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Set up a circuit as in Fig., consisting of a nichrome wire XY of length, say 0.5 m, an
ammeter, a voltmeter and four cells of 1.5 V each. (Nichrome is an alloy of nickel,
chromium, manganese, and iron metals).
First use only one cell as the source in the circuit. Note the reading in the ammeter I, for the current and the reading of the voltmeter V for the potential difference across the nichrome wire XY in the circuit.
Next connect two cells in the circuit and note the respective readings of the ammeter and voltmeter for the values of current through the nichrome wire and potential difference across the nichrome wire.
Repeat the above steps using three cells and then four cells in the circuit separately.
Calculate the ratio of V to I for each pair of potential difference V and current I.
In this activity, you will find that approximately the same value of V/I is obtained in each case. Thus the V-I graph is a straight line that passes through the origin of the graph Thus V/I is a constant ratio.
The electric current flowing through a metallic wire is directly proportional to the potential difference V, across its ends provided its temperature remains the same. This is called Ohm's law. In other wordsV I ? ….(i)
Or V I / constant ?
= ROr V = IR …(ii)
In equation (i) 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 ? . According to Ohm's law.
R = V/I …(iii)
If the potential difference across the two ends of a conductor is 1 V and the current
through it is 1A, then the resistance R , of the conductor is1? .
That is, 1 ohm = 1volt
Real Eyes Realize Real Lies.