Page 9 - 12-phy-13 Current Electricity

13. Current Electricity eLearn.Punjab

Symbolically Ohm’s law can be written as
V ∝ I

It implies that =V RI .....(13.2)

where R, the constant of proportionality is called the
resistance of the conductor. The value of the resistance

depends upon the nature, dimensions and the physical
state of the conductor. In fact the resistance is a measure

of the opposition to the motion of electrons due to their
continuous bumping with the atoms of the lattice.

The unit of resistance is ohm. A conductor has a resistance
of 1 ohm if a current of 1 ampere flows through it when a
Fig. 13.7: Current - Voltage graph of an ohmic
potential difference of 1 volt is applied across its ends. The material.
symbol of ohm is W.

If I is measured in amperes V in volts, then R is measured in
ohms i.e.

V (volts)
R (ohms) = .....(13.3)
I (amperes)

A sample of a conductor is said to obey Ohm’s law if its
resistance R remains constant that is, the graph of its V

versus I is exactly a straight line (Fig. 13.7). A conductor
which strictly obeys Ohm’s law is called ohmic. However,

there are devices, which do not obey Ohms’ law i.e. they are
non ohmic. The examples of non ohmic devices are filament

Fig. 13.8 bulbs and semiconductor diodes.
Let us apply a certain potential difference across the

terminals of a filament lamp and measure the resulting current passing through it. If we repeat
the measurement for different values of potential difference and draw a graph of voltage V versus

current I, it will be seen that the graph is not a straight line (Fig. 13.8). It means that a filament is
a non ohmic device. This deviation of I - V graph from straight line is due to the increase in the

resistance of the filament with temperature - a topic which is discussed in the next section. As the
current passing through the filament is increased from zero, the graph is a straight line in the initial

stage because the change in the resistance of the filament with temperature due to small current
is not appreciable. As the current is further increased, the resistance of the filament continues to

increase due to rise in its temperature.

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