A negative charged particle of known magnitude qis placed in a uniform electric field of known strength E. What additional information is needed to determine the magnitude of the force on the particle?

Force on a charged particle placed in an Electric field is given by the product of the magnitude of Electric Field and charge on the particle.

The direction of the force will be given by the nature of a charge on the particle. If the charge is positively charged then the force acting will be in the direction of the Electric field.

If the charge is negatively charged then force acting will be opposite to the direction of the electric field.

Thus no extra information is needed to determine the direction of force.