A) \[16\times {{10}^{8}}m/s\]
B) \[8.0\times {{10}^{6}}m/s\]
C) \[4.0\times {{10}^{6}}m/s\]
D) none of these
Correct Answer: B
Solution :
Key Idea: For no deflection magnetic force should equal electric force. The magnetic force acting on the electron beam is \[F=qvB\sin \theta \] ... (i) where \[q=e\] and \[\sin {{90}^{o}}=1\] The electric force acting on the electron is \[F=eE\] ... (ii) For no deflection. Electric force = magnetic force \[eE=evB\] \[\Rightarrow \] \[v=\frac{E}{B}=\frac{400}{0.0005}=8\times {{10}^{6}}m/s\].
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