ELECTRIC PHENOMENA

1
ELECTRIC PHENOMENA

• ELECTRIC CHARGE
• is a fundamental quantity is a property of
  matter which causes a force to act between
  particles (objects, bodies..) which have this
  property
• experimental evidence ? two types of charge
  (positive, negative)
• charge carried by constituents of matter
  electrons and protons
• Fundamental carriers of electric charge
• matter made up of atoms
• atoms made up of electrons and nuclei electrons
  have negative charge
• nuclei made up of protons and neutrons
• protons have positive charge whose magnitude is
  the same as that of the electrons
• neutrons have zero net charge
  
• normal matter
• is electrically neutral (has equal number of
  protons and electrons)
• electrons can be removed from one material,
  transferred to another
• net excess of negative charge where electrons
  added,
• net excess of positive charge where electrons
  removed.

2
Electric forces

• COULOMB'S LAW F k ? (q1? q2)/r2
• describes theelectric force(also electrostatic
  force,Coulomb force) between two bodies with
  charges q1 and q2 , where r is the distance
  between the two bodies k is a constant, ,
• unit of charge is 1 Coulomb, abbreviated 1 C
  1 C of charge corresponds to the charge of ?
  6.3 x 1018 protons
• the charge of an electron is ? 1.6 x 10-19
  Coulombs
• the force is a vector, its line of action is the
  straight line connecting the two charges
• Newton's third law ? the force exerted by charge
  q1 on charge q2 is equal and opposite in
  direction to that exerted by charge q2 on charge
  q1.
• for like charges, i.e. if the two charges q1
  and q1 have the same sign (both positive or both
  negative), the direction of the forces is away
  from the other charge (repulsive)
• for unlike charges (one positive, one
  negative), the direction of the forces is towards
  the other charge (attractive)
• strictly speaking, Coulomb's law as formulated
  above holds only for point charges, but is
  approximately also correct for charged bodies
  whose size is small compared to the distance r
  between them

3
Electric forces,contd

• superposition principle
• the force between any two charges is independent
  of the presence of all other charges.
• thus, the net force on any charge is the vector
  sum of all the forces due to each of the other
  charges interacting with it independently.
• electrostatic vs gravitational force
• Coulomb's law has same form as Newton's law of
  gravitation (mass ? charge, k ? G ),
  but electric forces much stronger.
• Example Forces in hydrogen atom the hydrogen
  atom has one electron in orbit around one proton,
  at a distance ra ? 10-10 m
• gravitational force between electron and proton
  Fg G me mp /ra2 ? 1.0 x 10 -47 N
  (remember G ? 6.7 x 10 -11 Nm /kg )
• electrostatic force between electron and
  proton Fc k qe qp /ra2 ? 2.6 x 10 -8
  N (k ? 9.0 x 10 Nm /C ) i.e. Fc
  /Fg ? 10 40
• therefore the gravitational force can be
  neglected at the atomic level
  

4
ELECTRIC FIELD

• field of force exists in a region of space
  when an appropriate object (called the test
  object or probe) placed at any point in the
  region experiences a force.
• force depends on a property of the test object
  (e.g. charge,..), the test charge
• field strength (force experienced by test
  object) divided by (test charge), force per
  unit test charge
• for electrostatic force, this field strength is
  called electrostatic field or electric field
• field can be visualized by lines of force or
  field lines, which give the direction of the
  field at every point, i.e. the force experienced
  by a test-charge at any point in space is in the
  direction tangent to the line of force at that
  point
• the density (concentration) of field lines
  corresponds to the magnitude of thefield
  strength the denser the concentration of lines,
  the stronger the field the farther apart the
  lines, the weaker the field
• electrostatic field lines begin on positive and
  end on negative charges
• field lines do not cross
• originally, field lines were invented (by
  Faraday) as means of visualization, but
  eventually were regarded as standing for an
  invisible physical reality - the electric field
• In modern view, all forces (interactions) are
  due to fields, described by gauge field
  theories.

5
Electric field of a point charge

• Coulomb's law gives force between two point
  charges formulation in terms of electric
  field the presence of a point-like particle of
  charge q causes a change in the space around
  it it generates an electric field which
  permeates all of space.
• When a test charge qtest qo is brought into the
  field, the field exerts a force on the
  test-charge, given by
• For a positive test-charge, the force points in
  the same direction as the field.
• Comparing with Coulomb's law, we find the
  electric field of a point-charge q is
  ( is a unit-vector (i.e. a
  vector of length 1), pointing from the charge q
  to the point at which the field is evaluated).
• Extended charged object
• Field due to extended distribution of charges
  sum of the fields due the individual point
  charges.

6
Electricity -- history

• Historical notes
• Greeks discovered about 600BC that amber, when
  rubbed with wool, attracts other objects
• Electric phenomena
• named after electron, Greek word for amber
• studied by many through ages
• real progress in understanding only gained in
  18th century
• Charles Dufay (1745) there are two types of
  electricity
• Charles Augustin de Coulomb (1736-1806)
• like charges repel, unlike charges attract each
  other
• discovered Coulomb's Law'', using torsion
  balance invented by him.
• André Marie Ampère (1775-1836) (Prof. Physics at
  École Polytechnique, Paris)
• La théorie des phénomènes électrodynamiques''
  (1826)
• attraction and repulsion of electric currents,
• direction of magnetic field of a current,
• explanation of magnetism as due to molecular
  currents.
• Benjamin Franklin (1706-1790) (US politician,
  diplomat, scientist, writer,printer)
• lightning as electrical phenomenon
• lightning rod
• coined name positive and negative for the two
  kinds of electric charge

7
History of electricity, contd

• Luigi Galvani (1737-1798) (Prof. of Anatomy at U.
  of Bologna)
• De viribus electricitatis in motu musculari
  commentarius (1791)
• electric phenomena in muscular motion
  (experiments with froglegs)
• Alessandro Volta (1745-1827)
• electrophorus (1775)
• straw electroscope (1781)
• condensator (1782)
• relation between chemical reactions and
  electricity (1796)
• Voltaic cell (battery) (1800)
• Michael Faraday (1791-1867) (bookbinder's
  apprentice, self-taught chemist and physicist,
  prof. of physics and chemistry)
• Experimental researches in electricity
  (1844-1845)
• Experimental researches in chemistry and
  physics (1859)
• concept of electric field, field lines (lines
  of force)
• induction (1831)
• basic laws of electrochemistry (1833-1834)
• investigations of dielectrics
• studies of gas discharges
• diamagnetism
• magnetic rotation of plane of polarization of
  light (1845)