Lab 6:

Circuit Fundamentals --
Electric Potential and Electric Current

Physics 204, Daniel A. Martens Yaverbaum

John Jay College of Criminal Justice, the CUNY

  Necessary Equipment:

    1. DC Power Supply ("Rectifier").

    2. One Prototyping Circuit "Breadboard".

    3.  Two (2) Incandescent Light Bulbs

    4. Assorted ("Jumper") Wire and ("Alligator") Clips

    5. Assorted Carbon Resistors

    6.  Digital Multi-Meter

        Ultimate Objectives:

    1. To grow familiar with the fundamental operating principles for circuit-building.

    2. To grow familiar with the basic techniques for taking measurements in a circuit.

    3. To discover and test the distinction between series and parallel configurations.

   4. To not get hurt.

One Possible/Suggested Research Question (with six related parts)

What is the quantitative relationship between the currents passing through two electrical devices that are connected to each other in series?
What is the quantitative relationship between the potential differences across two electrical devices connected to each other in series?
What is the quantitative relationship between the currents passing through two electrical devices that are connected to each other in parallel?
What is the quantitative relationship between the potential differences across two electrical devices connected to each other in parallel?
How can these quantitative relationships be understood in terms of the basic definitions of current, potential difference, series and parallel?
What is the fundamental relationship among the current, potential difference and resistance between any two locations in an electric circuit?

       Methods: Data Collection:

I.    SERIES.

    A. Using your power supply, wires,  bulbs AND ONE RESISTOR of your choice:  Design and build a circuit that causes TWO bulbs to light according to the following requirements:

            (1). Two two devices can be connected directly to each other.  Every time you attach a wire from a device, it must go into the breadboard.  Then, another wire must come out of the breadboard and go to the next device.

            (2). If one light bulb is unscrewed from its base and thereby turned off, THE OTHER LIGHT BULB MUST also "automatically" TURN OFF.

            Test that this works for either/both bulbs.  That is, test that they are dependent on each other.  Informally experiment with different resistors until you find one that allows for the most vivid (observable) results.

    B. Once you have created a situation such that one light bulb dies whenever the other is killed, you have successfully accomplished this:

            THESE TWO BULBS ARE IN "SERIES" WITH EACH OTHER.

AFTER YOU HAVE SUCCEEDED AT ACCOMPLISHING THE TWO ABOVE GOALS, GO BACK AND ACHIEVE PRECISELY THE SAME EFFECTS, BUT THIS TIME:

USE THE BREADBOARD IN ORDER TO MAKE EACH CONNECTION.
THAT IS, FIGURE OUT THE PATTERN OF CONDUCTING STRIPS THAT LIE UNDERNEATH THE WHITE GRID ON THE BOARD --
AND THAT ALLOW YOU TO CONNECT ALL DEVICES SECURELY YET TEMPORARILY.

C. Draw a complete and labeled picture of your circuit set-up. This should NOT be a standard "Circuit Diagram"; it should NOT be a schematic. It should be a physically detailed image of your physical set-up -- with all the relevant equipment and connections involved. If done properly, this picture will tell somebody how to reproduce your circuit research -- even if they are entirely unfamilar with the topic of electricity.

    D. Now using standard symbols (found in your text, on the web and quite possibly on the whiteboard of your lab), draw a CIRCUIT DIAGRAM for this circuit. This will distill and depict the information essential to your circuit.

    E. Familiarize yourself with the  digital multi-meter:

            One probe (wire, generally black)  must be inserted in the black socket marked "Com" (for "common" which means the same thing as "ground").
            The other probe (wire, generally red) will be inserted into a different socket.  The choice of socket depends on what you are measuring.  If you are measuring Volts of Potential or Ohms  of Resistance (abbreviated with the greek letter 'Omega'), that's one socket.  If you are measuring Amperes  of Current, that's one of two other sockets.  How do you know which one? The same way you determine where to point the dial of the meter before taking a measurement:

            The fundamental idea is that different scales for measuring equipment are appropriate for different scales of the quantities to be measured: A grape would not register ANYTHING on a scale (or balance) designed for trucks, while a truck would "peg" (break, overload, register infinity, or otherwise produce no meaningfulread-out) on a scale designed for small vegetables.  So you always need to choose a scale roughly on the order of the quantities you are roughly expecting.

            When in doubt, start with the highest possible scale (so that nothing breaks) and work your way down until you find a meaningful reading.

            F. Using this technique,  find and write down the CURRENTrunning through each of the devices:

        *** F1: Current through Bulb #1: ___________________ mA.

        *** F2: Current through Bulb #2: __________________ mA.

        *** F3: Current through Resistor: ___________________ mA.

        *** F4: Current through Battery: ___________________ mA.

                                    PLEASE NOTE: Once you have directly measured current readings for some devices, you might feel that you can infer current readings for others--without directly measuring.

                                    You might indeed be right.  If you have good reason for writing down certain values without measuring, then do so.  Just be prepared to justify.

    *** G. Which values did you to write without directly measuring? Why? Can you test these values just to make sure?

    *** H. How does your final current values relate to one other? Provide a plausible explanation for the relationship between the four current values you obtained.

    I. Using the meter, find and write down the POTENTIAL DIFFERENCE ("Potential Drop" or "Voltage Drop") across each of the devices:

    *** I1: Potential across Bulb #1: ____________________ V.

    *** I2: Potential across Bulb #2: ___________________ V.

    *** I3: Potential across Resistor: _____________________ V.

    *** I4: Potential across Battery: _____________________ V.

                                    PLEASE NOTE: Once you have directly measured potential readings for some, you might feel that you can infer potential readings for others--without directly measuring.

                                    You might indeed be right.  If you have good reason for writing down certain values without measuring, then do so.  Just be prepared to justify.

    *** K. Which values did you to write without directly measuring? Why? Please test these values just to make sure.

    *** L.  How does these potential values relate to one other? Provide a plausible explanation for the relationship among the four potential values you obtained.

II.    PARALLEL.

    A. Using your power supply, breadboard, wires,  bulbs AND TWO RESISTORS of your choice:  Design and build a circuit that causes TWO bulbs to light according to the following requirement:

            (1). Two two devices can be connected directly to each other.  Every time you attach a wire from a device, it must go into the breadboard.  Then, another wire must come out of the breadboard and go to the next device.

            (2) If one light bulb is unscrewed from its base and thereby turned off, THE OTHER LIGHT BULB STAYS ON!

            Test that this works for either/both bulbs.  That is, test that they are NOT dependent on each other.

    B. Once you have created a situation such that one light bulb SURVIVES even when the other is killed, you have successfully accomplished this:

            THESE TWO BULBS ARE IN PARALLEL WITH EACH OTHER.

*** C. Draw a complete and labeled picture of your circuit set-up. This should NOT be a standard "Circuit Diagram"; it should NOT be a schematic. It should be a physically detailed image of your physical set-up -- with all the relevant equipment and connections involved. If done properly, this picture will tell somebody how to reproduce your circuit research -- even if they are entirely unfamilar with the topic of electricity.

    *** D. Now using standard symbols (found in your text, on the web and quite possibly on the whiteboard of your lab), draw a CIRCUIT DIAGRAM for this circuit. This will distill and depict the information essential to your circuit.

*** E. Using standard symbols (depicted on chalkboard), draw a CIRCUIT DIAGRAM for this circuit.

    F. Using the digital multi-meter, find and write down the CURRENTrunning through each of the devices:

        *** F1: Current through Bulb #1: ___________________ mA.

        *** F2: Current through Bulb #2: __________________ mA.

        *** F3: Current through Resistor: ___________________ mA.

        *** F4: Current through Battery: ___________________ mA.

        *** F5: ALL appropriate UNCERTAINTIES: Absolute AND Fractional.  Compile all the data into a table.

PLEASE NOTE: Once you have directly measured current readings for some, you might feel that you can infer current readings for others--without directly measuring.

                                    You might indeed be right.  If you have good reason for writing down certain values without measuring, then do so.  Just be prepared to justify.

    *** G. Which values did you to write without directly measuring? Why? Can you test these values just to make sure?

    *** H. How does your final current values relate to one other? Provide a plausible explanation for the relationship between the four current values you obtained.

    I. Using the meter, find and write down the POTENTIAL DIFFERENCE ("Potential Drop" or "Voltage Drop") across each of the devices:

    *** I1: Potential across Bulb #1: ____________________ V.

    *** I2: Potential across Bulb #2: ___________________ V.

    *** I3: Potential across Resistor: _____________________ V.

    *** I4: Potential across Battery: _____________________ V.

    *** I5: ALL appropriate UNCERTAINTIES: Absolute AND Fractional.  Compile all the data into a table

                                    PLEASE NOTE: Once you have directly measured potential readings for some devices, you might feel that you can infer potential readings for others--without directly measuring.

                                    You might indeed be right.  If you have good reason for writing down certain values without measuring, then do so.  Just be prepared to justify.

    *** J. Which values did you to write without directly measuring? Why? Please test these values just to make sure.

    *** K.  How does these potential values relate to one other? Provide a plausible explanation for the relationship among the four potential values you obtained.

III. GENERALIZING.

    Complete the following sentences:

*** A. Two devices connected in series must yield identical values whenever  ______________________ is measured.

        Given the definitions of potential, current and resistance, the physical/logical reason for this is that :____________________________ ...  (provide as many complete English sentences as you feel necessary).

*** B. Two devices connected in parallel must yield identical values whenever ______________________ is measured.

        Given the definitions of potential, current and resistance, the physical/logical reason for this is that :____________________________ ...  (provide as many complete English sentences as you feel necessary).

*** IV. SUPPLEMENTAL PROBLEM.

    You are given one 9 Volt battery, two 30 ohm resistors, two 100 ohm resistors, and two 500 ohm resistors.

    Your goal is to create a circuit loop through which precisely 25 milliAmperes of current flows.

    In a complete and coherent page, explain what you would do and why.

        a) Include a clear circuit diagram.

        b) Include all relevant quantities and calculations.

        c) Determine and explain the amount of current going through every single resistor you use.  You may label your diagram with letters at relevant points if helpful.

        d) Provide a minimum of three - five (3 - 5) complete sentences explaining the reasoning behind your circuit design.

        e) Imagine that each of your resistors is, in fact, a light bulb.  Provide a minimum of two (2) complete sentences explaining what advantages and disadvantages your circuit would have--as compared to all six light bulbs being connected in series.