UNIVERSITY OF MASSACHUSETTS DARTMOUTH COLLEGE OF ENGINEERING EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
OHM'S LAW
Objective The objective of this lab is to demonstrate Ohm’s Law and practice making voltage and current measurements.
Background Since you will need to take voltage and current measurements on your circuit, you need to learn to do this correctly.
Voltage Measurement To measure voltage, 1) set your DMM on Volts, at a range setting greater than the voltage you expect to read; and 2) Put the + (red) probe on the positive side of the device you’re trying to measure and put the – (black) probe on the negative side (if you get a negative number that means the potential at the black probe is greater than the potential at the red probe). Refer to the photo below for an example of the proper way to measure the voltage across a resistor (the middle one). Current Measurement
To measure current, you will use an analog ammeter (continuous motion of a needle). 1) Turn off the system; and 2) put the meter in the current flow path (i.e. in series) with the other elements in the circuit. That means “breaking” the circuit between two elements and inserting the meter between them. Refer to the photo below for an example of the proper way to measure the current that is flowing through the meter. If the meter reading dial goes to the left, reverse the probes.
EMBED PBrush It’s essential that you let the current (charges) go through the ammeter in order to measure current. Think of cars on a highway that must go through a tollbooth that counts the rate of cars passing through. NEVER EVER connect the ammeter as you connect a voltmeter. That is, NEVER EVER measure the current by placing probes “across” the element! Part 1: Theoretical Approach 1. Using the nominal value as shown in the schematic below, calculate the current through and the voltage across the resistor. Tabulate your results by creating in EXCEL your own table similar to that shown below and entering your results in the “Theory” columns. Document all of your work electronically. CIRCUIT
2. Calculate the power dissipated by the resistor and report its value in “Theory” columns of the table.
Circuit Voltage Current Power (Volts) (mA) (mW) V I 1 1 P1
(across R1) (through R1) (Resistor R1)
Theory MultiSim Measured Theory MultiSim Measured Theory MultiSim
Resistor R1 ( W )
Nominal DMM
Part 2: Simulation Approach 1. Find your correct resistor by determining the color code. 2. Measure the resistance using the DMM, and report its value in the table. 3. Construct the circuit in MultiSim using the actual resistance as measured by the DMM. Simulate the circuit by using virtual Ammeters and Voltmeters to measure the current through and the voltage across the resistor. Report your data in the table appropriately in the MultiSim columns. 4. Print the MultiSim circuit with the ammeter and voltmeter values displayed and your team number and names printed on the schematic. 5. Use the Multisim Wattmeter to measure the power absorbed by the resistor. To use the wattmeter, connect the voltage portion where the voltmeter had been and connect the current portion where the ammeter had been. Record the reading in the table.
Part 3: Experimental Approach using the powered prototyping board You will use a prototyping breadboard, which has its own DC power supplies.
Ground terminal Power Switch
+ Terminal: Any voltage from 0 V to +12 V relative to Ground
Important information about the breadboard: The breadboard allows you to provide a constant voltage to your circuit, like a battery. Unlike a battery though, you can adjust the voltage to be exactly what you need. When the breadboard is plugged in and turned on, a voltage exists between the ground terminal and each of the voltage terminals. The ground terminal is also the negative terminal of the supply. You will construct the resistor circuit shown above on your breadboard, using the adjustable +12 V supply terminal and ground (you will need to connect wires from each end of the supply to your resistor).
1. BEFORE you connect wires to the supply terminals, use your DMM to make sure the voltage is set correctly for the circuit. 2. Build the resistor portion of the circuit on the breadboard neatly. 3. Before connecting the power supply to the resistor, demonstrate to the TA or instructor how you plan to measure voltage across the resistor and the current through it. 4. Apply the specified voltage using the power supply.
5. Measure the voltage across R1 using the DMM. 6. Measure the current using the Ammeter. – DO NOT use the DMM to measure current. 7. Calculate the power absorbed by the resistor based on your voltage and current measurements. 8. Report your measurements in the table appropriately.
Part 4: Experimental Approach using the powered prototyping board
Vary the voltage in the range 0 1. Record the voltage as read by the DMM in (V) and the current as read by the ammeter in (A). 2. Enter your measured values in Excel, as shown below: 3. Plot the voltage versus current in Excel. 4. Determine the slope from the graph. Number of Current recorded Voltage recorded by Measurements by ammeter DMM (A) (V) 1st 2nd ⋮ ⋮ 10th Deliverables Each team should hand in one printout of each table, your Multisim schematic with meter readings displayed, the graph created in EXCEL along with any hand calculations. In addition, answer the following questions: 1. Is Ohm’s law satisfied for resistor R1 ? In other words, does V1= I 1 R 1 ? Show your work. 2. How well do the MultiSim values compare with your theoretical calculations? 3. How well do the measured values compare with your theoretical calculations? 4. Based on your results, what is the power delivered by the supply and what is the relationship between it and the power absorbed by the resistor? 5. What is the physical meaning of the slope if part 4? Explain.