RESISTANCE OF A MATERIAL
In Figure 4.4, a
Nichrome wire was connected between X and Y. If you replace the Nichrome wire
with a copper wire and conduct the same experiment, you will notice a different
current for the same value of the potential difference across the wire. If you
again replace the copper wire with an aluminium wire, you will get another
value for the current passing through it. From equation (4.3), you have learnt
that V/I must be equal to the resistance of the conductor used. The variations
in the current for the same values of potential difference indicate that the
resistance of different materials is different. Now, the primary question is,
“what is resistance?”
Resistance of a material
is its property to oppose the flow of charges and hence the passage of current
through it. It is different for different materials.
From Ohm’s Law, V / I =
R.
The resistance of a
conductor can be defined as the ratio between the potential difference across
the ends of the conductor and the current flowing through it.
The SI unit of
resistance is ohm and it is represented by the symbol Ω.
Resistance of a
conductor is said to be one ohm if a current of one ampere flows through it
when a potential difference of one volt is maintained across its ends.
1 ohm = 1 volt / 1 ampere
Calculate the resistance
of a conductor through which a current of 2 A passes, when the potential
difference between its ends is 30 V.
Current through the
conductor I = 2 A,
Potential Difference V =
30 V
From Ohm’s Law: R = V/I.
Therefore, R = 30/2 = 15
Ω
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