Electricity (NCERT CHAPTER 12)
ELECTRIC CURRENT
 Electricity has an important role in modern
society.
 Electricity is a controllable and convenient form
of energy for a variety of uses in homes, schools,
hospitals, industries and so on.
 Electricity has made our life comfortable and
easy.
 Flowing electric charge through a conductor
constitute electric current.
 A continuous and closed path of an electric
current is called an electric circuit.
 Electric current is the rate of flow of electric
charge.
 When a net charge Q, flows across any cross
section of a conductor in time ‘t’, then the
current I, through the cross sections is:
I = Q/t.
 The SI unit of electric charge is coulomb which
is represented by ‘C’.
 One coulomb is equivalent to the charge
contained in nearly 6x10¹⁸ electrons.
 The SI unit of electric current is ampere(A).
 Current through a cross section of a conductor is
said to be one ampere when 1 coulomb of charge
flows through it in one second.
1 ampere = 1 coulomb/ 1 second
 Small quantities of current are expressed in
milliampere (1mA=10-3A) or in microampere
(1µA=10-6).
ELECTRIC POTENTIAL AND POTENTIAL
DIFFERENCE
 The electric potential difference between two
points in an electric circuit carrying some
current is defined as the work done to move a
unit charge from one point to the other.
 The chemical reaction within a cell generates the
potential difference across the terminals of the
cell.
 Potential difference (V) between two points =
work done (W)/ Charge(Q).
V= W/Q
 SI unit of electrical potential difference is
Volt(V).
 One volt is the potential difference between two
points in a current carrying conductor when one
joule of work is done to move a charge of one
coulomb from one point to the other.
Therefore, 1 volt = 1 joule/ 1 coulomb
 The potential difference is measured by an
instrument called Voltmeter.
 The voltmeter is always connected in parallel
across the points between which the potential
difference is to be measured.
CIRCUIT DIAGRAM
Ohm’s law
 Ohm’s law states, “The potential difference, V,
across the ends of a given metallic wire in an
electric circuit is directly proportional to the
current flowing through it, provided its
temperature remains the same.”
 V= IR or RI (mathematical relation of ohm’s
law).
 Resistance is the property of a conductor to
resist the flow of charges through it.
 SI unit of resistance is ohm, represented by the
Greek letter Ω.
 Resistance of a conductor is said to be one ohm
when a current of one ampere flows through the
conductor by applying a potential difference of
one volt across its ends.
1 ohm = 1 volt/1Ampere
 A good conductor is one that has low resistance.
On the other hand a good insulator has very high
resistance.
 Temperature plays a vital role in determining the
resistance of a conductor. At room temperature,
silver is the best conductor and copper is second
best.
Experimental verification of ohm’s law
 On increasing the value of electric current, value
of potential difference also increases.
 The ratio V/I of the various values of the V and
I at a given temperature is constant which is
equal to the resistance of the conductor.
 The value of current is different for different
resistance.
FACTORS ON WHICH THE RESISTANCE DEPENDS
 Materials that offer a low resistance are good
conductors of electricity. They allow electricity
to pass though them very easily. Copper, Silver,
Aluminium are good conductors.
 Materials that offer a higher resistance are
poor conductors of electricity. They allow very
less electricity to pass through them. Tungsten,
Bismuth, Stainless steel are poor conductors of
electricity.
 Materials that offer a very high resistance are
insulators, they allow no electricity to pass
through them. Plastic, paper, plastic, rubber,
glass are insulators.
 The resistance of a conductor is directly
proportional to the length of the conductor.
Resistance ∝ Length of conductor
R ∝ I
 The resistance of a conductor is inversely
proportional to the area of cross section of the
conductor.
Resistance ∝ 1/ area of cross section
R ∝ 1/A
 Resistance of a conductor depends on the nature
of material of the conductor.
Electrical resistivity
 Resistance of a uniform metallic conductor is
directly proportional to its length(l), i.e.,
resistance µ length of the conductor Rµl.
 Resistance of a unform metallic conductor is
inversely proportional to the area of cross
section (A). i.e.,
Rµ 1/ area of cross section
Rµ 1/A
R = ρ l/A
 Where ρ(rho) is a constant of proportionality and
is called the electrical resistivity of the material
of the conductor.
 The SI unit of resistivity is Ωm.
 The metals and alloys have very low resistivity in
the range of 10^-8 Ωm to 10^-6 Ωm. They are
good conductors of electricity.
 Insulators like rubber and glass have resistivity
of the order of 10^12 to 10^17 Ωm.
Combination Of Resistors in Series
 We connect a number of resistors in two waysSeries combination and Parallel combination.
 Three resistors having resistances R1, R2, and R3
respectively, are joined end to end in an electric
circuit, here the resistors are said to be
connected in series.
 In a series combination of resistors, the current
in the same in every part of the circuit or the
same current flows through each resistor.
 The total potential difference across a
combination of resistors in series is equal to the
sum of potential difference across the individual
resistor, i.e.,
V = V1 + V2+ V3