A Levels Physics

The diagram shows two parallel horizontal metal plates. There is a potential difference V between
the plates.
+V
liquid drop
A small charged liquid drop, midway between the plates, is held in equilibrium by the combination
of its weight and the electric force acting on it.
The acceleration of free fall is g and the electric field strength is E.
What is the polarity of the charge on the drop, and the ratio of charge to mass of the drop?
charge
polarity
mass
E
A
negative
negative
g
E
в
E
positive
g
D
positive
E


Choice B is correct because:

Find the ratio of charge/mass

Weight of the drop (W) = Electric Field Strength (E) 

mg = Eq

q/m = g/E

Ratio of charge / mass = g/E

Find the polarity of the charge on the drop 

Weight of drop acts downwards therefore electric force on drop must act upwards in order for equilibrium to be established.

The upwards plate is positive therefore the drop must have a negative charge in order for the electric force to be exerted upwards. 

Choices A & C are incorrect because they state the ratio of charge/mass to be E/g.

Choice D is incorrect because it states the polarity of the charge on the drop to be positive.

Electric Fields | 2014 - O/N - 9702/11 | Q28