A simple electric circuit
The battery rovides the
energy (through chemical
reactions)
The lamp consumes
energy not electrons.
Weight/clock example.
For many useful simulations on electric
circuit and not only GOTO:
http://phet.colorado.edu
Click on ‘Simulations’
Units of Electric Potential and Electric Currents
•  Electric Potential Units: VOLTS
–  your typical AA battery has 1.5 Volts potential
•  Electric Current Unit: AMPERE (=1 Coulomb/sec)
–  Ampere is the fundamental unit NOT charge
–  One amp is a lot of current. It will kill you if it goes through
your body
–  Every time you start your car the battery provides (for very
short time) a current of several hundred amps
A simple electric circuit
The battery rovides the
energy (through chemical
reactions)
The lamp consumes
energy not electrons.
Weight/clock example.
Resistance (R)
•  Charge doesn’t flow ‘ideally’, i.e. w/out
resistance (‘friction’ analog).
•  Resistors take energy away from the
charges and convert it to heat.
•  AGAIN: charges are not destroyed or
‘used up’ etc. only energy.
Resistance (R)
•  The resistance is defined as the ratio of the
applied potential at the ends of the resistor,
to the current flowing through the resistor
V
R=
I
Where:
R is the resistance of the object (in Ohms)
V the potential difference across the object, measured in Volts
I is the current passing through the object, measured in Amperes
Ohm's Law
•
For many conductors of electricity, the electric current which will flow through them is
directly proportional to the voltage applied to them. When a
microscopic view of Ohm's law is taken, it is found to depend upon the fact that the drift
velocity of charges through the material is proportional to the electric field in the
conductor. The ratio of voltage to current is called the resistance. Ohm’s law expresses
electric current I which will flow through a
resistor is directly proportional to the voltage V applied to it and
inversely proportional to its resistance R”
just the fact that “The
OHM’s Law and its derivatives
V
R=
I
V = I ⋅R
Examples
•  Simple circuit involving two resistors
(lamps)
– Resistors in series
– Resistors in parallel
Resistors in SERIES
The same current, I, flows through each resistor in SERIES
The total resistance is the SUM of the resistances
Resistors in PARALLEL
R2
Each resistor ‘sees’ the same potential
The total resistance is the sum of the INVERSES and it is always less
than the smallest one
Example: Two 10 Ohm resistors in series are equal to 20 Ohm
and in parallel equal to 5 Ohm.
Examples
•  Simple circuit involving two resistors
–  Resistors in series
–  Resistors in parallel
•  Simple circuits involving lamps and
switches.
Applications of Resistors
•  Electronic circuits with vast range of
applications
•  Non-fluorescent bulbs, Ovens, Heating
elements
•  Resistors sensitive to Temperature
–  Thermistors/thermometers
Superconductors
•  Superconductivity is a phenomenon of exactly zero
electrical resistance ( and expulsion of magnetic
fields) occurring in certain materials when cooled
below a characteristic critical temperature
•  It was discovered by Heike Kamerlingh Onnes on
April 8, 1911
•  It is a quantum mechanical phenomenon
•  It occurs abruptly at the critical temperature
•  Recently ‘high temperature superconductors’ show
promise for practical applications
Magnetism
•  made of “poles” (instead of “charge”)
•  also come in two flavors (“north” and “south”)
–  two flavors always come in equal amounts
•  made of “poles” (instead
of ‘charge’)
•  also come in two flavors
(“north” and “south”)
–  two flavors always come
in equal amounts
A similar law to Coulomb law exists for magnetic forces (but only
quantitatively)
s
Magnetic Field lines
Earth is a Magnet!!!!
In 1820 Hans Oersted found that a magnet is
deflected by an electric wire!
The unification of the
electric and the magnetic
force had begun
electric fields can generate magnetic fields
AND
magnetic fields can generate electric fields
•  Electromagnet
•  dynamo (generator)
•  so, which came first?
J.C. Maxwell (1875)
•  unified theory of electricity and magnetism
•  proved that they were different
“manifestations” of electromagnetic field
•  proved that E-M field could travel as a wave
•  showed that light is just one type of
Electromagnetic wave
THE NATURE OF LIGHT
We just answered the question of what is light?
•  James Clerk Maxwell (1875)
–  light is an electromagnetic wave
• but what is a “wave”?