PEE Notes

 Explain the direction of magnetic field in a straight current carrying conductors .

OR

     Magnetising force(MMF) on a long straight conductors .
Ans.

          

        In figure-3 a current carrying straight conductor (AB) is assumed to be extended to infinity in each direction .

Let it carry a current of I amperes upwards. Then the conductor will set up magnetic lines of force which encircle it, having their plane perpendicular to the conductor & their centre at the centre of the conductor.

     Suppose that the field strength at point C ( in the figure ) distance r meter from the centre of the conductor is H . Then it means that if a unit N pole is placed at C , It will experience a force of H newtons . The direction of this force would be tangential to the circular lines of force passing through point C. If a unit N pole is moved once round the circular path ( conductor ) against this force, the work done is…………………

                                                              W = Force × distance

                                                                   = H × 2πr

     According to work law this work done must be equal to the ampere-turns induced within the path.

     So ampere turns linked = NI                    

                                                = 1×I

                                                = I

So……..

                    H × 2πr = I

                   H = I ∕2πr    AT∕M

If there are N conductors, then

                                                    H × 2πr = NI

                                                    H = I ∕2πr    AT∕M  

         And……..

                     B = µ.NI∕2πr     Wb∕m ² ..................... in air

                     B = µ.µrNI∕2πr     Wb∕m ²  …………………….. in a medium