Current Electricity

It is the branch of electricity which deals with the study of cause and effect of moving charges.

Conductors And Insulators

Materials which allow an electric charge to flow through them are called conductors or good conductors of electricity. All metals in solid form and aqueous solutions of salts are examples of good conductors of electricity. Gases in general are not conductors of electricity. They conduct electricity only under certain special conditions of temperature and pressure.

           Materials which do not allow an electric charge to flow through them are called bad conductors of electricity or insulators. All plastic materials, glass, wood, rubber, ebonite, etc.,are examples of insulators in solid state. Pure water is an example is an example of insulator in liquid state. All gases in general are examples of insulators in gaseous state.

Electric Current

The rate at which electric charge flows through a conductor is known as electric current. It is denoted by 'i' and its SI unit is 'ampere'(A). Its CGS(Centimetre Gram Second) unit is 'biot'. 

           Electric current can be calculated mathematically by dividing the total amount of charge flowing through a conductor by the total time for which the current flows. Hence,

                                   i = Q(Total Charge)

                                         t(Total Time)

          Unit of electric Current =  Unit of Electric Charge 

                                                           Unit of a Time

                                   Ampere = Coulumb 

                                                    Second

Hence, if 1 Coulumb of charge flows through a conductor every second, then electric current flowing through the circuit is said to be 1 Ampere.

Cause of Electric Current 

Electric current through a conductor can be understood easily by an analogy of flow of water. Consider two cylindrical tanks as shown in the fig. The level of water in the first tank is greater than the level of water in the second tank. We will notice that the flow of water is from the first tank to the second tank. Hence, the level of water decides the direction of water flow. Also, the water flow ceases when the level of water in both the tanks becomes equal.

Similarly, in an electric conductor, the electric charge flows from one end to another end of conductor , only when an electric potential difference exists between the two ends of the conductor. Thus, an electric potential difference is the condition required for a conductor to have an electric charge through it from one end to the other and is measured in volts. If the potential difference across the two ends is zero, there exists no electric current through the the conductor.

Positively charged bodies are considered to be at a higher potential and the negatively charged bodies

are considered to be at a higher potential. So, when a positively charged body and a negatively charged are connected by means of a conductor, there exists s potential difference across the ends of the conductor and so  an electric charge flows through the conductor from a positively charged body to a negatively charged body. Initially, it was considered that the flow of positive charges constituted an electric current. Later, it was discovered that it is not the movement of positive charges that cause electric current, but the flow of electrons that causes electric current in a conductor.

When it was discovered that the flow of electrons constituted electric current in a conductor, many theories and laws regarding electricity were proposed. So, these laws were not changed again and electric current was classified into two types. One type of electric current is the flow of charge from a body at high potential(positively charged body), to a body at a low potential(negatively charged body) as assumed before and is called 'conventional electric current'. The second type of electric current is the actual current which is, due to movement of negative charges  from a negatively charged body to a positively charged body,called 'electronic current'. 

                             Introduction to Electricity

Introduction

Electricity has become an important part of our lives. Life without electricity is so hard to imagine. It can be easily converted into other forms of energy and transmitted over long distances in minimum energy loss. From lightning in a room to agriculture, communication and media to medical field, transport to computers, almost every field depends heavily on electricity. That's why it has become perhaps the most important branch of science and engineering.

Electrostatics and Current Electricity

For the better understanding of this form of energy, it is divided into electrostatics and current electricity. The study of electric charges at rest and their cause and effect constitutes  electrostatics.

The study of causes and effect of moving charges is called current electricity.

Electrification

A Greek philosopher and scientist named 'Thale'  rubbed a substance made of amber with fur cloth and noticed that both these materials started attracting small bodies like thread, pieces of paper, etc. Hence, amber and fur were considered to have acquired electrical property and process was called was called electrification (amber was called 'electron' in Greek language).

         When a body is rubbed with another body transfer of electrons from atoms of one body to that of the other body takes place. Electrons are negatively charged subatomic(found in the atom) particles, While rubbing, one body loses electrons and the other body gains the same number of electrons that are lost by first body, An atom is electrically neutral as it has the same number of electrons and protons, When some electrons are lost by the first body, it now becomes negatively charged because of presence of more electrons than protons while the other body becomes positively charged due to loss of electrons.

         This process of transfer of electrons or charges from one body to another is called electrification or charging.

Charges and properties of charges

'Charge' is defined as the amount of electricity present in a body. It is denoted by 'Q' or 'q'. The unit off electric charge in S.I. system is 'coulomb' named after the scientist Coulumb. It is denoted by 'C'.

Properties of charges 

The following are the properties of charges.

    1. There are two types of  charges, viz., positive and negative.

    2. Like charges repel and unlike charges attract each other.

    3.On losing electrons, a neutral body acquires a positive charge.

    4. On gaining electrons, a neutral body acquires a negative charge.

    5. The amount of charge present in a body, either positive or negative, is expressed in terms of an            integral  number of the charge of an electron, which is considered the elementary entity of an              electric charge.

    6. The magnitude of charge of an electron is equal to the magnitude of a proton.

    7. The charge on a body gives the information about the excess or deficiency of the total charge,              either positive or negative, present in the body.

    8. When no charge is supplied to or removed from a body, it is said to be electrically neutral and              then the amount of positive charge present in it is equal to the amount of negative charge                      present in it.

Methods of charging a body

The process of supplying an electric charge to a body is known as 'charging'. A body can be charged in different ways.

Charging by Friction

When a glass rod is rubbed with a silk cloth as shown in the fig., the glass rod loses some electrons and becomes positively charged whereas the silk cloth gains the electrons lost by the glass rod and becomes negatively charged. This method of charging two bodies by rubbing them is called charging by friction.

Charging by conduction

The method of charging a body as shown in the fig. is called charging by conduction.

Charging by Induction

In the induction process, a charged object is brought near but not touched to a neutral conducting object. The presence of a charged object near a neutral conductor will force (or induce) electrons within the conductor to move.

Detection and Presence of Charges

An electroscope is a scientific instrument used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. ... Electroscopes detect electric charge by the motion of a test object due to the Coulomb electrostatic force.

Working of an Electroscope

         It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf. A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied.^[9] To protect the gold leaves from drafts of air they are enclosed in a glass bottle, usually open at the bottom and mounted over a conductive base. Often there are grounded metal plates or foil strips in the bottle flanking the gold leaves on either side. These are a safety measure; if an excessive charge is applied to the delicate gold leaves, they will touch the grounding plates and discharge before tearing. They also capture charge leaking through the air that accumulate on the glass walls, and that increase the sensitivity of the instrument. In the precision instruments the inside of the bottle was occasionally evacuated, to prevent the charge on the terminal from leaking off through the ionization of the air.

When the metal terminal is touched with a charged object, the gold leaves spread apart in a 'V'. This is because some of the charge on the object is conducted through the terminal and metal rod to the leaves.^[9] Since they receive the same sign charge they repel each other and thus diverge. If the terminal is grounded by touching it with a finger, the charge is transferred through the human body into the earth and the gold leaves close together.

The electroscope can also be charged without touching it to a charged object, by electrostatic induction. If a charged object is brought near the electroscope terminal, the leaves also diverge, because the electric field of the object causes charges in the electroscope rod to separate the leaves. Charges of the opposite polarity to the charged object are attracted to the terminal, while charges with the same polarity are repelled to the leaves, causing them to spread. If the electroscope terminal is grounded while the charged object is nearby, by touching it momentarily with a finger, the same polarity charges in the leaves drain away to ground, leaving the electroscope with a net charge of opposite polarity to the object. The leaves close because the charge is all concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again.

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