and a positron enters via
opposite face with velocity 
. At this
instant,
(a) the electric forces on both the particles cause identical
accelerations
(b) the magnetic
forces on both the particles cause equal accelerations
(c)
both particles gain or loose energy at the same rate
(d)
the motion of the centre of mass (CM) is determined by
alone
Answer:
(b,c,d)Acceleration of
electron due to electric field is
.
Its
direction is opposite to the direction of 
.
Acceleration
of positron due to electric field is 
Its
direction is along the direction of 
.
Thus
. Hence, option (a) is
wrong.
Magnetic
force on electron, 
Acceleration
of electron, 
Magnetic
force on positron, 
Acceleration
of positron,
. Hence, option (b) is true.
As
both the particles (electron and positron) are of same mass and same charge in
magnitude having same acceleration due to magnetic field, hence they gain or
loose the energy at the same rate. Thus option (c) is true.
Due
to electric field, net electric force on electron-positron pair, 
Net
magnetic force on electron-positron pair
Therefore,
the motion of the centre of mass (CM) is determined by magnetic field alone.
Thus option (d) is true.
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