An ionised gas contains both positive and negative ions initially at rest. If it is subjected simultaneously to an electric field along the +x-direction and a magnetic field along the +z-direction, then

Options

A. Positive ions deflect towards +y - direction and negative ions towards -y - direction
B. All ions deflect towards +y - direction
C. All ions deflect towards -y - direction
D. Positive ions deflect toward -y - direction and negative ions towards -y - direction

Answer

C

Explanation

We can write `E=E.hati` and `B=Bhatk` velocity of the particle will be along q. E direction. Threfore, we can write `v=AqEhati` in E,B and v, A E and B are positive constnats while q can be positive or negative. Now, magnetic force on the particle will be `F_(m)=q(vxxB)=q{AqEhati}xx{Bhatk}=q^(2)AEB(hatixxhatk)F_(m)=q^(2)AEB(-hatj)`
Since `F_(m)` is along negative y- axis, no matter what is the sign of charge q. Therefore, all ions will deflect towards negative y- direction.