Electrostatics: Principles, Methods, and Applications, High school final essays of Physics

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 Electrostatics is a branch of physics that deals with static

electricity which is a non-moving electric charge on the

surface of an object.

 A gain of electrons produces negative static charges while a

loss of electrons produces positive charges.

 All substances are made of atoms .Three kinds of sub

atomic particles are:

 Protons

 Neutrons

 Electrons

 Protons have a positive charge. Electrons have a negative

charge. Neutrons do not have any charge (i.e. they are

neutral).

Electrostatics

• A hard rubber rod gains millions of electrons when it is

rubbed with a wool cloth. The cloth loses electrons to the rod and becomes positively charged. The rubber becomes negatively charged.

• When glass rod is rubbed with fur, the fur loses electrons

to the glass rod. Glass rod becomes negatively charged and fur becomes positively charged.

• A body is said to be positively charged if it has an excess of

positive charges or it has lost some electrons.

• A body is said to be negatively charged if it has an excess

of electrons i.e. If it has gained some electrons.

Electrostatic Charges of a Material

• It states that like charges repel and unlike charges

attract.

The following methods are used to charge materials:

The Basic Law of Electrostatics

Contact method

A neutral metal sphere

rests upon an

insulating platform.

When the+aluminium plate is touched to the metal sphere, electrons are drawn off the sphere and onto the aluminium plate.

The aluminium plate has less excess+charge and the metal sphere now has an excess of+charge.

Method of Separation

Two metal spheres are mounted on insulating stands

The presence of a- charge induces e-to move from sphere A to B. The two- sphere system is polarized.

Sphere B is separated from sphere A using the insulating stand. The two spheres have opposite charges.

The excess charge distributes itself uniformly over the surface of the spheres.

An electroscope is charged by bringing a charged polythene strip up to the electroscope and rubbing it along the edge of the cap. In this way charges are transferred from the polythene strip to the metal cap. The leaf and the metal rod get the same charge, repel each other and the leaf therefore remain diverged.

Note. The electroscope acquires the same charge as the

charging rod

Charging a Gold Leaf Electroscope by Contact

• When the charged rod (-) is brought close to the cap, the leaf rises. The electrons in the cap are repelled to the leaf and the metal rod and the repulsion in the rod and the leaf causes divergence of leaf
• When the cap is touched while the rod is in position, the leaf divergence decreases. Touching the cap with a finger or a conductor/earthing causes electrons on the leaf and the rod to move to the earth discharging the leaf and the rod hence decrease in divergence/leaf collapses.
• When the finger is withdrawn and later, the rod, the leaf diverges. The positive charges on the cap now redistributes to the leaf and rod causing them to repel hence the divergence increases.

Charging Electroscope Positively by Induction

Note. The electroscope acquires the opposite charge as the charge as the charging rod

-

+ +

+ +

electron s

Uses of Electroscope

1. To detect the presence of charge on a body

The material to be tested is placed on or brought close to the cap

of the electroscope. If it is not charged, the leaf does not diverge.

2. To test the quantity of charge on a charged body

Two pith balls coated with aluminium and having different radii

are placed on insulating handles.

They are charged by rubbing with a duster. The charged balls are

brought close to the cap of a charged electroscope.

The ball with a smaller radius causes a slight increase in

divergence while the larger ball causes a greater increase in

divergence.

3. To test for insulation properties of a material

Materials like copper, iron, aluminium, zinc and graphite make

the leaf divergence decrease. Materials like plastic, glass, wood,

do not affect the divergence of the leaf.

4. Testing the type of charge

Charge an electroscope negatively by contact method. Slowly bring a negative rod

to be tested close to the cap of the electroscope. The leaf diverges more. It does so because the negative charges on the rod repel more charges from the cap to the plate and the leaf. Similar charges in the plate and the leaf are repelled.

Effect of charges on electroscope

Charge on electroscope

Charge brought near cap

Effect on leaf divergence

+ + Increase

- - Increase

+ - Decrease

- + Decrease

+ or - Uncharged body Decrease

An increase in the divergence of the leaf is therefore the only

sure way of confirming the kind of charge on a body.

Charges in air

Air can also be charged. The presence of charges in air can

be shown by heating air above a charged electroscope. It is

observed that the leaf divergence decreases.

When a fuel burns, chemical reactions yield ionised

products. The ions move and collide with air molecules,

causing air to become ionised. Ionisation produces both

positive and negative charges. The ions carrying opposite the

charge to the electroscope are attracted to the cap of the

electroscope, resulting in the discharge of the electroscope.

Charges on insulators can be removed by ionised air.

Other than heating, air can also be ionised by

radiations.

Spray Painting

The spray gun can is filled with the paint and its nozzle charged.

During spraying, the paint droplets acquire similar charges and,

therefore, spread out finely due to repulsion. As they approach

the metallic body, the induce opposite charges which in turn

attracts them to the surface. Little paint is therefore used.

Applications of Electrostatic Charges

Applications of Electrostatic Charges

Photocopier machine

A photocopier produces paper copies of documents using heat and

electrostatic charges. Inside a photocopier is a cylindrical drum that is charged

negatively. The bright light is used to illuminate the original document. The

unprinted areas reflect light and become negatively charged while the toner is

positively charged and therefore, sticks on the negatively charged areas. The toner

image is transferred to the paper which is negatively charged. The hot drum melts

the toner to the paper and pressure rollers help to ensure complete and dry bonding to the paper. A copy of the original document, is therefore, produced.