Question

A wire is lying horizontally in the north-south direction and there is a horizontal magnetic field pointing towards the east. Some positive charges in the wire move north and an equal number of negative charges move south. The direction of force on the wire will be :

• A. East.
• B. Down, into the page.
• C. Up, out of the page.
• D. West

Answer 1

The correct option is B Down, into the page.

From figure, we get $\overrightarrow{I}=I\hat{y}$

Also $\overrightarrow{B}=B\hat{x}$

Magnetic force acting on the wire $\overrightarrow{F}=l(\overrightarrow{I}\times \overrightarrow{B})$

$\therefore$ $\overrightarrow{F}=lIB(\hat{y}\times \hat{x})$

$⟹$ $\overrightarrow{F}=-IlB\hat{z}$

Hence the force acts on the wire in the negative $z$ direction i.e. downwards, into the page.