An electric current runs counter clockwise in a rectangular loop around the outside edge of this page, which lies flat on your table. A uniform magnetic field is then turned on, directed parallel to the page from top to bottom, the magnetic force on the page will cause

Options

A. the bottom edge to lift
B. the top edge to lift
C. the left edge to lift
D. the right edge to lift

Answer

B

Explanation

The force on a linear conductor carrying current is `F=IlBsin theta`
where `theta` is the smaller angle between `Ivecl` and `vecB`. In figure, for arm PS and RQ, `theta=0^@` and `180^@`, and `sin theta=0`, so no force acts on them.
The force on arm `QP=IlB sin 90^@` acting perpendicular to the plane of paper upwards.
The force on arm `SR=IlBsin90^@` acting perpendicular to the plane of paper downwards.

Therefore, the top edge QP of loop will be lifted up.