Download Oscilloscope Problems

Oscilloscope Problems
PH304 Bennett
Homework due Friday 2/9/07 in class
In class, we learned how electrons move through different parts of an oscilloscope to produce a spot on
the screen. Refer to the diagram on the reverse side (or on the link on the website Assignments page) that
shows the internal parts of an oscilloscope. In the problems below, assume the following specifications
for the oscilloscope:
Voltage Difference between cathode and anode = 4000 Volts
Voltage Difference between vertical deflection plates = 3 Volts (top plate is at higher voltage)
Voltage Difference between horizontal deflection plates = 0
Vertical distance between vertical deflection plates = 10 cm
Horizontal length of vertical deflection plates = 20 cm
Assume that the electric field is uniform in the region between the vertical deflection plates and is zero
outside that region (except in the region between the cathode and anode, where similar comments apply).
Ignore gravity in all parts of the problem.
1. a. On the diagram, indicate the direction of the electric field in the region between the cathode and the
anode. Also indicate the location (cathode or anode) you will consider to be at zero voltage, and what
voltage the other location is at.
b. Assuming that an electron starts from rest at the cathode, determine the speed of the electron when it
reaches the anode. Be very careful about minus signs throughout this problem ! We hope your answer will
be 3.75 107 m/s. If not, use that number anyways in the rest of the problems. Solve this problem using the
work/energy principle (not DVAT’s !). This means you must also provide all the usual information
(define the system, list all outside forces, list relevant forms of energy of the system, indicate initial and
final locations).
c. Explain why your answer to part b is also equal to the electron’s velocity when it first enters the
vertical deflection plates. Refer to specific physics law(s) in your explanation.
2. a. On the diagram, indicate the direction of the electric field in the region between the vertical
deflection plates. Also indicate the location (top or bottom plate) you will consider to be at zero voltage,
and what voltage the other location is at. Finally, calculate the magnitude of the electric field in that
region.
b. Determine the vertical acceleration (magnitude and direction) of the electron while it passes between
the vertical deflection plates. Yes, you need a force diagram.
c. The horizontal acceleration will be zero since there are no horizontal forces on the electron. Use that
fact to calculate how much time the electron will spend traveling between the vertical deflection plates.
(Hint: this is a DVAT problem with motion in two dimensions. The vertical and horizontal motions are
independent, as we learned in PH302. Be sure your variables in this part are all horizontal variables !)
d. Use your results from parts b and c to calculate the electron’s vertical displacement from when it enters
the region between the vertical deflection plates until it leaves that region. This is another DVAT problem
- be sure your variables in this part are all vertical variables.
e. What will be the shape of the path taken by the electron while it passes through the constant electric
field between the vertical deflection plates ? Hint - think of the analogy to projectile motion of a mass in a
constant gravitational field.