Physics 104
Ohm's Law

Introduction
In this experiment you will become familiar with some basic circuit elements and electronic measurement instrumentation through investigation of Ohm's Law, using both direct current and alternating current circuits.

Equipment

Procedure
1. DC Ohm's Law - Inspect the circuit that has been set up on your breadboard. Before you turn anything on, sketch a schematic diagram of the circuit in your lab notebook. Follow the path you expect positive current to flow in the circuit and be sure this makes sense to you. If not, consult the instructor. Note that one DMM will measure the voltage drop across the resistor, and the other will measure the current through the resistor. Your resistor should have a value of approximately 2 kOhms.

Now set the appropriate DMM to read DC current in milliAmperes (1 mA = 0.001 A), set the other DMM to read DC volts, and turn on the green voltage supply, making sure that the knob is initially fully counterclockwise (at 0 V). Slowly turn the knob CW, and note that current is flowing. Consult with the instructor if you have any problems.

Slowly turn up the voltage, recording the current through the resistor as a function of the voltage across the resistor for voltage values of approximately 1 V, 2 V, 3 V, ... up to 10 V. Note that you don't have to set the voltage exactly at 1.00 V, etc., but you should record its value to three significant figures. Enter the data into a table in your lab notebook. Pay close attention to the scale marking on the meters! You could also directly record the data in Excel. At the 10 V setting, lightly touch the body of the resistor and record your observations.

2. AC Ohm's Law - Swap your function generator (FG) for the green transistor power supply, without changing anything else in your circuit. Set the DMM used to monitor voltage to "AC," and do the same for the current DMM.

Set the FG frequency to 100 Hz, and record the AC voltage drop across the resistor the FG source voltage is varied from 0.5 Volts to 5.0 Volts, in increments of 0.5 Volts. Use the "Vpp" key to set these voltages.  Note that you may have to adjust the DMM scales to get precise readings.

3. Series and Parallel Resistors - A second resistor is on your breadboard. Use the resistance function of the DMM to measure the equivalent resistance of this pair in series and in parallel. Record your results. Do this by setting up the resistors on the breadboard in an appropriate way and disconnecting them from any voltage source. Also measure the individual resistances of the two resistors.

4. Voltage Divider - Now set up the same circuit as in part 2 above, but use your series combination of resistors. Set the FG to a 1 kHz, 5 V amplitude wave and measure the voltage drop across each resistor. Sketch the circuit in your lab notebook.

5. Voltage of FG - Keep the FG settings at 1 kHz and 5 V amplitude. Now plug the FG into the oscilloscope and measure the amplitude of the wave produced by the FG.

6. Speaker Impedance - Measure the resistance of the loudspeaker at your station with the DMM in the same way you measured the resistors in number 3 above.

Analysis
1 & 2. Ohm's Law - Make two plots of the voltage drop across the resistor vs. the current through it, for both the AC and DC circuits. It's best to put voltage on the vertical axis and current on the horizontal axis. Determine the slope of the line from the graph by inserting a trendline using Excel, and choosing the option to display the trendline parameters on the graph. Attach both graphs to pages in your lab notebook. Compare the trendline slopes to what you expect, based on the resistor value.

3. Series and Parallel Resistors - Compare the measured values of the resistance for the series and parallel combinations with the theoretical values, calculated using the relations given in class, and comment.

4. Voltage Divider - Do the voltage drops across each resistor add up to the voltage drop across the FG? Compare the drop across each resistor to what you expect based upon the equation discussed in class.

5. Voltage of FG - Explain why the voltage drop you measured across the FG using the DMM is not the same as the amplitude of the wave produced by the FG. What voltage is the DMM displaying when it measures an AC Voltage?

Conclusion
Summarize your results, emphasizing what you've learned and how your experimental results compare to theoretical expectations. Discuss any substantial deviations from expected behavior.

Please be sure that all equipment is turned off before you leave. Thank you!