Q60S - Two uses for uniform electric fields

Answer all the questions in the spaces provided. The following data will be needed: 
	magnitude of the electronic charge = 1.6 ´ 10–19 C 		gravitational field strength = 9.8 N kg–1 

Questions 
Questions 1–5 are about an experiment designed to measure the charge on an electron. In this experiment, the ‘Millikan oil drop experiment’, two parallel metal plates, 3.2 ´ 10-2 m apart, are connected to a 600 V power supply. 

1.	Draw four arrowed lines on the diagram to show the electric field between the plates, well away from the edges of the plates. 
2.	Add dotted lines to the diagram to represent the 200 V and 400 V equipotentials between the plates. Indicate which is which. 
3.	Calculate the electric field strength between the two plates.

 
The electric field between the plates just supports the weight of an oil drop of mass 1.8 ´ 10 -15 kg, which has acquired a charge due to a few excess electrons. 

4.	Calculate the charge on the oil drop.

 
5.	What is the most likely number of excess electrons acquired by the oil drop?

Questions 6–12 are about an idea for separating minerals using an electric field. Two different minerals acquire opposite charges through crushing and friction. 

In an experimental arrangement, two such minerals fall from a conveyor belt through the electric field between two vertical conducting plates. A 54 kV potential difference is applied between the 2.0 m long plates which are 0.30 m apart. 

In a feasibility study, a 1.5 mg particle was found to take 1.1 s to fall 2.0 m in air. This is longer than would be expected from a calculation of the time taken in free fall.

6.	How would you account for the longer time?

 
7.	On the diagram, draw field lines to represent the electric field in the central region between the plates. 
8.	Calculate the magnitude of the electric field between the plates.

 
9.	The charge on one 1.5 mg particle is 106 electron charges. Calculate the magnitude of the electric force on this particle when it is between the plates.

 
10.	Use your answer to question 9 to calculate the sideways displacement after the 2.0 m fall through the electric field.

 
11.	Suggest why this method of separation will not work if the mass of the particles is too small.

 
12.	Suggest why this method of separation will not work if the mass of the particles is too large.