Lab Experiment No. 3 Ohm’s Law
I. Introduction
In this lab exercise, you will learn –
• how to connect the DMM to network elements,
• how to generate a VI plot,
• the verification of Ohm’s law, and
• the calculation of element power.
II. Experiment Procedure
Schematic diagrams for resistive networks N1 through N5 are shown in Figures 1 through 5 on the following pages. Current directions for each element are shown with line arrows. The actual element connections are also shown. The correct way to connect the DMM as an ammeter (AM) and as a voltmeter (VM) is shown in Figure 1(c) for reference.
(a) Resistor VI plot. In network N1, the 10KΩ resistor R1 is connected to the Agilent E3620A power supply. The supply voltage V1 is to be varied from 0 volts to 20 volts with the voltage steps shown in Table 1.
i. Measure and record the value of R1. Place the value in Table 1 where indicated.
ii. Use the digital multi-meter (DMM) to measure the voltage across and the current through R1 for each value of V1. Record these measurements in Table 1 where indicated.
iii. Use Excel to generate a graph of VR1 (linear scale vertical axis) plotted against IR1 (linear scale horizontal axis). Calculate the value of the slope of this plot and compare to the measured value of R1. Calculate the difference in percent (DiffR1) between these two values with the measured value as the base. Record these values in Table 1 where indicated.
(b) Verification of Ohm’s law. Networks N2 through N5 contain various combinations of resistors and voltage sources. Data tables are provided for each network.
i. For each network, use the digital multi-meter (DMM) to measure the voltage across and the current through each element (dc voltage sources and resistors), and the value of each resistor. Record these measurements in the tables where indicated. Again, the correct way to connect the DMM as an ammeter (AM) and as a voltmeter (VM) is shown in Figure 1(c).
ii. Verify the validity of Ohm’s law by calculating each resistor current from its measured voltage and the measured value of its resistance. That is, from Ohm’s law,
(1)
where VRi(meas) is the voltage measured across resistor Ri in volts (V), Ri(meas) is the measured value of Ri’s resistance in ohms (Ω), and IRi(calc) is the calculated value in amps (A) of the current through Ri. Record these calculated values in the tables where indicated.
iii. Verify the accuracy of Ohm’s law by calculating the percent difference (DiffI) between the measured resistor current (IRi(meas)) and calculated current (IRi(calc)) with the measured value as the base. In other words
(2)
Record these differences in the tables where indicated.
iv. Calculate the power dissipated by each resistor and delivered to or from each voltage source. The power in Watts (W) delivered to a network element e is computed from
(3)
where Ve is the voltage drop across e, Ie is the current through e, and Pe is the power delivered to the element. If Pe is negative, power is delivered from the element to the network. Calculate Pe using measured variables. Record these powers in the tables where indicated.
III. Lab Report
The report for this lab experiment must be word-processed and contain the following items –
• Title Page.
• Introduction.
• Procedure.
• Results.
• Discussions.
(a) Suggest useful applications for Ohm’s law as studied in this experiment.
• Conclusion.
(a) Are all measured and calculated currents within resistor tolerance? List those that are not.
(b) Explain how resistor variations produce differences between measured and calculated currents.
(c) Which method of determining resistor currents (measurement versus calculation) yields more accurate results? Explain.
(d) Which method is more convenient? Explain.
(e) Explain how you would convince your boss (via a sales pitch) to use on method over the other. Strengthen your sales pitch with solid engineering practice and mathematical reasoning.
• Appendix.
• References.
IV. Resistive Networks
1. Network N1.
Figure 1
(a) Network N1
(b) Component connections
(c) DMM connections
Table 1Measured variables from N1
V1 (V) / VR1 (V) / IR1 (A)
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
R1(meas)
(Ω) / Slope of VI plot
(Ω) / DiffR1
(%)
2. Network N2.
Figure 2
(a) Network N2
(b) Component connections
Table 2N2 measured and calculated variables
Element / Specified
value / Measure
value / Ve(meas)
(V) / Ie(meas)
(A) / Ie(calc)
(A) / DiffI
(%) / Pe
(W)
R1 / 1KΩ
R2 / 2KΩ
R3 / 3KΩ
V1 / 9V / N/A / N/A
3. Network N3.
Figure 3
(a) Network N3
(b) Component connections
Table 3N3 measured and calculated variables
Element / Specified
value / Measure
value / Ve(meas)
(V) / Ie(meas)
(A) / Ie(calc)
(A) / DiffI
(%) / Pe
(W)
R1 / 300KΩ
R2 / 150KΩ
R3 / 120KΩ
V1 / 5V / N/A / N/A
4. Network N4.
Figure 4
(a) Network N4
(b) Component connections
Table 4N4 measured and calculated variables
Element / Specified
value / Measure
value / Ve(meas)
(V) / Ie(meas)
(A) / Ie(calc)
(A) / DiffI
(%) / Pe
(W)
R1 / 47KΩ
R2 / 20KΩ
R3 / 100KΩ
V1 / 3V / N/A / N/A
V2 / 5V / N/A / N/A
5. Network N5.
Figure 5
(a) Network N5
(b) Component connections
Table 5N5 measured and calculated variables
Element / Specified
value / Measure
value / Ve(meas)
(V) / Ie(meas)
(A) / Ie(calc)
(A) / DiffI
(%) / Pe
(W)
R1 / 10KΩ
R2 / 30KΩ
R3 / 3KΩ
V1 / 10V / N/A / N/A
V2 / 15V / N/A / N/A