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ELECTRIC PHENOMENA

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Title: 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)
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