Download ELECTRICITY - Oscilloscope Problems

ELECTRICITY - Oscilloscope Problems
PH 308 STEFAN
Homework due Thursday 2/12/08
In class, we learned how electrons move through different parts of an oscilloscope
to produce a spot on the screen. Refer to the diagram below 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 V
Voltage Difference between vertical deflection plates = 3Volts (top plate is at a
higher voltage)
Vertical distance between vertical deflection plates = 10 cm
Horizontal length of vertical deflection plates = 20 cm
Use your RED pencil and correctly redraw the beam of electrons in the below picture,
according to the given signs of the vertical deflection plates specified in the text.
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 x E7 m/s. If not, use
that number anyways in the following problem.
2. Explain why vf anode = vi on vertical deflection.
3. 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 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.
d. Use your results from parts b and c to calculate the electron’s vertical
displacement from when it enters the vertical deflection plates until it leaves
them.