Positively charged particles are travelling in a vacuum through three narrow slits 
, 
and 
, as shown in the figure below.
Each particle has speed 
and charge 
. There is a uniform magnetic field of flux density 
and a uniform electric field of field strength 
in the region between the slits and .
State the expression for the force 
acting on a charged particle due to
the magnetic field,
the electric field.
The electric field acts downwards in the plane of the paper, as shown in the figure above. State and explain the direction of the magnetic field so that the positively charged particles may pass undeviated through the region between slits and .
due to the electric field must be opposite in direction to the magnetic force 
due to the magnetic field for the positively charged particles to pass undeviated through the region between slits 
