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Definitions:
Current: The rate of flow of charge .
I= current in Amperes
Q= charge in Coulombst= time in seconds
Voltage: is the potential difference between two points. Voltage
is equal
to the work which would have to be done, per unit charge,
against a static
electric field to move the charge between two points.
V= voltage in voltsW= work in Joules
Q= charge in Coulombs
Resistance: Is the obstruction to the flow of charge due to
collisions withthe vibrating atomic ions.
EMF: The terminal voltage providing no current is taken from the
cell. (i.e.
it is an open circuit)
Electriccharge: is a physical property of matter that causes it
toexperience a force when near other electrically charged
matter.
Internalresistance:A practical electrical power source which is
a linear
electric circuit may be represented as an ideal voltage source
in series with an
impedance. This resistance is termed the internalresistance of
the source.
When the power source delivers current, the measured e.m.f.
(voltage output) islower than the no-load voltage; the difference
is the voltage drop caused by the
internal resistance.
Volt: 1 volt is the potential difference between 2 points such
that the
energy converted from electrical to other forms is 1 J/C
Also =as the difference in electric potentialacross a wire when
an electric
current of one ampere dissipates one watt of power.
Ohm: Is the resistance of a conductor through which a current of
1Ampere is flowing when the voltage across it is 1 volt.
Ampere: the constant current that will produce an attractive
force of
2 107 newton per metre of length between two straight, parallel
conductors of
infinite length and negligible circular cross section placed one
metre apart in avacuum.
Also= In practical terms, the ampere is a measure of the amount
of
electric charge passing a point in an electric circuit per unit
time with
6.241 1018 electrons, or one coulomb per second constituting one
ampere.
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StateOhmslaw:
In a conductor, providing no change in the physical conditions
of it (e.g.
Temperature and cross sectional area) the current flowing
through the
conductor is directly proportional to the voltage across it.
Difference betweenohmnicandnon-ohmnicconductors
In an Ohmic conductor the current flowing is proportional to the
voltage applied.
In a non-Ohmic conductor there is no linear relationship.
Examples of Ohmic conductors are metals and carbon (as long as
no significant
heating occurs).
Examples of non-Ohmic conductors are semiconductors, thermistors
andfilament light-bulbs.
In both cases when plotting a V-I graph, the slope of the line
(or curve) is the
resistance of the conductor.
Resistivity
is a measure of how strongly a material opposes the flow of
electric current. A
low resistivity indicates a material that readily allows the
movement of electriccharge. The SI unit of electrical resistivity
is the ohmmetre (m).
= Resistivity in ohms per metre
R= Resistance of the conductor (ohms)
A= Cross sectional area of the conductor (m2)
L= Lenght of the conductor (m)
Temperaturecoefficientofresistance
= Temperature coefficient of resistance = Resistance at
(temperature2)
= Resistance at (temperature 1)
= Resistance at 0
