Final answer:
The electric field strength required to suspend a proton in a vertical electric field must be equal to the gravitational field strength.
Step-by-step explanation:
The electric field strength required to suspend a proton in a vertical electric field depends on the relative strengths of the electric and gravitational forces acting on the proton. In this case, the electric field strength must be equal to the gravitational field strength in order to balance the two forces and suspend the proton.
This means that the magnitude of the force exerted on the proton, when replaced by an electron, would be the same as the force exerted on a proton. The direction of the force on the proton would be opposite to the direction of the electric field.