In a certain region of space electric field →Eand magnetic field →B are perpendicular to each other. An electron enters in the region perpendicular to the direction of →B and →E both. If it moves un-deflected, then velocity of the electron is:
A
∣∣→E∣∣∣∣→B∣∣
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B
→E×→B
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C
∣∣→B∣∣∣∣→E∣∣
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D
→E→B
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Solution
The correct option is B∣∣→E∣∣∣∣→B∣∣ For the electron to pass undeflected, electric force on electron will equal and in opposite direction to that of the magnetic force. ∣∣→Fe∣∣=∣∣→FB∣∣ ∴∣∣q→E∣∣=∣∣q(→v×→B∣∣ ⇒|→v|=∣∣→E∣∣∣∣→B∣∣