A magnetic compass needle is placed in the plane of paper near point A as shown in the Figure. In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?

Hint:Right hand thumb rule explains the relation between the direction of current and magnetic field. According to right hand thumb rule, the direction of current is perpendicular to the direction of magnetic field. Use the relation and see when the needle will not show any deflection.


Complete answer:
According to right hand thumb rule, the direction of current is perpendicular to the direction of magnetic field. We will use this rule to calculate the deflection in the magnetic needle.So, if we place the current carrying conductor in the plane of paper where the needle is placed then the magnetic field and the current will satisfy the right hand thumb rule. And hence there will be no deflection in the needle.

Observe the diagram:

Therefore, from the above explanation, we should place the current carrying conductor in the same plane as of the magnetic field for the needle to show no deflection. Clearly, the deflection in the needle will be the maximum when the current carrying conductor is closest to the magnetic compass.

Note: Right hand thumb rule states that if we roll our fingers around the current carry wire in such a way that the direction of the fingers gives the direction of the magnetic field. Then the stretched thumb will give the direction of the current in the conductor. We cannot use the right-hand thumb rule to state the direction of the magnetic field and it is solely used to state the direction of either the current or the magnetic field.