Question

A very long straight wire carries a current $l$. At the instant when a charge $+Q$ at point $P$ has velocity $\vec{v}$, as shown, the force on the charge is

• A. along Oy
• B. opposite to Oy
• C. along Ox
• D. opposite to Ox

Answer 1

Answer: (A)

According to Fleming's left hand rule direction of force is along Oy axis which is perpendicular to wire.

$\vec{F}=e(\vec{v}\times \vec{B})$.
$B$ due to i is acting inwards i.e. into the paper. $v$ is along $Ox.$
$\therefore F=Q^{+}\left|\begin{array}{ccc}<br/>\hat{i}& \hat{j}& \hat{k}\\ <br/>v& 0& 0\\ <br/>0& 0& -B\end{array}\right|$

$\Rightarrow F=Q^{+}[-\hat{j}(-vB)+0]$
$\therefore \vec{F}=+QvB\hat{j}$.
i.e in Oy direction.