Question

The monoenergetic beam of electrons moving along + y direction enters a region of uniform electric and magnetic fields. If the beam goes straight undeflected, then fields B and E are directed respectively along

• A. -y axis and - z axis
• B. + z axis and + x axis
• C. + x axis and + z axis
• D. - x axis and - y axis

Answer 1

The correct option is C + x axis and + z axis

The total Lorentz force on the electron is
F = - e (E + v $\times$ B)
For the electron to be undeflected, $E=-v\times B$
When the options are checked, only option (C) satisfies this condition.