Practice Problems
Parallel Plates
Directions:
On this worksheet you will investigate properties of charged parallel plates.
omit
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?
+x
+z
-x
-z
+y
-y
omit
Question 2
Determine the total time it takes the electron to go from position A to position B based on the following values:
v = 5.7 x 10
6
m/sec
θ = 31º
E = 384 V/m
1.45 x 10
-7
sec
4.35 x 10
-8
sec
not given
2.35 x 10
1
sec
1.28 x 10
-2
sec
8.71 x 10
-8
sec
omit
Question 3
Determine the distance between position A and position B.
4.25 x 10
-1
m
2.56 x 10
-1
m
7.08 x 10
-1
m
not given
2.13 x 10
-1
m
omit
Question 4
A wall has a uniform charge distribution which produces a uniform horizontal electric field. A small plastic ball of mass 0.025 kg, carrying a charge of 30 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 0.0285 N. Is the wall positively or negatively charged?
negatively
not enough information has been provided to determine the wall's charge
positively
omit
Question 5
Calculate the tension in the string.
4.72 x 10
-3
N
2.47 x 10
-1
N
2.87 x 10
-2
N
4.05 x 10
-2
N
omit
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 2.5 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 0.768 g/cm
3
. The plates are 0.35 centimeters apart.
3.3 x 10
6
volts
4.04 x 10
3
volts
8.08 x 10
3
volts
not given
1.15 x 10
6
volts
omit
Question 7
Which drawing depicts the net field of a tiny negatively charged conducting sphere in the gap of two charged parallel plates?
not given
omit
Question 8
Electrons in a particle beam each have a kinetic energy of 4.8 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?
2.1 x 10
1
N/C
not given
4.29 x 10
3
N/C
1.03 x 10
7
N/C
7.19 x 10
5
N/C
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