Condition
(i)For directions of E,B,

→

v

(ii)For magnitudes of

→

E

,

→

B

,

→

v

(i)The velocity

→

v

, of the charged particles, and the

→

E

and

→

B

vectors, should be mutually perpendicular.
Also the forces on q , due to

→

E

and

→

B

, must be oppositely directed.
(Also accept if the student draws a diagram to show the directions.)

(ii) qE=qvB
‌ or ‌v=‌

E

B

[Alternatively, The student may write : Force due to electric field =q

→

E

Force due to magnetic field =q(

→

v

×

→

B

)
The required condition is
q

→

E

‌=−q(

→

v

×

→

B

)
‌ [or ‌

→

E

‌=−(

→

v

×

→

B

)=(

→

B

×

→

v

)