Experiment 9A: Magnetism/The Oscilloscope

Transcription

1 Experiment 9A: Magnetism/The Oscilloscope (This lab s "write up" is integrated into the answer sheet. You don't need to attach a separate one.) Part I: Magnetism and Coils A. Obtain a neodymium magnet from the instructor. Caution: Keep the magnet wrapped in tape and cardboard, as you found it. Without this padding, they have pinched people's fingers, and also gotten chipped. Connect a coil to a galvanometer (a sensitive ammeter). Do the things described on the answer sheet, record your observations and answer the questions. B. Generator. You have a device in which a coil is between the poles of a horseshoe magnet. Connect a galvanometer, then spin the coil. Answer the questions. C. Motor. A DC motor and a DC generator are basically the same device. In one case you put mechanical work in and get electricity out. In the other, you put electricity in and get mechanical work out. Get rid of the galvanometer and connect this circuit. Analog meters work best in this case. Use scales suitable for around 1 A and 3 V. Adjust the power supply so the motor has a moderate speed. (Avoid running it too fast.) You'll probably have to push it to get it started. Record the meter readings, and answer the questions. Part II: The oscilloscope: The purpose of an oscilloscope is to show graphs on its screen. The graph is drawn by a beam of electrons projected from behind. Energy from the beam makes the screen glow at the spot the beam hits. The beam passes between two pairs of plates; the electric fields between the plates deflect the beam, moving it around the screen to draw the picture. Procedure: Start with all of the oscilloscope s pushbuttons out. Push power at the upper left. A trace should appear after ten or fifteen seconds. If not, - Turn intensity up higher. - Turn y position 1 to midrange: The trace may be beyond the top or bottom of the screen. - Check that the AT/NORM button is out. Then, turn intensity to the minimum convenient level. Too bright can be bad for the screen. For a better idea of how the scope works, turn timebase fully counterclockwise. This turns down the sweep speed; there should now be an almost stationary spot where the electron beam hits the

2 screen. Slowly turning timebase clockwise should make the spot move faster and faster, the rate it moves being labeled next to the knob. Beyond a certain speed, the screen doesn t stop glowing before the spot passes over it again and you have a line. Adjust focus, x position, and y position 1 if necessary for a sharp, centered picture. A. Loudness and pitch. You have a function generator, which produces electronic signals. Run wires from its output to the scope's channel 1 input, red to red and black to black. Also connect a loudspeaker to its output. To read the function generator s frequency, multiply the setting of its frequency dial by the factor on its frequency multiplier knob. Notice that, depending on the setting of the function knob, it can put out a square wave, the familiar sine wave, or a triangle wave. Attenuation and variable attenuation are coarse and fine adjustments for the amplitude. Set the knobs on the function generator for a sinusoidal vibration, volume at some convenient level, and frequency somewhere in the hundreds of hertz. (Frequency multiplier on 100.) On the oscilloscope, adjust y amplifier 1 and timebase so that the picture is not too stretched out or squashed up, horizontally or vertically. If the picture is running along horizontally instead of standing still, try different positions for the trigger selector. (The triggering circuits are what tell the spot when to begin its trip across the screen. If it starts at the wrong time, the next wave it draws will not be on top of the previous one.) Draw two pictures from the scope to show the difference between loud and soft sounds, with the pitch kept the same. Draw two more to show the difference between high and low pitches, with the loudness kept the same. Don't change any knobs on the scope between pictures. B. Frequency of your voice. Replace the wires from the function generator with a microphone as shown. The red connector is for the signal wire, black is for ground. Once the bare metal ends are in, moderately tighten the nuts. Turn the y amplifier all the way clockwise. You should now see a graph of the sound if you sing or hum. To get the strongest signal, hold the microphone right under your nose or against your throat. Measure this sound s period and frequency: - Check that the three red knobs with arrows (variable timebase, variable y amplitude channel 1 and variable y amplitude channel 2) are fully counterclockwise. The numbers around the knobs don t mean anything otherwise, so ALWAYS check this just before taking a reading. - Determine f and T as in this example. (You do not have to have the knobs set the same as in the example. Rather, for the best possible look at the sound, set the timebase to stretch the wave horizontally as much as you can, and still see at least one complete period.):

3 EXAMPLE. From the display pictured, T = (6.2 cm)(.5 ms/cm) = 3.1 ms The 6.2 cm comes from the screen, as shown..5 ms/cm is the setting of the timebase knob. ms stands for millisecond. f = 1/T = 1/.0031 s = 323 Hz Convert 3.1 ms into.0031 s so f comes out in cycles per second, not cycles per ms. C. Speed of sound. The two little cans are transducers. They convert electrical signals into mechanical vibrations or mechanical vibrations into electrical signals. It doesn t matter which one is which. You will use one as a "loudspeaker" (source) and the other as a "microphone" (receiver). 1. Connect the wires as shown, attaching wires to the connectors the same way as with the microphone. Also get a wire that plugs into the ends of the nuts and run it from red on the function generator to the oscilloscope s red connector at the far right. 3. Set the function generator for Hz. Set the black attenuation, knob at 10 and the red one in the middle of it fully clockwise. 4. Hold the transducers so the source is aimed at the receiver. On the oscilloscope, adjust the Y AMPLIFIER 1 knob to somewhere around 50 mv/cm so the picture is not too stretched or squashed vertically. Put the TIMEBASE knob somewhere around 10 μs/cm so the picture is not too stretched or squashed horizontally. 5. The resonant frequency of the transducer is around 40,000 Hz, but not always exactly. Adjust the function generator to the frequency which gives the maximum amplitude on the oscilloscope. Then, do not disturb the frequency knob again. Record the frequency. 6. Adjust X POS, just to the right of INTENSITY, so that you can see the start of the trace at the left. Push in EXT TRIG, just below the TIMEBASE knob. Notice that now, if you move the receiver, you can see the waves go by on the scope. 7. Measure the wavelength: a. Start with the source and receiver a few centimeters apart. (Standing waves form between

4 them if they're too close.) Have a ruler under the receiver with the receiver at 0 cm. Be sure the ruler doesn't move after this. b. Move the receiver ten wavelengths along the ruler by counting the number of waves that go by on the scope. Record the distance moved by the receiver. (Not the distance between crests on the screen. The graph shows you the wave's period, not its wavelength.) Divide by ten. (This is more accurate than measuring just one wavelength.) 8. From the wave's frequency and wavelength, calculate its speed. D. Lissajous figures. Sometimes you want a signal to move the spot horizontally rather than vertically. To do this, press the xy button at the top, disconnecting the scope's sweep generator. Have intensity no higher than necessary - a stationary spot can damage the screen. Connect the signal from the function generator to the channel II input at the lower right. Set the function generator for a frequency of about 1 Hz. Put attenuation at 0 and variable attenuation around the middle of its range. Notice how the spot now moves horizontally rather than vertically. Two frequencies can be compared by putting one signal into the x input and the other into the y input. A good standard for comparison is commercial alternating current, since its frequency is an accurate 60 Hz. From the AC (yellow) terminals of a gray power supply, run wires to both red and black on channel 1. Start with the power supply s knob all the way counterclockwise. Leave the function generator on channel 2, and set it for a 60 Hz sine wave. Adjust the amplitudes of both signals and the y amplifier knobs on the scope so that the vertical size of the display roughly equals the horizontal size. If the two frequencies were exactly equal, you would see a stationary circle or ellipse. If they were nearly equal, the circle would seem to rotate. With unrelated frequencies, you just get a mess. Adjust the function generator's frequency until you get it as stationary as you can. How far off is your oscillator, both in hertz, and as a percent of the true frequency? The circle you were looking at is just the simplest of a whole family of curves called Lissajous figures. One of these is formed whenever the two frequencies are exactly in the ratio of two small integers. For example, you got the circle when they were 1 to 1. Adjust the function generator so that its frequency is exactly twice the power supply s. Sketch the 2:1 Lissajous figure. Also, find and sketch the 3:2 figure. These sketches should show all parts of the figure: As they appear to rotate, sometimes some parts get on top of other parts. The scope can also display both signals as sine waves. Put the xy button back out again, then press Dual at the bottom. Notice what happens. E. Antenna. Wires can pick up undesired signals, called electrical noise or interference, broadcast by our industrial civilization. (The reason coaxial cables are used for certain purposes is to shield against this. The signal wire in the center is surrounded by a grounded conductor which absorbs interference.) To look at this, put the xy button back out and connect a single wire to the red input of channel 1. Hold the other end in your hand by the bare metal. You and the wire are now acting as an antenna. Determine the signal's frequency. The frequency should suggest a probable source of this electrical noise. If you need another hint, watch the signal as you put your hand on the power cord of something that is turned on.

5 PHY 132 Part I: Report on Experiment 9A: Magnetism/The Oscilloscope Name Objective: To perform various demonstrations involving magnetism and induction. Procedure & Results: A) A coil is connected to a galvanometer. The difference between moving the magnet toward the coil along its axis vs. moving it away (keeping all else the same) is The effect of approaching the coil with opposite poles is The effect of approaching the coil with the same pole at different speeds (very quickly vs. barely moving) is (In each blank put "FLUX", "FLUX DENSITY", or "RATE THE FLUX CHANGES".) This last result shows that the induced emf depends on the, not or because and depend only on how far away the magnet is, but depends on how fast it's moving. B) When a coil is spun in a constant magnetic field, a galvanometer show an induced current. (does/ does not) Faraday's law says there has to be a changing flux for induction to happen. But, the coil stays the same distance from a permanent magnet, so B does not change. The explanation for what I saw is that

6 C) A simple DC motor is connected to a power supply and some meters. While running, V = and I = The power taken in by the motor is The energy it uses in one minute is Part II: Objective: To familiarize the student with the oscilloscope, and to make various measurements as described below: Procedures & Results: A. First, we used the oscilloscope to look at signals from a function generator. The signals also went to a speaker so we could hear them. The results were loud soft high pitch low pitch B. Next, we looked at signals from a microphone. Measuring the period and frequency for one of our voices, we obtained: T = f = C. To determine the speed of sound, we attached an ultrasonic transducer to the function generator and another to the oscilloscope. We set the function generator to produce sound with this frequency: f = Because the oscilloscope was triggered by a signal from the function generator, it effectively showed us the waves going by as we moved the microphone transducer through the sound waves.

7 Measuring the distance moved by the microphone with a ruler at the same time as we counted how many waves went by on the screen showed us the length of a wave. Length of 10 waves = 10 λ = λ = The speed was then found from v = f λ. v = This (does/ does not) match 343 m/s within 10%. D. Next, we connected the function generator to the scope's x input, and a signal at the line frequency (precisely 60 Hz) to the y input. The frequencies must match exactly for the circular display to stay still. In this way, we adjusted the function generator so that its frequency matched the 60 Hz. The reading on its dial was off by Hz, or %. We then adjusted the function generator to other frequencies, obtaining the Lissajous figures shown: 2:1 figure 3:2 figure E. Finally, we connected a wire to the scope's input, and held the other end, acting as an antenna. The scope showed the following signal: T = f = The source of this is apparently.