Phys
1020 Lab 2: Time-varying voltages:
The Electrocardiogram and Fluorescent lights.
NOTE: PRE-LAB ASSIGNMENT is at the end of this lab description. It will take a little more time than the
pre-lab for Lab 1.
Lab Logistics:
You and your group will work together to complete the lab and write up the group lab report. Remember, everyone will need to assume a new job. For instance, if you were the manager for Lab 1, then you should either be the recorder or the skeptic for this lab. Again, everyone should be helping with the hands-on stuff.
o The manager: This person is responsible
for making sure that the group follows the lab procedure and completes
everything that is asked for in the lab.
o The recorder: This person is
responsible for keeping the lab notebook for the day, recording the
observations observed by the group and the group’s answers to the questions
asked in the lab.
o The skeptic: This person is there to question the results of the lab. Is everything making sense? Are we taking the data correctly? Are the results and conclusions reasonable? Did we skip a step?
Begin
each lab report by titling the lab, listing your lab partners who are present,
and listing the jobs that each lab partner has assumed for the lab. Remember,
your lab report should give an explanation of all of your observations and
measurements. Also, you need to think of and try one additional experiment for either
Part 1, 2, or 3 of this lab that will further test your explanation of the
results that you found.
Lab Description
In this experiment, you will use an oscilloscope to measure the time-varying voltages from two quite different sources: 1) your heart, and 2) a photo-detector aimed at the fluorescent lights overhead.
Getting familiar with the Oscilloscope (This section should go fairly quickly)
An oscilloscope is a device that displays a graph of voltage vs. time (voltage on the vertical axis, time on the horizontal axis). If the input voltage is DC, that is, constant in time, then the oscilloscope displays a horizontal line, whose vertical position indicates the voltage. Your TA will introduce you to the use of the oscilloscope. The oscilloscope screen has 1 cm divisions on both axes. There are two separate input connectors to the oscilloscope labeled channel 1 and channel 2. Each input channel has a volts per division (volts/div) knob which sets the vertical (volts) scale. A single seconds per division (sec/div) knob sets the horizontal (time) scale. There are also position knobs for adjusting the vertical and horizontal position of the display.
Under the volts/div
knob is a 3-position switch that reads "AC
- ground
- DC".
In the ground position, the input to the oscilloscope is grounded (set to 0
volts), and the display becomes a horizontal line whose position is the zero
volts position and which can be adjusted with the vertical position knob. For
instance, one could set the middle line of the screen to be 0 volts. Then
positions above the middle line would be positive voltages, and positions below
the middle would be negative voltages. When the switch is in the DC position,
the signal is input to the oscilloscope unaltered. The DC position is the one
you will use to make all measurements in this lab. The AC position centers the
signal about it's overall DC shift from ground, so
that one can just measure AC fluctuations. There is a small knob in the center
of both the volts/div and sec/div knobs, called the
Electrical connections to the oscilloscope are made through a special kind of connector called a BNC connector. The BNC connector is used with coaxial cables (coax, for short). Coax cables have a central wire carrying the signal voltage and an outer cylindrical conductor which is usually grounded (0 volts). The outer conductor on the BNC connector on an oscilloscope is always grounded, and it is important to remember that the outer wire of a coax cable is always at zero volts when it is connected to an oscilloscope.
Experiment with a
We can use a 1.5 V battery to see how the signal of an oscilloscope responds to a DC voltage difference.
We will need the following:
- Oscilloscope
- 1.5 V battery
- BNC adapter
- 2 wires
First slide the oscilloscope mode switch to "peak-peak auto". Then
attach an adaptor to Channel 1 of the oscilloscope, and plug the 2 wires into
it. Hold one wire against one end of a 1.5 V battery, then
touch the other wire against the other terminal of the battery. Adjust the VOLTS/DIV control on Channel 1 until
you can see the new signal level. Release the contact, and see the level drop
back down to the initial 0 V.
Does this match your expectation from the pre-lab (Question 6)?
Measure the voltage on the oscilloscope in both conditions: when both ends of
the battery are connected to a wire, and when one wire is disconnected.
Alternate in time between connected and disconnected at a constant rate (for
example, make and break the circuit once each second) and see if you can make a
periodic signal on the oscilloscope. Change the time base (the TIME/DIV knob) and see how the signal
changes horizontally.
Now turn the battery around, so that each wire attaches to a different end of the battery. Measure both voltages again. Explain why the voltage changes when the battery is turned around. What determines which end will be at 0 V, or ground?
The electron beam
Several times in class (e.g. in the TV) we have encountered the cathode ray tube, with a fast-moving beam of electrons. The image on the oscilloscope is formed in a way very similar to that of your television. Here’s your chance to play with a beam of electrons. To see how the electron beam is deflected, use a strong permanent magnet. You will probably have the most success by bringing the permanent magnet close to the phosphor screen. Adjust the oscilloscope so that the electron beam is not sweeping across the screen but just hitting a single spot in the center of the oscilloscope screen (do this by turning the SEC/DIV knob all the way counterclockwise.)
Draw a sketch of the magnetic field lines for the permanent magnet you have. Check your drawing with your TA.
Hold the magnet near the magnet near the screen in various orientations. How can you make the dot shift up? shift down? right? left? How do you need to hold the magnet so that the dot is unaffected by it even though the magnet is near the electron beam?
Describe or draw a picture of how you got each
shift and explain your reasoning for why the electron
beam was deflected the way it is, and why it is possible to hold the magnet
near the screen in one particular orientation without deflecting the beam.
Part I. The EKG
Your heart is a complicated electrochemical machine that produces time-varying voltages as it beats. These heart voltages produce small voltage differences between points on your skin that can be measured and used to diagnose the condition of your heart. Usually, nine electrodes, positioned at various points of the patient's body, are used when recording a full electrocardiogram (EKG). However, in this lab, we will only use two electrodes to measure the voltage between your right and left hands.
A typical plot of voltage difference between two points on the human body vs. time is shown below. The P deflection corresponds to the contraction of the atria at the start of the heartbeat. The QRS group corresponds to the contraction of the ventricles. The T deflection corresponds to a re-polarization or recovery of the heart cells in preparation for the next beat. Every heart pattern is slightly different, and the interpretation of an EKG requires experience with many patients.
The EKG apparatus that you will use consists of two electrodes, an amplifier, and a storage oscilloscope. Signals travel from the hands, one placed on each electrode, to the EKG amplifier, and then on to the oscilloscope. The voltage that is measured is the potential difference between the two electrodes:
V = V1 - V2 (1)
Thus if you place your left hand on electrode 1 and your right hand on electrode 2, then the voltage measured will be the difference
Vinput = VLH - VRH (2)
The voltage difference between your hands is inconveniently small to measure directly. Therefore the signal from the electrodes is fed into an amplifier, which amplifies the signal. (The amplifier also filters the signal and contains an isolator to protect you from electric shocks.) Thus the voltage Vscope registered on the oscilloscope is larger than the input voltage at the electrodes by an amplification factor A:
Vscope = A Vinput (3)
EKG Procedure
Materials that you will need:
- Oscilloscope
- EKG amplifier
- Electrodes
- 3 cables (1 BNC cable)
First check that the cables from the electrode handles are plugged into the proper connections on the amplifier input – marked "left" and "right" on the amplifier, if not connect them this way. Then run a cable from the amplifier out jack to the Channel 1 input on the oscilloscope. Next plug the EKG amplifier power cord in to the power junction box.
Turn on the EKG amplifier on and switch its mode switch to "operate".
Next turn on the oscilloscope and make sure the switches are all set as shown on the handout taped to the scope. Be careful that the inner knobs marked "cal" on the CH1 VOLTS/DIV and the SEC/DIV controls are turned fully clockwise until a "click" is felt. You should now see a dot, or a short vertical line moving slowly across the screen of the oscilloscope. If it is not visible, ask your instructor for help.
2) Baseline Setting
Begin by sliding the input switch below the Channel 1 VOLTS/DIV knob from DC to ground. Then adjust the vertical position until the trace is on some horizontal reference line at or below the center of the screen. By setting the grounded signal to a known reference line on the display, subsequent measurements of voltage amplitude can be easily determined using the grid marks on the scope. Now, slide the input switch back to D.C. to continue with experiment.
3) Measure the Amplification Factor
The voltage displayed on the oscilloscope differs from the input voltage by the amplification factor A in Eq. (3). To find A, switch the amplifier mode switch to "calibrate"; then press the red button while the oscilloscope makes a single sweep in storage mode. This produces an input voltage whose peak (positive) value is 1 mV = 10-3 volts. By measuring the height of the peak voltage on the oscilloscope, you can calculate the amplification factor A of your amplifier. Include a careful picture of the trace, and be sure to indicate the horizontal and vertical scales (in SEC/DIV and VOLTS/DIV respectively).
4) EKG Trace
One person should sit down in front of the electrode assembly with their hands wrapped gently around the electrodes, palms down. It is important to have good skin contact and at the same time have relaxed muscles in the hands, arms, and shoulders. Muscle tension will cause voltage noise in the oscilloscope traces. With the hands in place on the electrodes, the EKG patterns will appear on the screen.
You will need a partner to operate the oscilloscope controls. Adjust the time base (TIME/DIV knob) and the VOLTS/DIV knob until you can see the EKG signal on the screen. Keep adjusting until you can clearly see the features of the signal. (ie. Make it big enough to fill most of the screen, without cutting any of it off). To store a trace, push in the "store" button on the upper right corner of the oscilloscope. Now when you press “save”, it will capture that trace of the repeating EKG pattern across the screen. Repeat this procedure many times. If you have trouble getting the oscilloscope started again once you have captured an image, ask your TA.
5) Voltage between the hands
Switch the amplifier back to "operate" and choose one member of your group to make an EKG trace with the left hand on electrode 1 and the right hand on 2. Once you have stored a good trace with at least two R peaks, make a careful drawing of the plot (be sure to label your axes). Measure the time between successive R peaks and then find your subject's pulse rate, which is simply the frequency f of the heart expressed in beats/min. (Remember that frequency = 1/ period: f = 1/T.) Measure the peak voltage (that is, the voltage at the R peak) on the screen and, using the known amplification factor A, calculate the actual peak value of VLH - VRH for your subject.
Repeat the preceding paragraph with a second member of your group and comment on the similarities and differences between the two traces.
6) Hands reversed
Have one of the subjects already used in part 6) above reverse his or her hands on the electrodes (RH on 1, LH on 2) and again make a careful drawing of the resulting EKG trace in such a way as to facilitate comparison with the previous plot.
7) Play around seeing what else you might discover about what changes the EKG signal, etc.
Questions:
How should your plots in 5) and 6) differ? Why? In what aspects should they be the same? How well do your plots fit these expectations?
Part II. The Photodetector
In the
Unfortunately or fortunately, the lights in the lab space are fancy new fluorescent lights which have compensated for this flicker (possible by using phosphors that glow for longer). So for this next section we will need to use the mercury vapor lamps you have on your bench.
PROCEDURE:
Disconnect the output cable from the EKG amplifier to the oscilloscope. Connect the short photo detector cable directly to the oscilloscope's Ch.1 input terminal. CAUTION: Hold the cable by the connector end, not by the fragile diode. Now bend the cable gently into a curve (near its center) so the detector points towards the mercury lamp. Set up the oscilloscope to display the signal from the detector. Increase the sweep rate and adjust the voltage scale until the periodic nature of the signal is evident.
When the scope is ready, store one sweep of the detector output and make a drawing of the sweep in your notebook, properly labeled. Measure the period and amplitude of the signal. From the period, compute the frequency. Is the measured frequency what you expect?
If you cover the detector with your hand, what happens to the signal Amplitude? What about its frequency?
From what you know about photoconductors, how do you think the photodiode is detecting light? What is changing with each cycle, the resistance, the current, or the voltage? And what is causing this change.
Additional Experiment
As usual you are to think up an experiment using the equipment from this lab
and record your procedure and results (time permitting).
Final Thoughts....
What are the main physical principles that tie Parts I & II
together? (What physics was used in both parts)
Did you find anything unexpected?
Do you think that your understanding of electricity has improved, especially
about the difference between AC and DC? Give an example of something you
learned or were able to explore more deeply.
Prelab Questions (due at the beginning of your lab session).
The graph below shows a sinusoidal voltage displayed on an oscilloscope screen. The volts/div setting is 50 V/div and the sec/div setting is 5 msec/div (5 x 10-3 sec/div). The vertical position knob on the oscilloscope has been adjusted so that the middle (dark) horizontal line is at zero volts.
1. What is the period T of this signal?
2. What is the frequency f of this signal?
3. Could this signal be the AC voltage from a
standard wall socket (in the
4. What is the amplitude of this signal?
5. How many times a second does this signal equal zero volts? That is, what is the frequency (in hertz) of the zero-crossings of this signal.
0 Volts
6. What would a DC voltage look like on an oscilloscope? (draw a simple sketch)