College Physics Laboratory - Winter Project Guide

During the autumn of 2006, your instructor designed the following circuit, built it, and if he can ever get it working reliably, will interface it to the Internet:

S2 S3 S4 S5 S6 S7 S8 S9

In the meantime, he wrote a program to mimic its behavior as closely as possible, and you will be using that program until the real thing comes along. In the foregoing, we will pretend that it is indeed working; that way there is less to change on this page when it does work.

Our College Physics Laboratory project for Winter is to completely analyze that circuit. You will be able to manipulate the switches remotely using the form above, and the program which controls the circuit will report to you the voltages measured (relative to ground) at points VA through VF. You will use that data in your analysis.

It may be useful to read the sections from the text on circuit elements and circuit analysis, since this laboratory project deals extensively with those subjects and you may not have completed them before undertaking your analysis.

We will be using many of the statistical techniques from last quarter in our analysis, as well as some new ones. And we will read several more historically important papers in physics in order to refine our understanding of the structure of effective experimental reporting. Your final report will be a culmination of this entire endeavor.

Taking Data

During the first class meeting, you will measure the resistances and capacitances of several components. Take down exact measurements because you will need these later in the project for error analysis.

Only one of the circuits above was constructed, so only one student at a time will be able to use it. But the experiment takes only a few seconds to perform, so if the circuit is in use by another student you will be notified, and as soon as it is free your data will be sent to you.

Each student will have a different set of configurations, numbered 1 through 8, which will be assigned by your instructor the first day of class. For each configuration, check the boxes above for the switches you want to close. Note that switch S1 is always closed during the taking of data.

Switch S2 allows us to include or exclude all of the capacitors at one time, while switches S3 and S4 allow us to exclude capacitors C2 and C3, respectively.

Resistors R1 and R4 are always in the circuit, while switches S5 and S6 allow us to remove resistors R2 and R3, respectively, and switches S7, S8 and S9 allow us to bypass resistors R5, R6 and R7, respectively.

Once you have chosen which switches you want to close for a particular data set, push the button labeled "Perform the experiment" and your data will shortly be returned to you in the form of a table. If all of the capacitors have been excluded from your circuit, a single line will be returned with the time since switch S1 was closed (in seconds), followed by the values of the voltages measured at the points VA through VF. If any capacitors have been included, you will receive 100 lines of data with the values of the voltages at those test points every 10 milliseconds (approximately) after the switch was thrown. Record all data without rounding.

The analog to digital converter which measures the voltages has 12 bit accuracy. This means that every voltage is a multiple of the reference voltage (2.5 V) divided by 212, or .00061 V.

Data Analysis

The following list of questions and assignments will guide you through the analysis of various circuit configurations.

  1. The resistors and capacitors used in the circuit are shown here:

    In order to get a sense of the size of these components, the resistors are each about 1/4 of an inch long, not including the wires, or leads, connected to each end. These resistors are rated at 1/4 Watt (meaning that if you push more power through them they will be damaged). C1 and C2 are polyester film capacitors, while C3 is a ceramic disc capacitor. Although it is not evident here, the longer lead on capacitors and LEDs is the positive lead.

    The switches are electronic CMOS switches, connected as shown in the implementation diagram.

    The resistors are color-coded with their values and tolerances. The tolerance is the percentage deviation that is allowed in the labeled value of the resistor. These resistors all have a gold tolerance color band (on the right hand side), signifying that their actual value may be within 5% of the value described by the other color bands. A silver tolerance band would signify 10%.

    Due to the size of these resistors, it is somewhat difficult to determine the color codes in some cases. Resistors R1 and R7 have brown, black and green bands; R2 and R4 have red, red and green bands; R3 has yellow, violet and green bands; R5 has brown, green and green bands, and R6 has orange, orange and green bands.

    The color codes on the resistors begin at the end where the colored band is on the edge of the resistor (on the left hand side in the photo). The first two (which we will call c1 and c2) encode the significant digits of a number between 10 and 99, and the last (c3) encodes the power of ten by which that number is multiplied:

    R = ((c1 * 10) + c2) x 10c3 W.

    The color codes are:
    black = 0red = 2yellow = 4blue = 6gray = 8
    brown = 1orange = 3green = 5violet = 7white = 9
    So a code of "yellow, violet, red" would signify a 4700 W resistor (also called "4.7 K").

    The measured values of the resistors are

    R1 = 980 KWR3 = 4.61 MWR5 = 1.49 MWR7 = 980 KW
    R2 = 2.21 MWR4 = 2.17 MWR6 = 3.3 MW
    Compute the labeled value of each resistor using the color chart above, and the relative deviation between its measured and labeled values, relative to the labeled value. Are they all within accepted tolerance?
  2. The capacitors are advertised to have the values
    C1 = .047 mFC2 = .022 mFC3 = .01 mF
    but they are labeled as
    C1 = 473KC2 = 223KC3 = 103K
    In each case, the 3 indicates 103 picoFarads (1000 pF = 1 nanoFarad, and 1000 nF = 1 mF). If there were no third digit, the one or two digit value would be in pF. The K indicates a tolerance of 10% (a J would indicate 5%). In fact, they have the following measured values:
    C1 = .0503 mFC2 = .0229 mFC3 = .00993 mF
    For each capacitor, compute the relative deviation between its measured and labeled values. Are they all within accepted tolerance?
  3. Submit the deviations for both resistors and capacitors to me at the beginning of class the 3rd week of the quarter.
  4. Select the switches for configuration 1 and perform the experiment. Note that Vin should be 1.53 V for every iteration of the experiment (a voltage regulator is used for this purpose). Compute the voltage drops across each resistor from the data (for instance, VA - VC = VR1). Compute the current flowing through each resistor (using its measured resistance) from Ohm's Law. Compute the power dissipated by each resistor.

    Compute the equivalent resistance of the entire circuit, the total current flowing through the circuit and the total power dissipated by it. Check that Kirchhoff's Laws (as formulated at the bottom of the section on circuit analysis) are obeyed. Are they obeyed exactly? If not, compute the relative deviations between the voltages or currents which should be the same.

    Use the data from the first class meeting to compute the relative error in the resistance measurements. Compute the relative error in the voltage measurements. Compute the RMS error of these two and compare it to the measured deviation for the current. Can the uncertainties in the resistance and voltage measurement account for the deviation from Kirchhoff's Laws?

  5. Submit the currents and relative deviations you used to check Kirchhoff's Laws in the last step to me at the beginning of class the 4th week of the quarter.
  6. Select the switches for configuration 2 and perform the experiment. Compute the voltage drops across each resistor from the data. Compute the current flowing through each resistor. Compute the power dissipated by each resistor.

    Compute the equivalent resistance of the entire circuit, the total current flowing through the circuit and the total power dissipated by it. Check that Kirchhoff's Laws are obeyed (as above).

  7. Select the switches for configuration 3 and perform the experiment. Compute the voltage drops across each resistor from the data. Compute the current flowing through each resistor. Compute the power dissipated by each resistor.

    Compute the equivalent resistance of the entire circuit, the total current flowing through the circuit and the total power dissipated by it. Check that Kirchhoff's Laws are obeyed (as above).

  8. Submit the currents and relative deviations you used to check Kirchhoff's Laws in the last step to me at the beginning of class the 5th week of the quarter.
  9. In the cases where Kirchhoff's Laws were not obeyed exactly, was the error random or systematic?
  10. Select the switches for configuration 4 and perform the experiment.

    Plot VA as a function of time. Identify the limiting value (Vlim) as t goes to 1 s. Compute the relative deviation of Vlim with respect to Vin.

    Plot VC on the same graph and describe its relation to the previous plot.

  11. Plot the charge stored on C1 as a function time. On a separate graph, plot the number of electrons stored on the negative side of C1 as a function of time. On a separate graph, plot the electrical energy stored in the capacitor as a function of time. How is this plot qualitatively different from the others?

    Plot the currents through R1 and R4 as functions of time, on the same graph. Describe the relationships between the various graphs.

    Note that the graph of charge vs. time is analogous to that of position vs. time. Therefore the slope of the charge graph has an important relationship to the currents through the resistors. What is it? This is one of the keys to understanding the behavior of the circuit.
  12. The voltage drop across C1 is theoretically described by the formula
    V(t) = Vin (1 - e-t / R C),
    where R is the equivalent total resistance of the circuit and C is the equivalent total capacitance of the circuit. If we divide the data for VA by Vin, subtract that quotient from 1 and then take the natural logarithm:
    f(t) = ln (1 - VA / Vin),
    we should obtain a straight line with slope -1 / R C. In order to compute this slope, we are going to perform a linear regression.

    If any of the VA values are equal to 1.53 V, eliminate them from the data set. This will make the data easier to fit, since the data is only accurate to .0006 V (the algorithm for linear regression is notoriously sensitive to rounding error). Be sure to eliminate the corresponding values of t. Compute f(t) for the remaining values of VA. Then compute the average of them (F), and the average of the time values (T). If we have N values of t and N values of f(t), the slope (m) and intercept (b) are given by

    m = (N * T * F - S (ti fi)) / (N (T2) - S ((ti)2))

    b = F - m T

    where the S denotes the sum taken from i = 1 to i = N.

    Compute m and b. Compute the relative deviation of the slope with respect to the expected value.

  13. Submit your slope and relative deviation from the last step to me at the beginning of class the 6th week of the quarter.
  14. Check that Kirchhoff's Laws are obeyed (as above) for the first, 50th and last set of voltages. Do you notice a pattern here? Why might that be?
  15. Select the switches for configuration 5 and perform the experiment.

    Plot the voltage drop across each capacitor as a function of time. Identify the limiting value (Vlim) of each as t goes to 1 s.

    Plot VC on the same graph. Describe the relationships between the various graphs.

  16. Plot the charge stored on each capacitor as a function of time (all on the same graph).

    Plot the current through R1 as a function of time. Describe the relationships between the various graphs.

  17. Do a linear regression for VA as before.

    Compute the relative deviation of the slope with respect to the expected value.

  18. Select the switches for configuration 6 and perform the experiment.

    Plot the voltage drop across each capacitor as a function of time. Identify the limiting value (Vlim) of each as t goes to 1 s.

    Plot VC on the same graph. Describe the relationships between the various graphs.

  19. Plot the charge stored on each capacitor as a function of time (all on the same graph).

    Plot the current through R1 as a function of time. Describe the relationships between the various graphs.

  20. Do a linear regression for VA as before.

    Compute the relative deviation of the slope with respect to the expected value.

  21. Submit your slope and relative deviation from the last step to me at the beginning of class the 7th week of the quarter.
  22. Select the switches for configuration 7 and perform the experiment.

    Plot the voltage drop across each capacitor as a function of time. Identify the limiting value (Vlim) of each as t goes to 1 s.

    Plot VC on the same graph. Describe the relationships between the various graphs.

  23. Plot the charge stored on each capacitor as a function of time (all on the same graph).

    Plot the current through R1 as a function of time. Describe the relationships between the various graphs.

  24. Do a linear regression for VA as before.

    Compute the relative deviation of the slope with respect to the expected value.

  25. Select the switches for configuration 8 and perform the experiment.

    Plot the voltage drop across each capacitor as a function of time. Identify the limiting value (Vlim) of each as t goes to 1 s.

    Plot VC on the same graph. Describe the relationships between the various graphs.

  26. Plot the charge stored on each capacitor as a function of time (all on the same graph).

    Plot the current through R1 as a function of time. Describe the relationships between the various graphs.

  27. Do a linear regression for VA as before.

    Compute the relative deviation of the slope with respect to the expected value.

  28. Submit your slope and relative deviation from the last step to me at the beginning of class the 8th week of the quarter.
  29. Summarize the behaviors of the circuit and its various components, with and without the capacitors, in terms of voltage drops, currents, power dissipation and the storage of energy and charge. How do conservation principles account for many of your observations? What were the sources of random and systematic error?

    One of the more interesting facets of this experiment is the dependence of the relative deviations for the slopes on the equivalent capacitance. How might you account for that? How does the concept of the time constant tie your results from configurations 4-8 together?

Reading Assignments

Over the course of the first seven weeks of the quarter, you are to read the following original scientific papers describing some more of the most important experimental results in physics in the last quarter of the 20th century. The links provided below should allow you to download the papers in PDF format from anywhere.

For each paper, do the following:

  1. Read it once to get the general idea of the paper, looking up terms and notation as directed below.
  2. Read it again and write down an outline of the paper, including the purpose and main idea of each part. Identify the paragraphs (by number) which belong to each part of your outline.
  3. In this (and last quarter's) laboratory project guide, there are a number of words and terms that appear in bold. These represent important concepts in experimental physics. Read the paper once more, identifying which of those terms appear in the paper, and noting in which section and context.
  4. Submit your outlines at the beginning of class the 4th and 8th weeks, respectively. Include your definitions for the terms you had to research.
    Do not simply cut and paste definitions from the web; write them in your own words!
The papers for this quarter are:

What are the elements common to both of these papers, and to those you read last quarter? Your project report will be expected to include these elements.


2007, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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