 # THERE ARE TWO INFINITE LONG PARALLEL RODS .CURRENT I THROUGH 1ST ROD FLOWS FROM TOP TO BOTTOM  AND SAME CURRENT I IN THE OTHER ROD FROM BOTTOM TO TOP.A POINT P LIES AT A DISTANCE r FROM SECOND ROD.IF THE SEPARATION BETWWEN THE RODS IS r metre,FIND MAGNETIC FIELD AT Q?Now i hav got the answer to the problem by subtracting the two magnetic fields due to rod 1 and 2.But still i m confused why did we take the minus sign ,i mean why do we need to subtract in this case and not simply add the two like we do if the pint would have been in betwwen the rods...Please try to explain me in little simpler language. Puneet Mittal AskiitiansExpert-IITD
22 Points
13 years ago

Dear Alina,

The Magnetic field because of each rod is vector quantity.

For a rod carrying current. The magnetic field points in upwards direction for all points to its left and downwards for all points to its right, according to right hand thumb rule. The left and right direction are directions with respect to the motion of current.

Here it is top to bottom direciton for the current for rod one so left direction is where point P is lying. Hence, For the rod one ( where the current is flowing from top to bottom direction )The magnetic field is upwards at point P.

Here it is bottom to top direciton for the current for rod two so right direction is where point P is lying. Hence,For the rod two ( where the current is flowing from bottom to top direction )The magnetic field is downwards at point P.

Since magnetic fields are in opposite directions, you are subtracting the two and getting the right result.

Now, If the point lies between the two rods then, for the first rod ( top to bottom current ) it is left direction and so magnetic field is upwards and for the second rod ( bottom to top current ) it is left direction too and so the magnetic field is upwards again. Since magnetic field is in same direction u will add them up to get the result.

I hope I am clear to you.

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