Q135 / 200 · NEET 2005 · Physics

A very long straight wire carries a current I. At the instant when a charge $+Q$ at point $\mathrm{P}$ has velocity $\bar{v}$ as shown the force on the charge is: Figure

  • Correct answer
    A
    along y
  • B
    opposite $y$
  • C
    along $x$
  • D
    Opposite $x$

Solution

According to Fleming's left hand rule direction of force is along $y$ axis when is perpendicular to wire: $$ \vec{F}=Q(\vec{V} \times \vec{B}) $$ $\mathrm{B}$ due to $\mathrm{I}$ is acting inwards is into the paper, $v$ is along or: $$ \begin{array}{ll} \therefore & F=a+\left|\begin{array}{ccc} \hat{i} & \hat{h} & \hat{k} \\ v & 0 & 0 \\ 0 & 0 & -\mathrm{B} \end{array}\right| \\ \Rightarrow & F=Q+[-j(-v B)+0] \\ \therefore & F=Q v B \hat{j} \end{array} $$ in $y$ direction.

Hence, the correct answer is option: 'A'.

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