Concept of E.M.F. and Potential Difference
There is a distinct between
e.m.f. and potential difference. The e.m.f. of a device, says a battery, is measure
of energy the battery gives to each coulomb of charge. Thus if a battery
supplies 4 joules of energy per coulomb, we say that it has an e.m.f. of 4volt.
This energy given to each coulomb in a battery is due to the chemical action.
The potential difference between to points,
say A and B, is a measure of the energy use by one coulomb in moving from A to
B. Thus if the potential difference between points A and B is 2 volts, it means
that each coulomb will give up an energy of 2 joules in moving from A to B.
Ohm’s Law:
The relationship between
voltage across and current through a conductor was first discovered by German
scientist George Simons Ohm. This relationship called Ohm’s law and may be
stated as:
The current (I) flowing through
conductor is directly proportional to the potential difference (V) across its
ends provided physical conditions (Temperature, Strain, etc.) do not change
i.e.,
I ∞ v
Or V/I = R = (Constant)
Where R is constant of proportionality
and is called resistance of the conductor. For example, If in figure (1) the
potential difference between point A and B of the conductor is V and the
current flowing is I, then V/I will be constant and equal to R, resistance of
the conductor between points A and B. If V is double up , current will also be
double up so that ratio V/I is constant.
If a *graph is drawn between applied
potential difference (V) and current (I) flowing through the conductor, it will
be straight line passing through the origin as shown in figure (2). Note slop
of the graph gives the resistance of the conductor (tanϴ = V/I = R).
Resistance:
The resistance
of a conductor is define as the ratio of potential difference applied across
its ends to the resulting current through the conductor i.e.,
R =V/I Resistance is an opposition
to the flow of current. If the resistance of the circuit is doubled, the
current is reduced to one half. If the resistance tripled, the current is
reduced to one third and so on. The SI unit of potential difference is V or 1V
and that of current is 1A. Therefore SI Unit of resistance is V/I which has
been given a special name ohm (symbol Ω).
1 ohm = 1Ω = 1 V/A
A conductor is said to have a
resistance of 1 ohm if a potential difference of 1V across its ends causes a current
of 1A to flow through it.
Calculating Resistance:
R = ρ L/A
Where ρ (Greek latter ‘Rho’) is called
Resistivity or Specific Resistance of the material, Its value depend upon the
nature of the material and temperature.
Resistivity or Specific Resistance:
R = ρ L/A
If L = 1m; A = 1m*m,
then R= ρ
Hence, specific resistance (or
resistivity) of a material is the resistance offer by 1m length of a wire
material having area of cross-section of 1m*m .
Conductance:
The reciprocal of
resistance of a conductor is called its conductance (G).
If a conductor has a resistance R, then the
conductance G is given by:
G = 1/R
SI Unit conductance is Siemens.
Conductivity:
The reciprocal of
resistivity of a conductor is called its conductivity.
Its denoted by
the symbol σ. If a conductor has resistivity ρ, then the conductivity is given
by:
σ =1/ρ
SI Unit of conductivity is Siemens per meter (S).
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