PhysicsLAB Practice Problems
Parallel Plates

Directions: On this worksheet you will investigate properties of charged parallel plates.

Question 1  An electric field E exists in the region between the two electrically charged parallel plates shown above. A beam of electrons of mass m, charge q, and velocity v enters the region through a small hole at position A. The electrons exit the region between the plates through a small hole at position B. Express your answers to the following questions in terms of the quantities m, q, θ, E, and v. Ignore the effects of gravity.

In which direction is the electric force acting on the electron after it enters the region between the plates?
Question 2  Determine the total time it takes the electron to go from position A to position B based on the following values:
  • v = 5.6 x 106 m/sec
  • θ = 38º
  • E = 333 V/m
Question 3  Determine the distance between position A and position B.
Question 4  A wall has a uniform charge distribution which produces a uniform horizontal electric field. A small plastic ball of mass 0.014 kg, carrying a charge of 50 µC, is suspended by an uncharged, nonconducting thread 0.35 m long. The thread is attached to the wall and the hall hangs in equilibrium, as shown below, in the electric and gravitational fields. The electric force on the ball has a magnitude of 0.028 N. Is the wall positively or negatively charged?

Question 5  Calculate the tension in the string.
Question 6  In a Millikan oil drop experiment, if the electric field between the plates is adjusted properly, it can exactly balance the weight of the drop. Suppose a tiny spherical oil droplet of radius 1.4 x 10-4 cm carries a charge equivalent to two electrons, or -2e. What voltage must be placed across the plates to balance the oil drop if the density of oil is 0.666 g/cm3. The plates are 0.35 centimeters apart.

Question 7  Which drawing depicts the net field of a tiny negatively charged conducting sphere in the gap of two charged parallel plates?
Question 8  Electrons in a particle beam each have a kinetic energy of 8 x 10-17 J. If the electrons are traveling parallel to the field lines, what magnitude electric field will stop them in a distance of 0.07 meters?

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Catharine H. Colwell
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