Studio Session 3

Electric potential and EKG measurements

In this studio session you will explore the connection between electric field lines and equipotential surfaces and you will use a PASCO EKG sensor to measure the action potential of your heart.

Equipment needed:
EKG sensor
Electrode patches
Rubbing alcohol

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.


Equipotential Surfaces

Objects with net electric charge attract or repel each other.  If you want to change the position of a charged object relative other charged objects, you, in general, have to do (positive or negative) work.  But sometimes it is possible to move a charged object relative to other charged objects along a surface without doing any work.  The potential energy of the charged object does not change as you move it.  If an electric charge can travel along a surface without the electric field or an external force doing any work, then the surface is called an equipotential surface.

pont chargeActivity 1

Assume you have a test charge at rest at a distance of 2 cm from the charge on the right.  You want to move it.

What path could you choose, so you would not have to do any work?  What is the shape of the equipotential surface?  (Remember that in general you can move in three dimensions.)  Explain your reasoning.

parallel platesFind some equipotential surfaces for the charge configuration shown on the right, which consists of two charged metal plates placed parallel to each other. 

What is the shape of the equipotential surfaces?  Remember you are trying to decide how a test charge could move so that the electric field does no work on it.  Sketch your predictions and explain your reasoning.

dipoleFind some equipotential surfaces for the electric dipole charge configuration shown on the right.

Sketch your predictions and explain your reasoning.


Activity 2

Use a spreadsheet to calculate the electric potential at grid points in the in the x-y plane due to 1, 2, 3, or 4 small, uniformly-charged spheres.  The x-y plane is divided into a 25 x 25 grid.  The upper left corner of the grid corresponds to x = 0.5 m, y = 0.5 m, and the lower right corner corresponds to x = 24.5 m, y = 24.5 m.  The charged spheres can be placed anywhere on the grid.  They will be located in the x-y plane.  The spreadsheet calculates the potential at each grid point and produces a surface and a contour plot of the potential.

The potential at r = (x,y,z) outside a uniformly charged sphere centered at r’ = (x’,y’,z’) is
V(r) = kq/|r - r'| = kq/((x - x')2 + (y - y')2 + (z - z')2)1/2.

In the x-y plane we have z = 0 and
V(x,y) = kq/((x - x')2 + (y - y')2)1/2.

The constant k has a value of 9*109 in SI units.  If we measure q in units of nC = 109 C, then kq = 9*q Nm2/C

Download this Microsoft Excel spreadsheet.

Examine the spreadsheet
Calls B1 -Z1 contain the x-coordinates and cell A2 - A26 the y-coordinates of the grid points
Cells A31 - C34 contain the x- and y-coordinates (in units of m) of the positions and the magnitudes (in units of nC) of four charges.
The spreadsheet initializes with a +10 nC charge at x = 13 m, y = 13 m and all the other charges have zero magnitude. 
(When you add more charges, let the x- and y-coordinates always be integers.  This avoids “divide by zero” errors, since the grid points have half integer x- and y-coordinates.)

tsble

Cell B2 contains the formula

=9*$C$31/SQRT((B$1-$A$31)^2+($A2-$B$31)^2)
+9*$C$32/SQRT((B$1-$A$32)^2+($A2-$B$32)^2)
+9*$C$33/SQRT((B$1-$A$33)^2+($A2-$B$33)^2)
+9*$C$34/SQRT((B$1-$A$34)^2+($A2-$B$34)^2)

This is the sum of V(x,y) = kq/((x - x')2 + (y - y')2)1/2 due to the four charges.
Cell B2 is copied into the other cells of the grid. The grid consists of cells B2 - Z26.
The spreadsheet shows two plots of the potential at the grid points. The contour lines are equipotential lines.  They are spaced in 5V intervals.

(a) Start with just the one +10 nC charge at x = 13 m, y = 13 m.

Describe the graphs.  What do they tell you about the potential outside a uniformly charged sphere?   Can you get information about the electric field outside a uniformly charged sphere from these graphs, i.e. can you draw field lines?  Estimate the magnitude and direction of the electric field in units of V/m = N/C at x = 20 m, y = 13 m.

(b)  Now change the positions and magnitudes of your charges.  Use the numbers below.

x y q
10    13    10   
16 13 10
0 0 0
0 0 0

Just type in the new numbers into the cells A31 - C34 and the spreadsheet and the graphs will update automatically.

Describe your graphs.  What do they tell you about the potential of this charge distribution?

(c)  Again change the positions and magnitudes of your charges.  Use the numbers below.

x y q
10    13    10   
16 13 -10
0 0 0
0 0 0

Describe your graphs.  What do they tell you about the potential of this charge distribution?

(d)  Again change the positions and magnitudes of your charges.  Use the numbers below.

x y q
10    10    10   
16 10 -10
10 16 -10
16 16 10

Describe your graphs.  What do they tell you about the potential of this charge distribution?

(e)  again change the positions and magnitudes of your charges.  Use the numbers below.

x y q
10    10    20   
16 10 -10
10 16 -10
16 16 20

Describe your graphs.  What do they tell you about the potential of this charge distribution?


Experiment

You will now use the PASCO EKG sensor to measure the action potential of your heart.
Link: What does the EKG measure?
Caution:  The sensor used in this laboratory will give you a good view of the electrical activity of the heart, but it is not an instrument to be used for medical diagnosis.  The interpretation of electrocardiograms for diagnosis requires significant training and experience, something that many of you can look forward to acquiring in the near future.

how to connect ekg sensorCollecting your data
Pick one person to be the subject in your group.  If you have time, you can repeat the experiment with another person as the subject. 
Use the following guidelines to make the EKG measurement.

Obtain a paper towel and a little rubbing alcohol.  With the dampened paper towel wipe off an area inside each elbow and inside of the right wrist.

Obtain three electrode patches from your instructor.  The patches have been designed to reduce the resistance of your skin.

Firmly place the electrode patches onto your skin - one on your right wrist, one on the inside of the right elbow, and one on the inside left elbow (as pictured).  Leave them in place until you have completed all EKG activities.
Caution:  A very small fraction of students may be allergic to the electrodes.  If you feel a burning sensation or are extremely uncomfortable, then remove the electrodes immediately and rinse the area.

Connect the EKG sensor unit to analog channel A of the Pasco 850 interface box using the cable with the DIN connectors.

Make sure the Pasco 850 interface is turned on.  Open the Capstone program.  The icon for this program is on the desktop.
In the Capstone program, click the Hardware Setup button on the left and add a Science Workshop Analog Sensor.  Chose the EKG sensor.  Click Hardware Setup again to close this window.

Drag a Graph icon onto the main display.  For the vertical axis choose Amplitude (mV).  Set the sample rate to 250 Hz.

Connect the EKG sensor to the electrodes.  The reference (black) lead should be connected to your wrist.  This lead will be the “flat line” potential on your EKG.  The positive (red) lead should be connected to the electrode on your left elbow.  Finally the negative (green) lead should be connected to the electrode on your right elbow.  Try to adjust the wires so that they are not twisting or pulling on the electrodes.

Once you are safely and securely connected to the EKG sensor, remain fairly still and breathe normally.  A lab partner should operate the computer and start collecting the data.

Start collecting data.  Collect enough data for about 5-10 heartbeats.
Save the file with your data for further analysis and paste the graph into your log.

Disconnect the EKG sensor from the electrodes, but leave the electrodes attached to your arms.


example EKGAnalyzing your data

Using your data, answer the following questions for your heart.  Explain how you arrive at your answers.  You must justify all answers.

(a) Peak-to-peak value of the voltage between the R wave and the S wave.
(b) The P-R time interval.
(c) The Q-R-S time interval.
(d) The Q-T time interval.
(e) The frequency of your heart during data collection (in beats/min and in Hz)?

Guess what would happen if you switched the red and green leads?  Try it out experimentally and make a sketch of your results in your logbook.  Explain any differences that may occur due to this change.

Collecting more data

EKG after mild exercise
Have your subject stand up and exercise for three minutes (jog in place, “step in time”, walk up and down the stair case, walk briskly around the hallway, ...).   After the three minutes are up, have the person sit back down and get reconnected to the EKG sensor as quickly as possible.  Collect a new set of data.  Save the file with the data for further analysis and paste the graph into your log. 

Using your data, answer the following questions for your heart.  Explain how arrive at your answers.  You must justify all answers.

(a) Peak-to-peak value of the voltage between the R wave and the S wave.
(b) The P-R time interval.
(c) The Q-R-S time interval.
(d) The Q-T time interval.
(e) The frequency of your heart during data collection (in beats/min and in Hz)?

Briefly describe the exercise that was completed.  What effect did the exercise have on your EKG?
What things seemed unaffected?


If you have time for some extra lab credit:

Measure the electrical potential of a Bicep MuscleMeasure the electrical potential of a Bicep Muscle

Connect the positive and negative electrode to your subject's upper biceps as shown on the right.

Use a damp paper towel to wipe off the area at the top and at the bottom of the bicep muscle on the arm without an electrode on the wrist.

Firmly place electrodes on your skin so that you have 1 on your right wrist, 1 on the upper portion of the left bicep, and 1 on the lower portion of the left bicep (as pictured).

Connect the sensor to the electrodes.  The reference (black) lead should be connected to your wrist.  The red and green leads should be connected to the bicep muscle area.  Try to adjust the wires so that they are not twisting or pulling on the electrodes.

Prepare to collect data when the subject is relaxing his/her bicep muscle and when he/she is flexing the muscle.  A good technique for flexing the muscle is to lift up on the table.  Whether flexing or relaxing the muscle, try to stay as stationary as possible.

Data Analysis

Describe the behavior of the data for this muscle and paste the graph into your log.  Are you convinced that this muscle generates a voltage when it is flexed?  Explain!
How is the signal from the bicep muscle similar to or different from the signal from the heart muscle?


Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.