Question

A small bar magnet is placed lengthwise with its north pole pointing east along the eastern arm with its centre 25 cm away from the centre of the needle of the magnetometer. The needle is then found to be deflected through an angle of 30${}^0$. If the arms of the magnetometer are rotated anti-clockwise through 90${}^0$, the deflection of the needle would be :

• A. 30${}^0$
• B. 60${}^0$
• C. 45${}^0$
• D. 0${}^0$

Answer 1

The correct option is D 0${}^0$

$B=B_{H}tan{30}^o$
If the arms of the magnetometer are rotated through ${90}^o$ , both B and $B_H$ are along the same line , hence the needle would not be deflected .