Page 181 - NCERT Science Class 10 English Medium

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 –
Figure
Figure 11.311.3
Figure 11.3
Figure
Figure 11.3
11.3
V–I graph for a nichrome wire. A V ∝ I (11.4)
straight line plot shows that as the or V/I = constant
current through a wire increases, the = R
potential difference across the wire or V = IR (11.5)
increases linearly – this is Ohm’s law.
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
1 volt
is 1 Ω. That is, 1 ohm =
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.

Activity 11.211.2
Activity
Activity 11.2
Activity 11.2
Activity 11.2
n Take a nichrome wire, a torch bulb, a 10 W bulb and an ammeter (0 – 5 A range), a plug
key and some connecting wires.
n Set up the circuit by connecting four dry cells of 1.5 V each in series with the ammeter
leaving a gap XY in the circuit, as shown in Fig. 11.4.

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