ch 15 static electricity

8/9/2019 Ch 15 Static Electricity

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15

Static Electricity

1. Static electricity—electric charge; charging by friction; simple orbital model of the atom;

detection of charge (pith ball or electroscope); sparking; lightning conductors.

A Concise

Static Electricity : The bodies which on rubbing acquire the

property of attracting light bodies (such as paper pieces etc.) are

said to be electrified or charged. If this charge is not allowed toflow, it is called the static electricity.

Static electricity is produced by friction.

The positive or negative charges have nothing to do with

mathematical operations, but are symbols to represent different

kinds of electric charges.

Electrostatics or Static Electricity is that branch of electricity

which deals with the study of the properties and production of

electric charges that remain at rest on a body.

A charged body attracts an uncharged body.

Like charges repel each other. Unlike charges attract each other.

Methods of Charging

(i) Conduction : In this method an uncharged body is brought in

contact with a charged body.

(ii) Induction : In this method the uncharged body is kept near

the charged body.

Coulomb’s Law :

f ∝ q 1q 2d 2

Where f is the force, q 1, q 2 the charges and d is the distancebetween the two charges.

Some Common Good and Poor Conductors and Insulators



No. Good conductors Poor conductors Insulators

1. Metals; especially

gold, silver, copper

and aluminium

1. Water

2. Human body

3. Earth

1. Rubber

2. Glass

3. Dry air

4. Moist air 4. Plastics

2. Carbon 5. Semi-conductors;

silicon and germanium

The energy associated with moving charges is called current

electricity.

If a body gains electrons, it is said to be negatively charged.

If a body loses electrons, there is a deficiency of electrons.

Hence, body is said to be positively charged.

The static electricity is produced by friction.

Properties of Charge

(i) There are two type of charges : (a) Positive (+) Negative

(–)

(ii) Similar charges repel each other and opposite charges

attract each other.



(iii) Charge is always conserved i.e. it can neither be created

nor be destroyed.

(iv) Charge is quantized i.e., transfer of charge takes place inwhole number.

Q = ne

Where Q is total charge on a conductor

n is the number of electrons that are transferred and

e is the charge present on one electron (e = – 1·6 ∞

10–19 coulomb)

Conclusion Regarding Electrification

(i) There are two kinds of electric charges. The charges which

are similar to charges on a glass rod, are positive and

those which are similar to the charges on an ebonite rod

are negative.

(ii) Like charges repel each other.

(iii) Unlike charges attract each other.

(iv) The force of attraction or repulsion operates even if the

experiment is conducted in vacuum.

It must be remembered that during positive or negative electrif-

ication, it is the electrons and not the protons which get

transferred.

If one body gets positively charged due to rubbing, then the body

which is used for rubbing gets negatively charged at the same time.

Thus, equal and opposite electric charges are produced at the same

time.

How does Glass Rod get Positively Charged and Silk Negatively

Charged

When we rub a glass rod with silk, then some of the electrons from

the glass rod are transferred to silk. Since the glass rod has now a

deficiency of electrons as compared to protons in its nucleus,

therefore it gets positively charged. At the same time silk atoms

tend to have an excess of electrons as compared to protons and

therefore get negatively charged.

How does Ebonite Rod get Negatively Charged and Cat’s Skin

Positively Charged

When we rub ebonite rod with Cat’s skin, some of the electrons

from Cat’s skin are transferred to ebonite rod. As ebonite rod has

excess of electrons as compared to protons, therefore it gets

negatively charged. At the same time as Cat’s skin has deficiency of

electrons as compared to protons, therefore it gets positively

charged.

It is to be remembered that a conductor can be charged only if it

is mounted on some insulated stand as otherwise the charges are

earthed away.



Electric Induction

The process in which a conductor gets charged in the presence of a

charged body without touching and it gets uncharged on removal ofcharged body is called electric induction

Lightning Conductor

It is a device which is used to protect the large buildings

against lightning during a thunder storm. In rainy season, the roof

and walls of a building are wet that is why become good conductor of

electricity. The lightning from clouds may pass on to the building

may cause a great damage. A lightning conductor is made of copper

and has sharp points of platinum. It is connected to the earth by

means of a metal band running down the building and terminating in aburied metal plate.

ch 15 static electricity

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