Purpose: Today we will be introduced to the oscilloscope, which is fundamental to our study of physics. We will also study the motion of an electron in a uniform electric field.
What is a Oscilloscope?
The oscilloscope is basically a graph-displaying device - it draws a graph of an electrical signal. In most applications the graph shows how signals change over time: the vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The intensity or brightness of the display is sometimes called the Z axis. This simple graph can tell you many things about a signal. Here are a few:
- You can determine the time and voltage values of a signal.
- You can calculate the frequency of an oscillating signal.
- You can see the "moving parts" of a circuit represented by the signal.
- You can tell if a malfunctioning component is distorting the signal.
- You can find out how much of a signal is direct current (DC) or alternating current (AC).
- You can tell how much of the signal is noise and whether the noise is changing with time.
We began class with the discussion of a Cathode ray tube. This is an electron tube in which electrons are accelerated by high voltage anodes, formed into a beam by focusing electrodes, and projected toward a screen that forms on the face of the tube; which is noticeable because it leaves a bright spot wherever it hits the screen. To form a display, the electron beam is deflected in the vertical and horizontal directions by magnetic field produced by coils located around the neck of the tube.
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| Here we see Professor Mason hooking it up. |
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| Here we see the beam of electrons on the face of the Cathode ray tube. |
Here we have the Oscilloscope, we begin by making a prediction. We were asked what would happen to the electrons in a lit filament in a light bulb. We said they would go in every direction (pictured below). However, this was not the case. The oscilloscope (pictured below) is suppose to show a dot of light traveling horizontally. We then apply a voltage of 1.2, this made the beam jump (the graph drawn in the second picture below). This relates to Professor Masons question.
Then we were asked "what is the expression for acceleration?" Our answer was a=qV/md (pictured below). The second question we were asked was "how much time it needs for the electrons to pass through?" Using the equation we derived for acceleration, we came up with an equation to solve for time, which is t=L/V (pictured below). In order to make these calculations we used the two plates of the CRT to find the relationship between electric field and force of the electron. In conclusion, we find that velocity in proportional to voltage.
We were then asked what would be the change in y. After some calculations (pictured below) we for that it was equal to (qVLD)/(mv_0/x^2d).
Our next activity is entitled:
Producing Voltage Changes for Input to a Scope
In order for us to understand how a oscilloscope works we must conduct a series of experiments using it along with a generator and a speaker(for the first part of the experiment). We will put voltages into the oscilloscope that change over time and see what happens.
Activity: Sounds from a Function Generator
a. Describe the sound you hear when you have the sine wave output set at 96 Hertz.
Deep rhythmic waves
b. Describe what happens to the sounds when you use the triangle and square wave outputs instead.
Square: higher pitch waves
Triangle: deep rhythmic waves
c. What happens to the sounds when you change the frequency of the function generator output?
faster higher pitch waves.
d. How do changes in amplitude affect the sounds?
It affects the volume
We continue our study of the oscilloscope by discussing further theory of how this devise works. We are told that the oscilloscope uses a CRT to display the effects of two voltages at the same time, one voltage is the voltage in the circuit and the other is provided by the oscilloscope itself and is proportional to time. Thus the CRT acts to plot the voltage needed against time.
Activity: The Oscilloscope Controls
a. Play with the intensity control and the power/illumination controls. What do they do?Intensity makes it brighter/ dimmer. Voltage causes the line to move up and down.
b. Use the intensity control to adjust the brightness of the spot on the oscilloscope screen so that it is just comfortably visible. Play with the Focus control. What does it do?
The focus control make the line either blurry or sharp.
c. With the auto button depressed, set the time base knob (Time/Div) to 0.5 seconds per centimeter so that the beam of electrons sweeps horizontally across the phosphor at the rate of one-half second for each box on the grid. All other push buttons should be in the normal (out) position. You should see a trace moving across the screen. Play with the time base control. What does it do? What do the position controls do?
Moves faster as you increase time.
Activity: Measuring Changing Voltages
a. With the function generator set at 96 Hz, use your oscilloscope to determine the period of a sinusoidal wave form. Hint: Make sure the inner knob of the time base control is in the calibrate position and then read the time base setting.5 to -5 (picture below)
b. How does this period compare with the period calculated on the basis of the frequency reading on the dial?
Both values are similar when calculating (pictured below)
c. Experiment with the “DC Offset” control on the function generator and the “AC/DC” button on the oscilloscope. Explain how these controls affect the oscilloscope display.
It goes from a sine wave to a straight line.
d. Switch over to the sinusoidal wave and square wave outputs of the function generator. Take a picture of each waveform for your blog.
Pictures are below
e. Experiment with the frequency dial and multipliers on the function generator and the time base control on the oscilloscope. How do changes in these settings affect the wave forms?
Square: when the frequency was decreased the lines got longer and when the frequency was increased the lines got shorter and multiplied.
Sine: when frequency is increased the wave compresses.
MYSTERY BOX:
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