UNIVERSITY OF MASSACHUSETTS DARTMOUTHCOLLEGE OF ENGINEERING

EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I

OHM'S LAW

ObjectiveThe objective of this lab is to demonstrate Ohm’s Law and practice making voltage and

current measurements.

BackgroundSince you will need to take voltage and current measurements on your circuit, you need

to learn to do this correctly.

Voltage MeasurementTo 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 Approach1. 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

(Volts)Current

(mA)

Power

(mW)

(across R1)

(through R1)

P1

(Resistor R1)

Theory MultiSim Measured Theory MultiSim Measured Theory MultiSim

Resistor R1 ( )

Nominal DMM

Part 2: Simulation Approach1. 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.

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.

Ground terminal

+ Terminal: Any voltage from 0 V to

+12 V relative to Ground

Power Switch

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 , in increments of 1 Volt. Use the DMM to set

the voltage close to 1 V, 2 V, 3 V etc. In other words, it is quite all right to have the

DMM reads 0.98 V when you try to set the voltage to 1 V.

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

Measurements

Current recorded

by ammeter

(A)

Voltage recorded by

DMM

(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 ? In other words, does ? 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.