Question

A charged particle enters into a space and continues to move undeflected then in that space.Which of the following is true for this case ?

• A. A uniform horizontal electric field and a vertical magnetic field may be present
• B. A vertical electric field alone may be present
• C. Uniform electric and magnetic fields, both directed vertically downwards, may be present
• D. A uniform horizontal magnetic field alone may be present

Answer 1

The correct option is D A uniform horizontal magnetic field alone may be present

The force on a charged particle by an electric and magnetic field is
$F=q(\overrightarrow{E}+\overrightarrow{v}\times \overrightarrow{B})$
One can see that the force exerted by the electric field is independent of direction but the force by the magnetic field can vary with the relative orientation of v and B. So if a uniform horizontal magnetic field alone may be present then only the $(\overrightarrow{v}\times \overrightarrow{B})$ term will be zero and the particle will move unreflected.