Transcript Chapter-16

Chapter-16
ELECTRICITY
DEFINITIONS
 COULOMB
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It is SI unit of electric charge. One coulomb (1C) of charge being that
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quantity of charge which when placed one meter from an identical charge
in vacuums repels it with a force of 8.99 x 109 N.
INSULATORS
Insulators are those materials, which do not allow electric charges to pass
through them. In other words, insulators are materials that do not allow
electrical current to pass. In insulators electrons are tightly bounded to
their atoms. Insulators do not have free electrons.
Examples Plastic, rubber, wood, glass etc.
CONDUCTOR
Conductors are those materials, which allow electric charges to pass
through them. In other words,
conductors are materials that allow
electric current to pass. In conductors electrons are loosely bounded to
atoms. Conductors have free electrons.
Examples:
Copper, Gold, Aluminum, Silver etc.
ELECTRIC FIELD
 Space or region surrounding a charge or charged
body within which another charge experiences
some electrostatic force of attraction or repulsion
when placed at a point is called Electric Field.
ELECTRIC INTENSITY
It is the strength of electric field at a point. Electric
intensity at a point is defined as the force experienced
per unit positive charge at a point placed in the electric
field.
Mathematically,
 E=F/q
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It is a vector quantity. It has the same direction as that
of force.
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Units
N/C or Volt/m
E=1/4pe x q/r2
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ELECTRICAL POTENTIAL
Electric potential at a point is defined as the
amount of work done in moving unit positive charge
against the direction of electric field from a point
to that point.
Electrical potential = work done/charge
or
U=work/q
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unit of electric potential in SI system is Volt .
1 volt = 1 Joule/coulomb
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VOLT
Unit of electric potential and potential difference
in SI system is called Volt.
 It is defined as
"in an electric field potential b/w two points
is 1 volt if the amount of work
done in moving 1 Coulomb charge from one
point to another point is 1 Joule."
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POTENTIAL DIFFERENCE
Potential difference b/w two points A and B is
equal to the amount of work done by moving a unit
positive charge from point A to point B against the
electric field
 VB-VA=VAB
or
VAB= (work)AB/q
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Unit
Volt or Joule/Coulomb
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ELECTRIC CURRENT
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The rate of flow of electric charge through a cross
section of a conductor is called Electric Current
or Electric charge passes through a cross section of a
conductor is called Electric Current.
It is denoted by I.
FORMULA
I = Q/t
UNITS
Ampere in SI system.
OTHER UNITS
mA (milli Ampere) = 10-3A
m A (micro Ampere) = 10-6A
AMPERE
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If one coulomb of electric charge passes through a cross
section of a conductor in one second, the amount of current
passes through it is called Ampere. 1A = 1c/1sec.
RESISTANCE
opposition offered by the atoms of a conductor in the flow of
electric current is called Resistance. It is a hurdle in the flow
of electric current. Different substances have different
resistance. Resistance of a conductor increases with the
increase in temperature.
SYMBOL
It is denoted by R.
UNIT
Oh
CAPACITOR
Capacitor is an electronic device, which is used to store
electric charge or electrical energy. A
capacitor stores electric charge on its plates. There are a
number of types of capacitors available.
 STRUCTURE OF CAPACITOR
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A capacitor consists of two identical conducting plates
which are placed in front of each other. One
plate of
capacitor is connected to the positive terminal of power
supply and the other plate is connected to negative
terminal. The plate, which is connected to positive terminal
acquired positive charge, and the other plate connected to
negative terminal. Separation between the plates in very
small. The space between the plates is field with air or any
suitable dielectric material
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Capacitor
PRINCIPLE OF CAPACITOR
Electric charge stored between the plates of a
capacitor is directly proportional to the
potential difference between the plates.
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Let the potential difference between the plates
is V and the charge stored on any one of the plates
of capacitor is Q then,
 Q directly prpportional V
Q = CV
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where
C= Capacitance of the capacitor
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CAPACITANCE
Charge storing capability of a capacitor is called
capacitance of capacitor.
Definition: Capacitance of a capacitor is defined
as the ratio of the charge stored on any of the plates
of capacitor to the potential between the plates.
COMBINATION OF RESISTORS
Resistance can be joined to each other by two ways:
1. Series combination
2. Parallel combination
 SERIES COMBINATION
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Characteristics:
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1. If different resistances are joined with each other such that
there is only one path for the flow of
electric current then the
combination of such resistances is called Series Combination.
2. In series combination current through each resistor is
constant.
3. In series combination Potential difference across each resistor
is different depending upon the value of
resistance.
4. Equivalent resistance of circuit is equal to the sum of individual
resistances.
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Series combination
Re = R1 + R2 + R3 + R4 + …………….. Rn
DISADVANTAGE
If one component is fused, then the other components of circuit will not
function.
EQUIVALENT RESISTANCE IN SERIES
COMBINATION
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Consider three resistances R1, R2, & R3 connected in series
combination with a power supply of voltage.
Potential difference of each resistor is V1, V2, & V3 respectively.
Let electric current I is passing through the circuit.
Now
V = V1 + V2 + V3
According to Ohm’s law V = IR
thus
IRe = IR1 + IR2 + IR3
IRe = I(R1 + R2 + R3)
IRe/I = R1 + R2 + R3
Re = R1 + R2 + R3
This shows that in series combination equivalent resistance of
circuit is always greater than individual resistances.
PARALLEL COMBINATION
 Characteristics:
1. If there are more than one path for the flow of
current in a circuit then the combination of
resistances is called Parallel Combination.
2. In parallel combination current through each
resistor is different.
3. Potential difference across each resistor is
constant.
4. Equivalent resistance of circuit is always less
than either of the resistances included in the circuit.
Parallel Combination
ADVANTAGE
In parallel combination of resistors, if one component of circuit (resistor) is
damaged then rest of the
component of the circuit will perform their work
without any disturbance. It is due to the presence of
more than paths for the
flow of electric current.
EQUIVALENT RESISTANCE IN
PARALLEL COMBINATION
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Consider three resistances R1 , R2 & R3 connected in
parallel combination with a power supply of voltage V.
Now
I = I 1 + I2 + I3
according to Ohm’s law
V/R = I
Therefore,
V/Re = V/R1 + V/R2 + V/R3
V/Re = V(1/R1 + 1/R2 + 1/R3)
V/ReV = 1/R1 + 1/R2 + 1/R3
OR
JOULE'S LAW
INTRODUCTION:
When an electric current passes through a wire
heat energy is produced. It is due to the collision
of electrons with the atoms. In order to continue
steady current, work has to be done on electric
charges.
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STATEMENT:
Amount of work done on electric charge on steady
current is directly proportional to amount of heat.
 Work directly proportional to Heat
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PROOF
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Consider a conductor through which electric current q is passing in
time t let the potential difference between two ends of wire is V.
We know that
v = W/q
or
W = q x V_(i)
According to Ohm’s law V = IR
putting the value of V in equation (i)
W = q x IR_______(ii)
But
I = q/t
Or
Q = It
putting the value of q in equation (ii)
W = It . IR
W = I2Rt
OHM'S LAW
 INTRODUCTION
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Ohm’s law is a quantitative relation b/w potential difference and electric
current.
STATEMENT
According to Ohm’s law,
"The electric current passes through a conductor is directly
proportional
to the potential differences between the ends of conductor,
if physical conditions of conductor remain constant."
i.e.
I directly proportional to V
I = kV
K =constant and is called "conductivity of material"
I/K = V or V = I/K or V = I x 1/K so, Let [1/K = resistance]
V=IxR
GRAPHICAL REPRESENTATION