Physic II Lab M3A1 Experiment: DC Circuits
PART I – Current, Electrical Potential and Resistance
· Download the following file, to your desktop by:
· right-clicking on M3-Activity-1 : File attached
· selecting Save target as…
· saving the file to your computer where you can easily access it again
This file will be used within the lab simulation.
Start the simulation “Circuit Construction Kit (DC Only) ” (if you haven’t done so already) by clicking on the image below. : http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-dc_en.jnlp
The simulation has a set of controls on the right hand side of the screen. These are divided vertically into different areas. Make the selections indicated.
· From “Circuit,” select load.
· When prompted, upload the file M3-Activity-1.
· From “Visual,” select “Life Like.”
· From “Tools,” select noncontact ammeter.
· Position the voltmeter and ammeter as shown in the image below.
http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-dc_en.jnlp
Image retrieved from the PHET “Circuit Construction Kit (DC Only)” simulation.
Inside your notebook, draw a graph with potential (V) on the vertical axis and current (I) on the horizontal axis. Use Ohm’s Law to make a numerical prediction of the shape that this graph will take. The default values are 9.0 V for the battery, and 10.0 Ohms for the resistor, but you will want to right click on both components to verify this. Plot at least 5 prediction points, and sketch in the graph in your notebook. Now use the Circuit Construction Kit to measure the relationship between battery potential and current.
· Leave the resistor value constant.
· Record the battery voltage and the resulting current in the circuit for the default values.
· Right click on the battery, and change the voltage.
· Click on the “Run” arrow to start the simulation.
· Record the new values of potential and current.
· Take at least 5 data points.
How does the graph produced through your simulation data compare to the predicted graph in your notebook? Continue your investigation. Can you find any factors that might explain any discrepancies? Try turning on the voltmeter and measuring the battery terminal voltage as shown below. Does the battery terminal voltage match the voltage you set using the simulation controls?
Describe in detail any changes that need to be made to the procedure in order to make the predicted graph, so that it will reflect results of the simulation. Use your revised procedure, and place both graphs in your lab report. Be sure to provide a discussion of the differences between a real source of EMF V.S. and an ideal source of EMF.
Part II – Kirchhoff’s Rules for Current and Potential
Your basic mathematical tools for analyzing a circuit are Ohm’s Law, and Kirchoff’s loop rule for potentials and junction rule for currents. We are going to use this activity to gain confidence in the accuracy of these latter two rules, and also to give you some practice in applying these rules.
· Download the following file, to your desktop by:
· right-clicking on M3-Activity-2 : File attached
· selecting Save target as…
· saving the file to your computer where you can easily access it again
· Load the circuit file M3-Activity2 (as you did for M3-Activity1 in Part I).
· Activate the voltmeter.
When the simulation starts, the switch in the circuit is open. There is no conductive path, so the far right left branch of the circuit is not active. Initially, we will only concern ourselves with the portion of the circuit containing batteries and the middle branch.
The loop rule states that the sum of changes in voltage around any closed path in a circuit is zero. In this activity, you are going to use the voltmeter to follow a closed path in the circuit provided. You will record the change in voltage across every component, and then take the sum of these changes.
· Draw the circuit in your laboratory notebook.
· Draw arrows to indicate a sense of direction through the circuit.
· Traverse the loop of the circuit with the voltmeter, measuring the change in potential across each component.
Tip: If you start with the red probe ahead of the black probe make sure this orientation is maintained throughout the data taking. Why might this be important? Record the results in your notebook.
When you close the switch, you will have three circuit loops. The second is obvious, but the third conductive loop may take a moment or two to identify. (Hint: A conducting loop is not required to have a battery.)
· Close the switch.
· Repeat the analysis with all three circuit loops.
How were the potentials across the components in the first loop affected by closing the switch? How might the interior lights of your car be affected by turning on the radio, or engaging the starter motor?
The junction rule states that the algebraic sum of all currents entering or leaving a junction is zero. This is actually just a restatement of the conservation of charge in that a wire junction cannot store charge. Designate the top junction as junction 1.
· Measure the magnitude of all currents entering or leaving the junction.
· Assign a “+” algebraic sign to all currents leaving a junction.
· Assign a “-” algebraic sign to all currents entering a junction.
Be sure to record your observations and analysis in your laboratory notebook. Does your analysis support or fail to support the Kirchhoff’s loop and junction rules?
PART III – The Equivalent Resistance of Series and Parallel Resistors
Use the Circuit Construction Kit to create a circuit with at least three series resistors, or load file M3-Activity-3: Download the following file, to your desktop by:
· right-clicking on M3-Activity-3 : File attached
· selecting Save target as…
· saving the file to your computer where you can easily access it again
Use different values for each resistor. Calculate the equivalent resistance of the system using the series rule for resistors.
Describe in detail how you will measure the equivalent resistance of the entire network of resistors. Where will you place the red and black probes of the voltmeter? Where will you place the noncontact ammeter?
Record your data in your laboratory notebook, and perform your analysis. Do your results support or fail to support the series rule for equivalent resistance?
A single loop circuit with three series resistors. Note the position of the voltmeter and the ammeter. (Image retrieved from the PHET “Circuit Construction Kit (DC Only)” simulation).
Use the Circuit Construction Kit to create a circuit with at least three parallel resistors, or load file M3-Activity-4: Download the following file, to your desktop by:
· right-clicking on M3-Activity-4 : File attached
· selecting Save target as…
· saving the file to your computer where you can easily access it again
Use different values for each resistor. Calculate the equivalent resistance of the system using the parallel rule for resistors.
Describe in detail how you will measure the relevant voltages and currents. Record your data in your laboratory notebook and perform your analysis. Do your results support or fail to support the parallel rule for equivalent resistance?
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