Activity . Take a nichrome wire, a torch bulb, a W bulb and an ammeter ( – A range), a plug key and some connecting wires. Set up the circuit by connecting four dry cells of . V each in series with the ammeter leaving a gap XY in the circuit, as shown in Fig.
. . 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.
. . Thus, V/I is a constant ratio. In , a German physicist Georg Simon Ohm ( – ) 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 ( . ) or V/I constant or V IR ( .
) In Eq. ( . ), 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 ( . ) If the potential difference across the two ends of a conductor is V and the current through it is A, then the resistance R, of the conductor is Ω . That is, ohm = volt ampere Also from Eq.
( . ) we get I = V/R ( . ) It is obvious from Eq. ( .
) 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.