Current Electricity (Part=2) Ohm's Law and Resistance

    Ohm's Law

     is there any relation between current and potential difference? Let us perform an experiment for this.


    Activity

                Connect one meter long nichrome wire to a variable power supply as shown in Fig.(8.6). Also connect an ammeter in series with the circuit. Connect a voltmeter parallel to the nichrome wire. Increase the voltage step by from the power supply. Keep on noting the       

Fig. 8.6



reading V of voltmeter and the reading I of ammeter. You will observe that the value V-I remains constant i.e.,V is proportional to I. The relation between potential difference and current, first discovered  by George Simon Ohm in 1826is that

 






The current flowing through a conductor is directly proportional to the potential difference provided that the temperature and the physical state of the conductor does not change.

          This is called Ohm's law. Mathematically, we can write it as:


                                           V    OC     I

                                           V     =       RI                    ..........................        (8.2)

Where R is a constant called the resistance of the conductor. From Ohm's law, we can determine the value of any unknown quantity buy knowing the other two quantities.


Resistance 

The opposition to the flow of charges is called resistance. Resistance is equal to the ratio of potential difference and current . From equation 8.2






                                                         R   =   V-I




In a circuit diagram, resistance is shown by a zig-zag line . (8.7). The SI unit of resistance is ohm.


If one volt potential difference applied across the ends of a conductor gives rise to a current of one ampere, the resistance of the  conductor is one ohm.


The cause of resistance is that when a potential difference is provided across the ends of a conductor the free electrons in the conductor start moving from negative to positive end. In their way the free electrons collide with the atoms of the conductor due to which hindrance is produced in their motion. Because every conductor contains atoms in it, therefore every conductor does have resistance how so ever small it nay be.

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