Transcript ELECTRIC PHENOMENA

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;
particle
electron
proton
neutron

symbol
e, ep, p+
n
Mass (kg)
9.11x10-31
1.67x10-27
1.67x10-27
Mass (MeV)
0.511
938.3
939.6
Charge (C)
-1.6x10-19
+1.6x10-19
0
“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.
Electric forces

COULOMB'S LAW:
F = k  (q1 q2)/r2








describes the”electric 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
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
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”.
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)