Interactive Lecture Demonstration 2 – Oscilloscope

 

 

Refer to section 15.9 of Serway and Faughn for a description of how an oscilloscope works.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. When the oscilloscope is turned on, you will see a bright green dot in the center of the oscilloscope screen. What is that dot ? Are you seeing electrons ?

 

 

 

 

  1. There is a high voltage difference between the cathode and the anode inside the oscilloscope. That voltage difference is used to accelerate the electrons towards the screen. On the diagram, label which location (cathode or anode) is at the higher voltage, and draw the electric field line pattern in the region between the cathode and anode.

 

  1. The oscilloscope has two input wires, one black and one red. Observe what happens to the dot when a D-cell (1.5 Volt) battery is connected to the input wires of the oscilloscope. The + end of the battery is connected to the red wire, the – end of the battery is connected to the black wire. What do you think the red wire is connected to inside the oscilloscope ? What is the black wire connected to ? Remember the sign of an electron's charge !

 

 

 

 

 

  1. On the diagram, sketch the electric field line pattern in the region between the vertical deflection plates when the battery is connected as in part 3. Also label which plate (top or bottom) has the higher voltage.
  2. Starting with the definition of an electric field, explain in words why the vertical deflection plates cause the electrons to be deflected in the way you saw.

 

 

 

 

 

  1. Predict (and give reasons for your prediction) what will happen if two batteries are now connected to the oscilloscope wires.

 

 

 

 

 

 

We will now calculate how much vertical deflection the electrons should have inside the oscilloscope when the oscilloscope is connected to a 1.5 Volt battery. We will assume the following additional information is known: voltage difference between the cathode and anode is 1000 Volts; separation of the vertical deflection plates is 5cm; length of the vertical deflection plates is 20cm.

 

  1. Using the law of conservation of energy, calculate the speed of the electrons when they arrive at the anode. Assume that the electrons are at rest when they leave the cathode.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. Next, determine a) the magnitude of the electric field and b) the vertical acceleration of the electrons while they travel between the vertical deflection plates.

 

 

 

 

 

 

 

 

  1. Now we want to find the vertical displacement of the electrons while they travel between the plates. This is a problem in 2-dimensional motion, with constant acceleration in the vertical direction. We hope that you remember how to do that kind of problem from PY211 ! As a reminder, you will need 5 known DVAT variables out of the 9 total variables (those are vertical and horizontal displacement, initial velocity, final velocity and acceleration, plus time). Write down the 5 known variables for this problem:

 

 

 

  1. Now use an equation of motion for constant acceleration (DVAT equation) to solve for the time it takes the electrons to move between the vertical deflection plates.

 

 

 

 

 

 

 

 

 

  1. Finally, use another DVAT equation to solve for the vertical displacement of the electrons.

 

 

 

 

 

 

 

 

 

 

  1. What shape is the path of the electrons while they travel between the deflection plates ? What shape is the path once they are outside the deflection plates ? Sketch (and label) the path of the electron beam on the diagram above. Is your answer to part 11 consistent with what you actually saw the electron beam do ?