INTRODUCTION: However,most of the practical applications of electricity involve charges in motion.For example,an electric bulb glows when charge flows through it,an electric fan rotates when charge is made to pass through its coil.Again,our radio and T.V. sets operate on flowing charges, and so on.The flowing charges are said to constitute electric current.We shall study later that electric current is of two types: direct current (d.c.) and ‘alternating current (a.c.).Most of the partial devices use alternating current where charges flow back and forth through conductors.However,we start studying ”charges in motion’ through conductors in one direction.In this chapter,we shall confine ourselves to the study of direct current circuits.
The branch of Physics,which deals with the study of charges in motion is called Current Electricity.
The charged entities whose flow in a definite constitutes the electric current are called current Carriers.For example,
(i)In solids,which are good conductors,e.g.,metals,the current caare the valance electrons.These electrons are not firmly attached to individual atoms, are free to move throughout the volume of conductor.
In semiconductor like silicon and germanium,current carries are negatively charged electrons and positively charged holes.
In insulators like glass,ebonite etc.,there are no free charge/current carries.
(ii)In liquids like NaCl,current carries are positively charged Na+ ion (called cation) and negatively charged Cl- ion (called anion). Similarly,in CuSo4 solution,the current carries are Cu++ ion and So–4 ions.Such liquids through which electricity can pass are called electrolytes.Thus,in an electrolyte,electric current due to similtasimul drifting of cations and anions.
(iii)In Gases under normal conditions,there are no charge/current carries as they are insulators.However,gases can be ionnized by applying high potential difference at low pressure or by exposing them to X-rays.In that event,current carries in gases would be the electrons and positive ions of the gas.
ELECTRIC CURRENT AND RESISTANCE
Electric current is possible in a closed circuit if there is a source of external force (infact a source of energy) which compels the current carriers (electrons or ions) to move in a definite direction (i.e. from lower potential energy to higher potential energy).Tis external force which makes the current carriers to move in a definite direction is called ELECTROMOTIVE force (emf).The emf is a force at all.It is a special case of potential difference.Due to this,it has the nature of workdone per unit charge.
Thus the ELECTROMOTIVE force is associated with an arrangement or mechanism which can supply energy or does work to move the electric charge from lower potential to higher potential energy.Such an arrangement is called a source of emf,which may be a cell,a battery,a generator or dynamo.It is a seat of power.
E.M.F is a cell is defined as the maximum potential between the two electrodes of the cell when no current is drawn from the cell or cell is in the open circuit.
It may also be defined as the energy supplied by the cell to drive a unit positive charge once around the complete circuit
where,vector E is the electric field (I.e., force per unit charge) and vector do is the small path segment vector.
Symbolically a cell is represented by two unequal parallel limeslhaving a small separation.The taller line represents the positive electrode P and smaller line represents the negative electrode P and smaller line represents the negative electrode N.
The SI unit of emf of a cell is volt (V) or joule per Coulomb (JC-¹).The emf of a cell is said to be one volt,if 1 joule is supplied by the cell to drive one Coulomb of charge once around the whole circuit (including the cell).
The emf of a cell depends upon the nature of electrodes,nature and the concentration of electrolyte used in the cell and its temperature .
ELECTRIC CURRENT AND RESISTANCE