The acceleration of an electron at a moment in a magentic field $\vec B\, = \,2\hat i + 3\hat j + 4\hat k$ is $\vec a\, = \,x\hat i - 2\hat j + \hat k$. The value of $x$ is
A$0.5$
B$1$
C$2.5$
D$1.5$
Medium
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B$1$
b $ \overrightarrow{\mathrm{B}} \cdot \overrightarrow{\mathrm{a}} =0 $
$ 2 \mathrm{x}-6+4 =0 $
$ 2 \mathrm{x}=2 $
$x=1$
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