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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) ;
  • 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.
  • COUL
  • Electric forces
  • COULOMB'S LAW: F = k  (q1q2)/r2
  • describes the”electric force”(also “electrostatic force”,”Coulomb force”) between two bodies with charges q1andq2 , 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
  • Electric forces,cont’d
  • 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
  • “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”.
  • 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.
  • 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
  • History of electricity, cont’d
  • 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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