Ohm's law and resistance


 To determine the resistance per cm of a given wire by plotting a graph of potential difference versus current.

The Theory:

What does Ohm's Law state?

According to the Ohm's law, "The current flowing through a conductor is directly proportional to the potential difference across its ends provided the physical conditions (temperature, dimensions, pressure) of the conductor remains the same." If I be the current flowing through a conductor and V be the potential difference across its ends, then according to Ohm's Law,

I ∝ V

V ∝ I or V = RI

where, R is the constant of proportionality. It is known as resistance of the conductor. Or

                                                                             V/I = R  

                                                                                                        ..the working formula

R depends upon the material, temperature and dimensions of the conductor.

In S.I. units, the potential difference V is measured in volt and the current I in ampere, the resistance R is measured in ohm.

How do we establish the current-voltage relationship?

To establish the current-voltage relationship, it is to be shown that the ratio V / I remains constant for a given resistance, therefore a graph between the potential difference(V) and the current (I) must be a straight line.

How then do we find the unknown values of resistance?

It is the constant ratio that gives the unknown values of resistance, 

                                                                       V/I = R

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