Moving Charges and Magnetism - Result Question 69

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

[2005]

(a) along $O Y$

(b) opposite to $O Y$

(c) along $O X$

(d) opposite to $O X$

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Answer:

Correct Answer: 70. (a)

Solution:

  1. (a) The direction of $\vec{B}$ is along $(-\hat{k})$

$\therefore$ The magnetic force

$\vec{F}=Q(\vec{v} \times \vec{B})=Q(v \hat{i}) \times B(-\hat{k})=Q v B \hat{j}$

$\Rightarrow \vec{F}$ is along $O Y$.