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 ELECTROSTICS WHY? Section 1 Electric charges and forces Matter is made up of atoms Electrons can be transferred from one atom to another When an electron is added to an atom, the atom contains more negative The quantity of electric charge is expressed in the unit coulomb (C) Experiment l.l Discussion of Experiment 1.1 Like charges repel; unlike charges attract In all cases, no new charges are produced in the rubbing process Experiment 1.2 When both strips are connected to the positive terminal, the EHT power Charging by sharing The Van de Graff generator generates charges Earthing Charging by Contact Dsicharging through air A charged object discharges easily when the air contains more ions Section 2 Conductors and insulator Objects are classified as electrical conductors ( Conductors and insulators are not two distinct class of material Charge distribution Charging by induction Charging by induction: Negative charges are induced on the side closer to the rod and I understand now I know the same phenomenon can be observed when a charged rod is Section 3 Applications of electrostatics Electrostatic precipitation Photocopying Electrostatic hazards #Flying aeroplanes and moving vehicles accumulate charges too #Charges accumulated in clouds are carried to the earth through THE

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## ELECTROSTICS

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### WHY?

When I was a primary student,my teacher told me to rub my plastic ruler by using a piece of cloth. As a result, the ruler attracted pieces of small paper automatically.

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### Matter is made up of atoms

An atom contains positive charges and negative charges. The negative charges are due to the electrons. The atom as a whole is neutral; the quantity of positive charge and negative charge are equal.

There is empty space. The electrons are easily to lose away.

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### Electrons can be transferred from one atom to another

When an electron is removed from a molecule, the molecule contains more positive charges than negative charges. In other words, it contains net positive charges . It becomes a positive ion (??).

+

Positive ion

Neutral Atom

electron

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### When an electron is added to an atom, the atom contains more negative

charges than positive charges. It has a net negative charge. We say the atom is negatively charged; it becomes a negative ion.

+

Negative ion

Electron

Neutral Atom

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### The quantity of electric charge is expressed in the unit coulomb (C)

An electron carries a charge of e = -1.6 x 10 -19 C The negative sign means the charge is negative.

-19

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### Experiment l.l

Charging by friction

Rub two acetate strips with a dry woolen cloth and hold them side by side. The strips repel each other. Repeat with two rubbed polythene strips. The strips also repel each other.(Fig 1.4) Repeat with an acetate strip and polythene strip. The strips attract each other.

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### Discussion of Experiment 1.1

Before an acetate strips is rubbed, it contains the same quantity of positive and negative charges. The strip is neutral because there is no net charge. The same is true for the woolen cloth. When the acetate strip is rubbed, some electrons in the strip are transferred to the cloth . As a result, the strip contains more positive charges than negative charges. It has a net positive charge. On the other hand, the cloth is negatively charged because it contains more negative charges than positive charges. When a polythene strip is rubbed, it is negatively charged while the cloth is positively charged. Some electrons in the cloth have been transferred to the strip.

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### Like charges repel; unlike charges attract

Different material strips attract each other.

Same material strips repel each other.

*Fig 1.5

There are forces called electric forces (??), which cause the strips repel or attract each other.

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### In all cases, no new charges are produced in the rubbing process

The charges are merely separated. They are transferred from one object to the other through direct contact of the two objects. The quantity of charge obtained by one object is equal to the quantity of opposite charge obtained by the other. The net charge of the two objects taken together remains zero.

Charge cannot be created or destroyed.

Principle of conservation of charge

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### Experiment 1.2

Experiment 1.2

Charging by pushing

Connect one strip to the positive terminal and the other to the negative terminal (Fig 1.7). The strips attract each other.

Connect two aluminium strips to the positive terminal of the extra high tension (EHT) power supply as shown below. The strips repel each other.

*Fig 1.6

*Fig 1.7

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### When both strips are connected to the positive terminal, the EHT power

supply draws electrons from the strips, which then become positively charged. For simplicity, we may say that the EHT power supply pushes positive charges onto the strips. When the strips are connected to different terminals, the power supply pushes positive charges onto one strip through the positive terminal and negative charges onto the other strip through the negative terminal. The charge are transferred by conduction.

Discussion of Experiment 1.2

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Experiment 1.3

*Fig 1.7

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### The Van de Graff generator generates charges

The Hairs share the same type of charge with the generator by conduction.

Discussion of Experiment 1.3

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### Earthing

Fig 1.8 shows a charged metal ball electrically connected to the ground. This can be done by connecting the metal ball and the ground with a wire, or simply by touching the ball with the finger. Since the earth is a huge object, it practically obtains all the charges from the ball, which then becomes neutral. In fact, any charged metal will be completely discharged as result of earthing or grounding.

+

+

+

+

*Fig 1.9 Symbol for earth

*Fig 1.8

ground

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### Charging by Contact

*Fig 1.10 An uncharged sphere becomes positive after touching positively charged sphere

*Fig 1.11 An uncharged sphere becomes negative after touching negatively charged sphere

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### Dsicharging through air

Air contains positive ions and free electrons. When a positively charged rods is left in the air, the rod gradually attracts free electrons into it. The positive charge is then neutralized by the electrons. When a negatively charged rod is left in the air, the excess electrons are gradually lost into the air and neutralize the positive ions. In both cases, the rod is said to discharge through air.

-

+

-

+

-

+ + + + +

- - - - -

+

-

+

-

+

*Fig 1.12

Positive rod

Negative rod

Positive ions and free electrons in air

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### A charged object discharges easily when the air contains more ions

This can be done by heating the air with a flame(Fig l.12) or hot body, or irradiating it with a radioactive source. In these processes, many ions and free electrons are produced.

A charged object also discharges more easily when the air contains a lot of moisture. It discharges less easily when the air is dry.

*Fig 1.12

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### Objects are classified as electrical conductors (

?) or insulators (???). In a conductor, the charges can move easily from one place to another

On the hand, in an insulator, the charges cannot move easily.

Electrons are free to move in conductor.

*Fig 2.1

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### Conductors and insulators are not two distinct class of material

Good insulators are bad conductors and vice versa. Even the best insulator can allow a very small flow of charge. Similarly, charges are not completely free in a conductor. Metals are the most common conductors while non-metals are usually insulators. Some common conductors and insulators are listed below.

Conductors

Insulator

Silver copper aluminium graphite water

Vacuum plastics wax dry air wood

Earth, human body

In decending order of goodness

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### Charge distribution

For a conducting sphere, the excess charges settle evenly on the surface (Fig 2.2). For a conductor of irregular shape, the charge density is higher at sharp points.

*Fig 2.2 It is the same of positive ions

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### Charging by induction

The distribution of charges on a conductor is also affected by charges outside. Fig 2.3 shows a charged rod (which may be a conductor or an insulator) brought near a neutral conductor. The positive charges in the rod attract unlike charges in the conductor to the side closer to the rod and push the like charges to the far side. The conductor is charged locally. But on the whole it still remains neutral.

This phenomenon is called electric induction (???). The charges separated in the conductor are called induced charges. The more charges the cod contains and closer is the rod to the conductor, the more charges are induced

rod

+ + + +

-

-

- -

++

+ +

conductor

*Fig 2.3

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Alternatively,

*Fig 2.4

*Fig 2.5

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### Negative charges are induced on the side closer to the rod and

positive charges on the far side. The attractive force between the positively charged rod and the negative charges on the aluminium piece is larger than the repulsive force between the rod and the positive charges. As a result, there is a net attractive force between the rod and the aluminium piece.

*Fig 2.6 The pieces are attracted towards the rod

Attraction of neutral objects

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### I understand now

Thank you!

*Fig 2.7

For the same reason, tiny paper strips are attracted towards a charged object.

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### I know the same phenomenon can be observed when a charged rod is

brought near a stream of running water.

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*Fig 3.1

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*Fig 3.2

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### Electrostatic hazards

Static charges accumulate easily on insulators due to friction. This causes a nuisance in many respects.

~ When you comb your hair on a dry day. You can feel the pull on you hair by the static charges produced.

No need to mention this point!!

*Fig 3.3

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### #Flying aeroplanes and moving vehicles accumulate charges too

Aeroplanes are fitted with conductive tyres so that the charges are carried to the earth upon landing. If not, a spark may occur during refuel and may cause fires and even explosions. Oil trucks prevent such accidents by carrying a conductive belt or metal chain touching the ground.(Fig 3.4)

*Fig 3.4

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### #Charges accumulated in clouds are carried to the earth through

lightning during a thunderstorm. To prevent the lightning from hitting tall buildings, lightning conductors (???) are installed on the rooftop.

Help!

*Fig 3.4 The lightning conductor is a lonfg copper strip .

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### THE

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