The charged particles whose flow in a definite direction constitutes the electric current are called current carriers. e.g. electrons in conductors, ions in electrolyte, electrons and holes in semiconductor.

1. CURRENT ELECTRICITY
CH 2(A) ELECTRIC CURRENT AND RESISTANCE
#) Current Carriers. The charged particles whose flow in a definite direction constitutes the
electric current are called current carriers. e.g. electrons in conductors, ions in electrolyte,
electrons and holes in semiconductor.
#) Electromotive force of a cell is defined as the maximum potential difference between the
two electrodes of a cell when the cell is in the open circuit. E.M.F of a cell depends upon
nature of electrodes, nature and concentration of electrolyte used in the cell and its
temperature.
#) Electric Current. The flow of charge in a definite direction constitutes the electric current.
S.I unit of current is ampere. The arrow head marked in circuit represents the direction of
conventional current I.e. direction of flow of positive charge, whereas the direction of flow
of electrons gives the direction of electronic current which is opposite to that of
conventional current. Current is a scalar quantity.
#) Drift Velocity. It is defined as the average velocity with which free electrons get drifted
towards the positive end of the conductor under the influence of an external electric field.
#) Relation between current and drift velocity mobility.
1= nAe v d = A ne µ e E
Where n is the electron density or no. Of electrons per unit volume of the conductor and A is
the area of cross-section of the conductor. The small value of the drift velocity ( - 10-5 ms-1 )
produces a large amount of current due to presence of large number of free electrons in a
conductor (- 1029 m-3). The propagation of current is at the speed of light.
#) Ohm's law. It states that the current (I) flowing through a conductor is directly
proportional to the potential difference ( V ) across the ends of the conductor, provided
physical conditions of the conductor such as temperature, mechanical strain etc. are kept
constant I.e. V * 1 or V = IR
Where R is known as resistance of the conductor, which depends upon the nature and
dimensions of the conductor. The S.I. unit of R is ohm.
#) Resistance of a conductor. It is the obstruction posed by the conductor to the flow of
current through it. Resistance of a conductor is due to the collisions of free electrons with
the ions or atoms of the conductor while drifting towards the positive end of the conductor.

2. #) Specific resistance or electrical resistivity of the material of a conductor is defined as the
resistance of unit length and unit area of cross, section of the conductor. The S.I unit of
resistivity is m. Resistivity of a conductor depends upon the nature of the conductor but is
independent of the length or area of cross section of the conductor.
#) Current density at a point is defined as the amount of current flowing per unit area of
cross-section of the Conductor, provided the area is held in a direction normal to the
current.
The S.I. unit of current density is Am -2
#) Electrical conductivity of a conductor is the inverse of its resistivity. The S.I unit of ó .
#) Effect of temperature on resistance. The resistance of a metal conductor at a
temperature t 0 C is given by R t = R 0 (1 + a t )
Where R o is the resistance of a conductor at 0o C and a is the temperature co-efficient of
resistance.
For metals a is positive I.e. resistance increases with rise in temperature.
For semi conductor and insulators a is negative I.e. resistance decreases with rise in
temperature.
For alloys like manganin, eureka and constantan, the value of a is very small as compared to
that of conductors. That is why these alloys are used in making standard resistances.
#) Non-ohmic conductors. Those conductors which do not obey Ohm's law are called non-
ohmic conductors e.g. vacuum tube, liquid electrolyte etc.
#) Super-conductors. Those materials which offer least resistance to the flow of current
through them are called super-conductors. Examples: mercury at temp. 4.2 K, lead at 7.25 K
and niobium at temperature 9.2 K become super-conductors.
The conductivity of superconductor is called superconductivity.
#) Color code for carbon resistors. The number attached from 0 to 9 to the various colors
can be recollected by the sentence B.B. ROY Great Britain Very Good Wife.
Black -0, Brown-1, Red-2, Orange-3, Yellow-4, Green-5, Blue-6, Violet-7, Grey-8, White-9. The
strip of gold, silver and no color shows the accuracy of 5%, 10% and 20% of the given carbon
resistor.
#) Resistance in series. The total resistance (R s ) is given by. R s = R 1 + R2 +R3 + ......
#) Internal resistance of a cell is defined as the resistance offered by the electrolyte and
electrodes of a cell when electric current flows through it. Internal resistance of a cell

3. depends upon: (I) distance between the electrodes, (ii) the nature of electrodes (iii) nature
of electrolyte and (iv) area of the electrodes immersed in the electrolyte.
#) Terminal potential difference of a cell is defined as the potential difference between the
two electrodes of a cell in a closed circuit. Terminal potential difference of a cell decreases if
the current drawn from the cell increases. Terminal potential difference of a cell ( V ) is less
than the e.m.f. of a cell by an amount equal to potential drop across the internal resistance
of the cell.
Where R is the external resistance in the circuit and r is the internal resistance of a cell.
#) Ch 2 (B) ELECTRIC MEASUREMENT
#) Kirchhoff's Laws.
First law: The algebraic sum of the current meeting at a junction is zero. The current
reaching a function if taken positive then the current leaving the junction is taken negative.
This law supports the concept that moving charges are not accumulated at a junction.
Second law: In a closed loop, the algebraic sum of all the potential difference is zero.
While traversing a closed loop. If negative pole of the cell is encountered first then its emf is
-ve , otherwise +ve. The product of resistance and current in an arm of the circuit is taken
positive if the direction of current in that arm is in the same sense as one moves in a closed
loop and is taken negative if the direction of current in that arm is opposite to the sense as
one moves in the closed loop.
#) Wheatstone bridge Principle: it states that if four resistances P,Q,R,S are arranged to
form a bridge as shown in Fig . 2.(b). 3 ,on pressing battery key K1 first and then
galvanometer key K2 , if the galvanometer shows no deflection, then the bridge is balanced.
#) Principle of potentiometer. It is based on the fact that the fakk of potential across any
portion of the wire is directly proportional to the length of that portion provides the wire is
of uniform area of cross section and a constant current is flowing.
CH 2 (C) HEATING EFFECT OF CURRENT
#) Joule's law of heating. It states that the amount of heat produced in a conductor is
directly proportional to the:
(I) square of the current flowing through the conductor, (ii) resistance of the conductor and
(iii) Time for which the current is passed.
#) Electric power. It is defined as the rate at which work is done in maintaining the current in
electric circuit.

4. Electric power, P = VI = 12 R = V2 /R watt or joule / second
#) Electric energy. The electric energy consumed in a circuit is defined as the total work done
in maintaining the current in an electric circuit for a given time.
Electric energy = VIt = P t = 12 Rt = v2 t / R
S.I unit of electric energy is joule (denoted by J )
Where 1 joule = 1 watt * 1 second = 1 volt * 1 ampere * 1 second
Commercial unit of electric energy is kilowatt hour ( k Wh ) where 1kWh = 1000 Wh = 3.6 *
106 J.