An electron enters with a velocity `vecv=v_0hati` into a cubical region (faces parallel to coordinate planes) in which there are uniform electric and magnetic fields. The orbit of the electron is found to spiral down inside the cube in plane parallel to the x-y plane. Suggest a configuration of fields `vecE` and `vecB` that can lead to it.

Options

A. `vecE=E_(0)hatj, vecB=B_(0)hatk`
B. `vecE=E_(0)hati, vecB=B_(0)hatk`
C. `vecE=E_(0)hati, vecB=B_(0)hatj`
D. `vecE=E_(0)hatj, vecB=B_(0)hatj`

Answer

B

Explanation

The orbit of the electron spirals down inside the cube in plane parallel to x-y plane. Therefore, `vecB` must be along `+z` direction and `vecE` should be along x-axis.
i.e. `vecB=B_(0)hatk` and `vecE=E_(0)hati` where `E_(0) gt0`