PhysicsLAB Practice Problems
Electric fields

Directions: On this worksheet you will investigate properties of electric fields.

Question 1  Which set of equations correctly describes the electric fields in this diagram? The central +4 point charge has no dimension.

 r < r1r1 < r < r2r > r2
AE = 0E = k(12q)/r2E = k(8q)/r2
BE = k(4q)/r2E = 0E = k(12q)/r2
CE = k(4q)/r2E = k(8q)/r2E = 0
DE = k(4q)/r2E = 0E = k(8q)/r2
Question 2  

If a charge of +21 µC is placed in an uniform electric field having a strength of 9000 N/C, it will experience a force of
Question 3  If the central charge is +5 µC, and point B is 21 cm away, what is the magnitude of the electric field strength at point B?

Question 4  According to the diagram shown below, what is the ratio of the magnitude of the left charge compared to the magnitude of the right charge?

Question 5  What magnitude electric field is required in a cathode ray tube to accelerate electrons from 0 to 2.1 x 107 m/sec in a distance of 2.7 cm?
Question 6  In Question 5, through how large a potential difference does the electron travel while accelerating?
Question 7  In Millikan’s oil drop experiment, if the electric field between the plates was adjusted to the right magnitude, it would exactly balance the weight of the drop. Suppose a tiny spherical oil droplet of radius 2.1 x 10-6 meters carries a charge equivalent to two electrons. What electric field is required to balance the weight? (The density of oil is 850 kg/m3.)

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