11.4 OHM’S LAW
Is there a relationship between the potential difference across a conductor
and the current through it?
In this Activity, you will find that approximately the
same value for V/I is obtained in each case. Thus the V–I
graph is a straight line that passes through the origin of
the graph, as shown in Fig. 11.3. Thus, V/I is a constant
ratio.
In 1827, a German physicist Georg Simon Ohm
(1787–1854) found out the relationship between the current
I, flowing in a metallic wire and the potential difference
across its terminals. The potential difference, V, across the
ends of a given metallic wire in an electric circuit is directly
proportional to the current flowing through it, provided its
temperature remains the same. This is called Ohm’s law. In
other words –
V ∝ I (11.4)
or V/I = constant
= R
or V = IR (11.5)
In Eq. (11.4), R is a constant for the given metallic wire
at a given temperature and is called its resistance. It is the
property of a conductor to resist the flow of charges through it. Its SI
unit is ohm, represented by the Greek letter Ω. According to Ohm’s law,
R = V/I (11.6)
If the potential difference across the two ends of a conductor is 1 V
and the current through it is 1 A, then the resistance R, of the conductor
is 1 Ω. That is, 1 ohm = 1 volt
1 ampere
Also from Eq. (11.5) we get
I = V/R (11.7)
It is obvious from Eq. (11.7) that the current through a resistor is
inversely proportional to its resistance. If the resistance is doubled the
current gets halved. In many practical cases it is necessary to increase
or decrease the current in an electric circuit. A component used to
regulate current without changing the voltage source is called variable
resistance. In an electric circuit, a device called rheostat is often used to
change the resistance in the circuit. We will now study about electrical
resistance of a conductor with the help of following Activity.
In this Activity we observe that the current is different for different
components. Why do they differ? Certain components offer an easy path
for the flow of electric current while the others resist the flow. We know
that motion of electrons in an electric circuit constitutes an electric
current. The electrons, however, are not completely free to move within a
conductor. They are restrained by the attraction of the atoms among
which they move. Thus, motion of electrons through a conductor is
retarded by its resistance. A component of a given size that offers a low
resistance is a good conductor. A conductor having some appreciable
resistance is called a resistor. A component of identical size that offers a
higher resistance is a poor conductor. An insulator of the same size offers
even higher resistance.
