

(1) By Conduction.

(2) By Induction.

1) Charging by Conduction:

� When a rubber rod is rubbed with fur, rubber rod acquires negative charge.

� Suppose this rod is brought in contact with the metal disc of the electroscope then some

electrons will be able to move in the electroscope. Thus, the electroscope will attain a

negative charge and excess electrons are being conducted through stem to the foils.

� Therefore, the foils repel with each other.

� Similarly for the positive glass rod brought to the metal disc, same mechanism happens in

reverse manner.

2) Charging by Induction:

� Take an ebonite rod and charge it by rubbing that with flannel (animal fur). Subsequent to

this effect, ebonite rod is charged negatively and flannel is charge positively.

� Bring the ebonite rod near the disc of gold leaf electroscope without touching the disc. It is

observed because of the induction, a positive charge is produced on the disc, because it is

near to the ebonite rod. Therefore, some part of the stem and gold leaves acquire negative

charge. Therefore the leaves repel each other.

� Now, touch the disc with your finger. Then it is observed that gold leaves collapse, because

the induced negative charges on the leaves run through the finger to the ground.

� Remove the earthing from the charged body (i.e. from the disc). It is observed the leaves

remain collapsed for some time and

� Now remove the charged body. The positive charge is now not bounded and distributed

over the electroscope. Leaves acquire the positive charges and they repel.

Similarly we can do the same experiment by bringing positively charged glass rod near to

the disc of electroscope. The phenomenon observed is same as it is seen in the ebonite

rod, but the electroscope acquires negative charge in this case.

� To detect nature of a charge:

� Pre-requisite of electroscope: For finding out the

nature of charge of the unknown body, the

electroscope should be charged with known charge

(either positive or negative).

� When the charged body is brought near the charged

electroscope, leaves of the electroscope may diverge

or repel.

� When a charged body is brought near the electroscope, if leaves collapse we may regard

that the charged body possesses the opposite charge than the charge on electroscope. This

happens because unlike charges attract and the charge on the unknown body tries to

attract the charge on electroscope and hence the charge from the leaves move towards

the disc and hence they collapse.

� When an unknown body of similar charge brought near to the disc of electroscope, then it

is observed the gold leaves further diverge. Hence the charge of the unknown body can be

assessed.

� If an unknown neutral body brought near to the electroscope, it is observed that the

golden leaves remain uncharged.

� Uses of Gold Leaf Electroscope:

1) To test the type of charge. (i.e. positive or negative)

2) To distinguish between insulators and conductors.

Consider that initially the electroscope is negatively charged. Now place the sulfur in

contact with the metal disc of electroscope. Nothing happens to the leaves of

electroscope. They remain in diverged condition. No change is seen in the position of

leaves of electroscope because the sulfur is an insulator and hence it does not allow

electricity to pass through it.

Now keep copper rod in contact with the metal disc of electroscope. Leaves collapse.

Copper is a conductor and it allows electricity to pass through it. Hence when it is touched

to the disc of electroscope, charge from the leaves travels towards the copper rod through

the metal disc and charge now reduces on the leaves and they repel.

3) To show that the charge on hollow conductor resides on the outside.

� Keep an uncharged ice pail in contact with the metal disc of uncharged electroscope.

� Now suspend insulated charged metal sphere from a silk thread such that it is lowered

into the ice pail without touching it.

� Charge on the metal sphere induces the charge on the ice pail. Because of that the

inner surface acquires the opposite charge than the charge on metal sphere and the

outer surface acquires the same charge as the charge on metal sphere.

� Ice pail is in contact with the electroscope and hence electroscope also acquires the

charge and leaves repel.

� If the sphere is moved to touch the ice pail then the charge on metal sphere combines

with the charge on ice pail and metal sphere becomes neutral.

� But the divergence of the leaves remains as it is. This proves that still the charge is

there on the outer surface of the ice pail.

LIGHTNING

Lightning is the spectacular bluish white light produced in the clouds.

OR

Lightning is a gigantic electric discharge occurring between two charged clouds or between

a cloud and the earth.

� Lightning is unpredictable. It occurs randomly and instantaneously. It last for less than half

a second.

� American inventor, Benjamin Franklin first showed that lightning is a static electricity.

� Cause of Lightning.

� During the formation of cumulonimbus (rain/storm) cloud, a separation of charge takes

place and generally the bottom of the cloud acquires negative charge.

� As a result of this an opposite charge is induced on the surface of the earth.

� Sometimes the negative charge on the bottom of the cloud gets large enough to

overcome air’s resistance and electrons begin flowing toward earth along a zigzag path

at about 60 miles per second.

� While going downward, electrons collide with air molecules and ionize them and

produce more free electrons.

� As electrons approach the ground, the ground becomes more and more positively

charged.

� This positively charged region moves up through any conducting objects on the ground

fro example houses, trees and people – into the air.

� When the downward moving electrons meet the upward flowing positive charge at an

altitude of around hundred meters, they form a complete circuit and lightning occurs.

� When lightning occurs, the air suddenly expands and it creates sound known as

thunder.

� Lightning and thunder are produced simultaneously, but the sound of the thunder is

heard a few seconds after because the speed of sound is 330 ms-1 and speed of light is

3 × 10 ms-1

and hence sound wave take more time to reach earth.

� Lightning passing through a building can cause serious damage to the building.

� Buildings can be protected from such damage if a lightning conductor is installed.

� Lightning Conductor.

� It is a device used to protect a building from damage by lightning during a

thunderstorm.

� Lightning conductor prevents lightning by neutralizing the charge on the clouds with

opposite charge sent up from its pointed spokes.

� In spite of this if lightning strikes, it provides a safe path for the discharge to flow down

to the earth, leaving the building unaffected.

� A lightning conductor consists of a thick copper strip on the outside of the building.

� It reaches above the highest part of the building. Top of this conductor is in the form of

spikes.

� Its lower end is terminated on a metal plate buried in the ground.

� When lightning strikes, the lightning conductor allows the entire discharge from the

clouds to flow into the earth.