Question

1a. Write an algebraic expression for the gravitational force on a spherical oil drop of radius r and density ρ in the earths' gravitational field g.

b. The electrostatic force is applied by two parallel plates (similar to a capacitor) with a potential difference V between them.

Write an algebraic expression for the electric field between two such plates separated by a distance d

c Write an algebraic expression for the electrostatic force that acts on a charge q due to this electric field in terms of potential difference V.

d. Combine these two expressions to write an algebraic expression for the charge q in terms of r,ρ etc.

.

Answer 1

a)Gravitatioal force=mg

m=mass=volumeXdensity=(4/3)X

r=radius,   =density

Gravitatioal force=mg=(4/3)Xg

b)Electric field E=V/d

V=potential difference, d=distance between plates

c) Electric force on charge q

F=qE=qV/d

d) If E is acting upwards, electric force is upwards. But gravitational force is downwards. When the 2 forces are equal, the charge is at rest

Equating the forces

mg=qE

(4/3)Xg=qE

q=(4g)/(3E)