An ionized gas contains both positive and negative ions . If it is subjected simultaneously to an electric field along the ` +x` - direction and a magnetic field along the ` +y` - direction and the negative ions towardws `-y` - direction

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 towards ` - y-direction` and negative ions towards `+ y direction`

Answer

C

Explanation

Case of positively charged particle :
Two forces are acting on the positively charged particle
(a) due to electric field in the positive ` x- direction`
(b) Force due to magnetic field.
` vec(F) = q (v hat(i)xxB hat(k)) rArr vec(F) = qvB hat(-j)`
this forces will move the positively charged particle towards ` Y - axis `.
Case of negatively charged particle.
Two forces are acting on the negatively charged particle
(a) due to eletric field in the negative ` x- direction` .
(b) due to magnetic field
` vec(F) = -q(vec(v)xxvec(B))`
vec(F) = -q[ v( hat (-i) xxB hat (k))]`
` vec (F) = -qvB[ hat(i) xx hat(k)] , vec(F) = qvB hat(-j)`
Same direction as that of positive charge .
(c) is the correct answer.