TO THE EXITING WORLD OF

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TO THE EXITING WORLDOF
WELCOME
PHYSICS

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PHYSICS

• CLASS XII

B.SESHA SAI PGT PhysicsKV NO1.BHUBANESWAR
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DEVELOPED BY

• B.SESHA SAI
• PGT PHYSICS
• KENDRIYA VIDYALAYA NO1,BHUBANESWAR

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UNIT 1
ELECTROSTATICS
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ELECTROSTATICS

• THE BRANCH OF PHYSICS DEALING WITH CHARGES AT
  REST AND THEIR PROPERTIES
• STATIC ELECTRICITY WAS FIRST OBSERVED BY THALES
  OF MILETUS IN 600 BC WHEN HE FOUND THAT AMBER
  WHEN RUBBED WITH FUR ACQUIRED THE PROPERTY OF
  ATTRCACTING TINY PIECES OF SAW DUST ETC.
• ELECTRICITY PRODUCED BY RUBBING IS CALLED
  FRICTIONAL ELECTRICITY
• SINCE THE CHARGES SO PRODUCED ARE AT REST IT IS
  ALSO CALLED STATIC ELECTRICITY
• CHARGES ARE PRODUCED BY TRANSFER OF ELECTRONS

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IN 1600 AD, DR. WILLIAM GILBERT, COURT PHYSICIAN
TO QUEEN ELIZABETH I OF ENGLAND, PUBLISHED THE
BOOK (DE MAGNETO) IN WHICH HE MADE AN ACCOUNT OF
ALL THE EXPERIMENTS AND OBSERVATIONS MADE SO FAR
IN THE FIELD OF ELECTROSTATICS. GILBERT FOUND
THAT THERE ARE TWO KINDS OF CHARGES AND THAT LIKE
CHARGES REPEL AND UNLIKE CHARGES ATTRACT. HE
NAMED THE TWO KINDS OF CHARGES AS RESINOUS AND
VITREOUS. THE CHARGE ACQUIRED BY AMBER OR EBONITE
(WHEN RUBBED WITH WOOL OR FUR) WAS CALLED
RESINOUS AND THE OTHER KIND OF CHARGE WAS CALLED
VITREOUS.
EWN GSP
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BENJAMIN FRANKLIN, AN AMERICAN SCIENTIST

• Introduced the convention according to which
  resinous charge was called negative and the other
  was called positive

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CONSERVATION OF CHARGES

• THE TOTAL CHARGE IN ANY SYSTEM IS ALWAYS
  CONSERVED
• NET CHARGE CAN NEITHER BE CREATED NOR BE
  DESTROYED IN ISOLATION
• CHARGES CAN ONLY BE PRODUCED OR DESTROYED IN
  EQUAL AND OPPOSITE PAIRS
• THE TOTAL CHARGE BEFORE AND AFTER ANY REACTION
  REMAINS THE SAME.

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QUANTIZATION OF CHARGE

• THE CHARGE PRESENT IN ANY BODY IS ALWAYS THE
  INTEGRAL MULTIPLE OF FUNDAMENTAL CHARGE ? THE
  CHARGE OF AN ELECTRON (1.6 X 10-19C)
• NO BODY CAN POSSESS FRACTIONAL ELECTRONIC CHARGE
  (IN THE MACROSCOPIC WORLD)

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QUARKS

• ARE PARTICLES CONSIDERED TO POSSESS FRACTIONAL
  ELECTRONIC CHARGES -- 1/3 e, 2/3 e ..
• THERE ARE SIX TYPES OF QUARKS? UP, DOWN, TOP,
  BOTTOM, CHARM AND STRANGE
• BUT THE EXISTENCE OF QUARKS DONOT VIOLATE THE LAW
  OF CONSERVATION OF CHARGE. IT ONLY CHANGES THE
  MAGNITUDE OF FUNDAMENTAL CHARGE TO THAT OF THE
  LOWEST POSSIBLE CHARGE ON QUARKS.
• ALSO, QUARKS CANNOT EXIST FREELY. THEY ARE ALWAYS
  FOUND COMBINED TO FORM INTEGRAL MULTIPLES OF
  ELECTRONIC CHARGE.

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DETAILS OF QUARKS
Quark Symbol Spin Charge BaryonNumber S C B T Mass
Up U 1/2 2/3 1/3 0 0 0 0 360 MeV
Down D 1/2 -1/3 1/3 0 0 0 0 360 MeV
Charm C 1/2 2/3 1/3 0 1 0 0 1500 MeV
Strange S 1/2 -1/3 1/3 -1 0 0 0 540 MeV
Top T 1/2 2/3 1/3 0 0 0 1 174 GeV
Bottom B 1/2 -1/3 1/3 0 0 1 0 5 GeV
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COULOMBS LAW

• THE FORCE OF ATTRACTION OR REPULSION BETWEEN TWO
  POINT CHARGES IS DIRECTLY PROPORTIONAL TO THE
  PRODUCT OF THE AMGNITUDE OF THE CHARGES AND
  INVERSELY PROPORTIONAL TO THE SQUARE OF THE
  DISTANCE BETWEEN THEM.
• MATHEMATICALLY

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RELATIVE PERMITIVITY

• Is defined as the ratio of the force between two
  point charges separated in vacuum to the force
  between the same two charges separated by the
  same distance while kept in the medium.
• i.e. ?r F0 /Fm

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PRINCIPLE OF SUPER POSITION

• States that when there are a number of point
  charges, the net force on any one of the charges
  is equal to the vector sum of the forces due to
  the individual charges.
• i.e.
• F1 F12 F13 F14

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DEFINE 1 COULOMB

• One coulomb is defined as that charge which when
  kept one metre apart from an equal and similar
  charge in vacuum, repels it with a force of 9 x
  109N.

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ELECTRIC FIELD

• Qualitatively
• The region of space around a charge where it can
  exert a force of electrical origin on another
  charge.
• Quantitatively
• The intensity of ELECTRIC FIELD at any point is
  defined as the force exerted per unit charge by a
  positive test charge kept at that point.

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ELECTRIC LINES OF FORCE

• Are imaginary lines of force such that the
  tangent to it at any point gives the direction of
  electric field at that point.
• A positive point charge free to move will move in
  the direction of electric field and a negative
  point charge will move in a direction opposite to
  the direction of electric field along an electric
  line of force.

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The lines of force to represent uniform electric
field are as shown below
The electric lines of force due to point charge q
gt 0 are as shown below
The electric lines of force due to point charge q
lt 0 are as shown below

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PROPERTIES OF ELECTRIC LINES OF FORCE

• Start from a positive charge and end in a
  negative charge.
• The tangent to it at any point gives the
  direction of electric field at that point.
• They never intersect each other
• They tend to contract longitudinally and expand
  laterally.
• They always enter or emerge normal to the surface
  of a charged conductor.
• They are close together in regions of strong
  electric field and far apart in regions of weak
  electric field.

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WHY TWO ELECTRIC LINES OF FORCE NEVER INTERSECT?

• If they intersect two tangents can be drawn from
  the same point( i.e. at the point of
  intersection) indicating two directions of
  electric field at the same position which is
  impossible.

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ELECTRIC DIPOLE

• Two equal and opposite point charges separated by
  a very small distance constitute an electric
  dipole.
• Electric dipole moment of a dipole is defined as
  the product of the magnitude of either of the
  charges and the distance between the charges.
• Dipole moment,

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ELECTRIC FIELD AT A POINT DUE TO A DIPOLE

• On the axial position
• On the equatorial position

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TORQUE ON A DIPOLE

• ? pE sin?
• Or
• ? p X E
• where p is the electric dipole moment and E is
  the intensity of electric field.

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DERIVATION (? PE sin?)

• Force on charge q at A .
• force on charge - q at B
• Forces F A and FB equal and opposite form a
  couple which tends to rotate the dipole
• torque acting on dipole is

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• so from -------- ( 1 )
• No torque acts when dipole moment aligns parallel
  to electric field ( i.e ? 0 )
• from ( 2 ) 0

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ELECTRIC FLUX

• Is the total lines of force passing normal to a
  given surface
• ?E E A for uniform electric field
• Electric flux is a scalar quantity

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GAUSS THEOREM

• States the total electric flux through a closed
  surface (surface integral of electric field over
  a closed surface) is equal to 1/?o times the
  total charge enclosed by the surface.
• Mathematically

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ELECTRIC FIELD AT A POINT DUE TO DIFFERENT CHARGE
DISTRIBUTIONS

• E due to a point charge
• E due to a line of charge
• E due to a plane sheet of charge
• E due to a sphere of charge

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ELECTRIC POTENTIAL

• Electric potential at any point is defined as the
  work done per unit charge in bringing a positive
  test charge from infinity to that point without
  any acceleration.

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POTENTIAL DIFFERENCE

• Potential difference between two points is
  defined as the work done per unit charge in
  carrying a positive test charge from one point to
  other without any acceleration.

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POTENTIAL ENERGY OF A SYSTEM OF CHARGES

• Potential energy of a system of charges is
  defined as the total work done in assembling all
  the charges constituting the system from infinity
  to their respective positions.

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WORK DONE IN ROTATING A DIPOLE IN A UNIFORM
ELECTRIC FIELD
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POTENTIAL ENERGY OF A DIPOLE IN A UNIFORM
ELECTRIC FIELD
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ACTION OF POINTS

• The surface charge density is not uniform in the
  case of uneven metal surfaces. It is maximum at
  sharp points and hence the intensity of electric
  field will also be maximum at these points. This
  is known as action of points.

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CORONA DISCHARGE

• When a metal with sharp points is charged, the
  sharp points acquire a high electric field and
  ionizes the air molecules nearby and then repels
  them away. The charged air molecules moving away
  from the sharp points constitute an electric wind
  and the discharge of electricity from sharp
  points like this is known as corona discharge.

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LIGHTNING CONDUCTOR

• Is a device made of metal with sharp points fixed
  on the top of huge buildings and earthed by thick
  strips of conductor.
• They protect the building in two ways.
• They avoid the occurrence of lightning by corona
  discharge and neutralizing the clouds.
• Even if lightning strikes, it provides a low
  resistance conducting path for the charges coming
  from the clouds and protects the building from
  damage.

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VAN DE GRAFF GENERATOR
Is a device used to produce very high potential
by the action of points. It works on the
principle that whenever a charge is given to a
hollow conductor, the charge is immediately
transferred to the outer surface.
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A Van de Graff Generator
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CAPACITANCE

• The ratio of electric charge to electric
  potential of a conductor or a device is called
  capacitance
• Capacitance C Q/V
• Unit is farad (F)
• 1 farad 1 coulomb / 1 volt

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PRINCIPLE OF A CAPACITOR

• Capacitor is based on the principle that the
  capacitance of an isolated charged conductor
  increases when an uncharged earthed conductor is
  kept near it and the capacitance is further
  increased by keeping a dielectric medium between
  the conductors.

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CAPACITANCE OF A PARALLEL PLATE CAPACITOR

• Electric field between the plates,
• E ?/?0
• But ?Q/A
• ?EQ/A?0
• Potential difference between the two plates , V
  Ed Qd/A ?0
• Capacitance, C Q/V
• CA ?0/d

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CAPACITANCE OF A PARALLEL PLATE CAPACITOR WITH A
DIELECTRIC SLAB

• When a dielectric slab is kept between the plates
  COMPLETELY filling the gap
• E E0/K where K is the dielectric constant of
  the medium.
• Potential difference
• V Ed E0d/KQd/K ?0A
• Capacitance C Q/V K ?0A/d KC
• ?when a dielectric medium is filled between the
  plates of a capacitor, its capacitance is
  increased K times.

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CAPACITANCE OF A PARALLEL PLATE CAPACITOR WITH A
DIELECTRIC SLAB

• When a dielectric slab is kept between the plates
  PARTIALLY filling the gap

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CAPACITANCE OF A PARALLEL PLATE CAPACITOR WITH A
METAL SLAB OF THICKNESS t
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COMBINATION OF CAPACITORS

• SERIES COMBINATION
• When capacitors are combined in series, the
  reciprocal of effective capacitance
• PARALLEL COMBINATION
• When capacitors are combined in series, the
  effective capacitance

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DEFINE DIELECTRIC CONSTANT ON THE BASIS OF
CAPACITANCE OF A PARALLEL PLATE CAPACITOR

• Dielectric constant of a medium is defined as the
  ratio of the capacitance of a capacitor
  completely filled with the medium to the
  capacitance of the capacitor without any
  dielectric.

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DIELECTRIC STRENGTH

• Dielectric strength of a dielectric is the
  maximum electric field that can be applied to it
  beyond which it breaks down.

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PRACTICE PROBLEMS

• Calculate the number of electrons in excess in a
  body with 1 coulomb of negative charge.
• Q ne
• Q 1C
• e 1.6 X 10-19C
• n Q/e 1/(1.6 X 10-19C) 6.25 X 1018