ELECTROSTATICS

CONTENT

1. Definition of Electrostatics
2. Types of Charges
3. The Law of Electrostatics
4. Gold leaf Electroscope
5. Ways of Producing Charges
6. Charge Distribution in a Conductor
7. Lightning Conductors
8. Electrophorus

Definition of Electrostatics

Electrostatics is the study of charges at rest. It is electricity that does not move from one point to another in the substance in which it is produced.

Types of Charges

Positive charge

A body becomes positively charged if it losses electron. This can be obtained in the Laboratory if glass rod is rubbed with silk and there is a net transfer of surface electrons from glass to the silk. The glass becomes positively charged and the silk becomes negatively charged.

Negative charge

A body is negatively charged if it gains electron. This is obtained by rubbing ebonite rod with fur and there is a transfer of electrons from the atoms of fur to the ebonite rod. The fur becomes positively charged.

The positively charged protons deep in the nucleus are not free to be transferred. Hence bodies do not become electrically charged by transfer of protons. They become charged by transfer of electrons

The Law of Electrostatics

Like or similar charges repel each other; unlike or opposite charges attract each other.

Gold leaf Electroscope

An electroscope is an instrument used for the detection and testing of small electric charges. It consists of a flat brass disc or cap, a brass rod with a gold leaf. The metal case is made draught-proof and connected to the earth to prevent accumulation of charges due to external influence.

Uses of the Gold Leaf Electroscope

1. To detect charges: If a charged body is placed on the cap of a charged  electroscope an increase in divergence or collapse of the leaf shows the body is charged. If there is no change in the divergence, it means the body is not charged.
2. To determine the nature of charge on the body: If a charged body is placed on a charged electroscope, increase in divergence means the charge on the electroscope and the body are the same. If there is collapse of the leaf, it means they have opposite charge or the body is uncharged.
3. To determine the conducting properties of a body: If a good conductor is placed on the cap of an electroscope, the leaf collapses immediately. If it is a semi conductor, it collapses gradually and if an insulator, there is no alteration of the leaf

EVALUATION

1. What is electrostatics?
2. Explain three functions of a Gold leaf electroscope

Ways of Producing Charges

1. Electrostatic Induction:

Electrostatic induction is the act of charging a neutral body by placing a charged body near it without any contact between the two.

STEP 1: A negatively charged body is brought near the uncharged body, free electrons from the metal sphere are repelled by the excess electrons on the rod. They shift towards the right. They can not escape from the sphere because the stand and the surrounding air are insulated.

STEP 2: These excess charges called induced charges are released to the earth by touching the right part of the sphere with a wire and the other part of the wire to the earth.

STEP 3: The wire is disconnected.

STEP 4: The negatively charged rod is removed. A net positive charge is left on the rod.

2. Friction:

Charges can also be produced by friction. By rubbing as in ebonite and fur, glass rod and silk, charges are transferred from one by either of the two bodies involved. Equal and opposite charges are produced by friction.

Effects of Charging by Friction

1. Passengers stepping out of cars and buses complain of a slight electric shock as soon as their feet touch the ground.This is because friction between the air and the body of the fast moving car makes the body of the vehicle to be charged.
2. A chain is often left hanging from the rear of a petrol tanker to discharge the charges acquired on the body during movement as this may cause a sparkwhen inflammable vapour is present.
3. Contact: This is done by bringing a charged body in contact with an uncharged body. Charges are transferred from the charged body to the uncharged body.

Charge Distribution in a Conductor

Charges are usually concentrated at places where the surface is sharply curved. The charge density is highest at the sharpest point of the conductor. Because of this high charge density, air molecule close to this point get ionized ( i.e broken down into positive and negative ions). Those with charge opposite to the conductor will be attracted to the conductor. Those with charge opposite to that of the conductor will be repelled. As these ions move, they collide with other molecules and knock off electron from them thereby ionizing those molecules. This process could continue leading to a geometrical increase in the number of ions around the conductor.

For a hollow conductor, charges reside only on its outside surface, no charges reside inside the conductor.

EVALUATION

1. What is electrostatic induction?
2. Explain the three methods of charging.

Lightning Conductors

Lightning conductors are used to prevent tall buildings from being damaged when being struck by lightning. They are made from a copper with a sharp point edge or spike at the top. It helps to conduct the charges generated harmlessly to the earth. When electrical charges in thunderclouds build up, attraction between unlike charges within a cloud increases steadily until a heavy spark  and sound is produced as the charges approach one another. This spark is observed as lightning and the sound is thunder. The heat generated can set a building or tree on fire.

The charge on the cloud induces electrical charges on the lightening conductor. This buildup at the sharp edge and cause ionization of air molecule around it. Some of the charge avalanche result from the  ionization of air around the lightening conductor travels toward the cloud and help to neutralize some of the charge on the cloud thereby reducing the possibility of a lightening

Electrophorus

Electrophorus is used for storing and transferring electric charges. It consists of a metal disc fitted with an insulating handle and another flat disc made of insulating material such as ebonite

GENERAL EVALUATION

1. Explain the use of the following: (i) Lightning conductor (ii) Electrophorus