Aproton is suspended in the air by an electric field at the surface of earth. what is the strength of this electric field?

The strength of the electric field is

Explanation:

By Newton's first law tells us that to maintain suspended the proton (at rest), the net force on it should be zero, assuming there's only gravitational interaction between earth and proton we should apply a force in the opposite direction equal to the weight of the proton (, at earth surface) to make net force zero.

So, the electrostatic force on the charge should be:

(1)

with m the mass and g the gravity

but electrostatic force of a charge on an electric field is:

(2)

with E the magnitude of electric field and q the charge of the proton

Equating (1) and (2):

Solving for E:

A fast search on google give us the mass of the proton and its charge:

mass and volume are the answers you are looking for