Name ________________________________
Partner(s): ________________________________
Experiment 3 – Ohm’s Law and Resistance
Objectives
To demonstrate how Ohm’s Law is used to define resistance.
Equipment
1 DC Regulated Power Supply: a source of variable voltage
1 Resistor (different values for different groups)
Wires with alligator clips and banana ends
2 Digital Multimeters per group
Preparation
You will be pressed for time during the lab. Since successful completion of all lab
activities counts towards your final lab grade it will be important to be well
prepared by doing Pre-Lab assignments and reading the entire lab before
attending the lab.
Pre-Lab
Read the Pre-Lab introduction and answer the accompanying questions and
problems before this Lab.
Points earned today
Pre-Lab
____
Lab
____
Total
____
Instructor Initials
____
Date
____
Physics 1201
III-1
Pre-Lab for LAB 3
A Short Description of the Meters used for Electrical Measurements
An ideal meter should not influence the circuit on which it is used, but a real meter does
influence the behavior of a circuit. A good meter is designed to influence the circuit as little as
possible.
A voltmeter is used to measure the voltage V across a battery, resistor, or other circuit
component while the circuit is in operation. An ideal voltmeter has infinite resistance, and
therefore would draw no current. Real voltmeters have large but finite resistances of typically
several x 104 ohms to several x 105 ohms.
An ammeter is used to measure the current I in some connecting wire in a circuit while the
circuit is in operation. An ideal ammeter has zero resistance, so that there is no voltage drop IR
across it no matter how high I is. Real ammeters have small resistance, typical resistances of a
few x10−3 ohms to a few x 10−1 ohms, and thus behave like a piece of connecting wire. Never
connect an ammeter directly across a battery or
other circuit component because it will shortcircuit the component, and possibly blow a fuse
inside the ammeter.
An ohmmeter is a packaged combination of a
battery of known voltage and an ammeter with
a specially-calibrated scale.
You use an
ohmmeter by connecting it across a resistor that
is not connected to anything else.
The
ohmmeter has its own battery, and thus may
disturb any circuit connected to it.
A multimeter is a multipurpose instrument that
can serve as a voltmeter or ammeter for DC or
AC, or as an ohmmeter for measuring
resistance. On a multimeter (see the Figure
below), the function is selected by a rotary
switch in the instrument. One input terminal
(COM) on the multimeter is common to all
functions (that is, you always connect one wire
there). There are separate second terminals for
current and voltage measurement.
2.30
Display
Ω
mA
V
Rotary
Switch
A
10A max mA °C
COM
Hz V Ω
Terminals
Figure. Multimeter.
For this lab you will NOT be using the AC capability of the instrument (the switch positions
labeled V~ and A~).
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Physics 1201
Pre-Lab for LAB 3
Problems
1. A resistor carries 0.40 A when connected to a 9.0-V battery. What is the
resistance of the resistor?
2. An incandescent light bulb is rated at a power of 60 W when connected to a
120-V source.
a) What current passes through the light bulb when it is connected to a 120V source?
b) What current would pass through the light bulb when it is connected to a
250-V source?
3.
Physics 1201
A circuit consists of a resistor connected to a battery. Draw below three
pictures of this circuit showing:
1. the circuit alone.
2. the circuit with an ammeter connected to measure the current in the
resistor.
3. the circuit with a voltmeter connected to measure the voltage across in
the resistor.
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Laboratory
Activity 1
Limit the current the power source can provide to protect the multimeter.
When the multimeter is used as an ammeter, a fuse is needed to protect its circuitry:
current over 600 mA (milliamperes) will blow this internal fuse. To avoid replacing the
fuse, you can limit the current the power source can provide.
To limit the current, do the following:
1) With the power source OFF, turn both VOLTAGE and CURRENT knobs to their
full counterclockwise positions.
2) Connect a wire lead to the (+) and (–) terminals of the power source (see the diagram
below): this “shorts” the power source – there is no danger, there is no voltage to
“short” yet.
“Shorting” the power supply
3) Turn the power source ON: the VOLT and AMP.: the displays should both read
0.00.
4) Rotate the VOLTAGE knob about half a turn clockwise, then slowly turn the
CURRENT knob clockwise until the AMP. display reads about 0.25 – a small LED
(next to the words “CURRENT LIMITED”) should glow red. If the LED glows red, you
have now limited the current the power source can supply to (about) 0.25 A = 250
mA!
5) Apply a piece of tape to the knob to prevent the knob from turning causing the
setting to change.
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Physics 1201
Activity 2
Learn to measure current, voltage, and resistance
In this Activity, first you will learn how to measure voltage, current, and resistance
using a multimeter as a voltmeter, ammeter, or ohmmeter. Later you will make
measurements of voltage and current in a resistor and investigate the mathematical
relationship between them.
Part 1
How to Use the Multimeter to Measure Current and Voltage
The multimeter has several terminals and dial settings.
terminals and dial setting to use to make measurements.
You need to know which
1. Your instructor will show how to use the multimeter as a voltmeter, ammeter, or
ohmmeter. In the figures below, make a drawing for each type of meter showing the
position of the rotary switch and where leads are inserted into the terminals of the
multimeter. Be clear in your drawings because you can use these later as a reference.
Note: one lead always is inserted in the terminal marked COM (common).
To use the multimeter as a
voltmeter:
Ω
Physics 1201
Ω
mA
V
10A max mA °C
To use the multimeter as an
ammeter:
A
COM
Hz V Ω
Ω
mA
V
10A max mA °C
To use the multimeter as an
ohmmeter:
A
COM
Hz V Ω
mA
V
10A max mA °C
A
COM
Hz V Ω
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2. Your instructor will next demonstrate how to attach leads from a meter to the
components of a circuit to measure voltage, current, or resistance. Make a record
showing how the leads (wires) are connected to the components (parts) of the circuit by
completing the diagrams below. Be clear in your drawing because you will use these
diagrams later as a reference.
Use these symbols in your drawings.
Voltmeter: To measure the voltage across a
resistor in a circuit, attach the voltmeter’s
leads on each side of the resistor.
Voltmeter: To measure the voltage across a
battery in a circuit, attach the voltmeter’s
leads on each side of the battery.
Ammeter: To measure the current entering or leaving a resistor in a circuit, direct the
current from the circuit into the ammeter and then back to the circuit.
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Physics 1201
Ohmmeter: To measure the resistance of a resistor, attach the ohmmeter’s leads on each
side of the resistor. The resistor must not be connected to anything else!
Have your instructor check your progress at this point.
Part 2 Measure the resistance of a resistor.
Use the multimeter to measure the resistance of the resistor, which is fixed in a holder,
which you will use in Activity 3. Record its resistance here:
Resistance =
Physics 1201
III-7
Activity 3
Construct a Simple Circuit and Attach Meters to Measure Current and
Voltage in a Resistor
1. Before you proceed, make sure the DC Regulated Power Supply is turned OFF.
2. Examine the control knobs and displays of the DC Regulated Power Supply. You
will use a DC Regulated Power Supply as a battery (voltage source) for your circuit.
The voltage of this power source can be varied, unlike the voltage of a regular
battery; also, the current from this power source can be limited to protect the
equipment connected to it and to the circuit.
A) Note that the power source has two control knobs:
VOLTAGE: sets an upper limit on the voltage provided by the power source.
CURRENT: sets an upper limit on the current provided by the power source.
B) Note that the power source has two meter displays:
VOLT: displays the voltage in volts (V) provided by the power source.
AMP.: displays the current in amperes (A) provided by the power source.
3. Pre-set the voltage to zero on the power supply. Turn the VOLTAGE knob to its full
counterclockwise position: this will set the voltage so that it will be zero when the
power supply is turned ON.
4. Study the figure and diagram of the circuit shown below. At left the figure shows
how the leads (wires) are to be connected to the components (parts) of the circuit.
The components consist of a resistor, which is fixed in a holder, and a battery, which
is the DC Regulated Power Supply. At right is the diagram of the circuit.
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Physics 1201
5. Study the figure and diagram of the circuit showing how the meters are attached. At
left the figure shows the same circuit with meters attached. Note some lead
connections have been altered to divert current to the ammeter and back to the
circuit. At right is the diagram of the circuit.
6. Construct the circuit with the meters attached. Use the resistor whose resistance you
measured in Part 2 of Activity 2. Some people find it useful: first, to contruct the
circuit without meters; and second, to attach the meters, altering the lead
connections as needed.
7. Your instructor will check your circuit, and also verify that the current is limited on
your power supply. After you and your lab partner(s) have agreed that you have
constructed the circuit, have your instructor check it.
Instructor’s OK to proceed:
If your instructor has verified that the circuit is correctly constructed, and that the
current is limited, you may proceed to the next step.
Warning:
From this point onward, through the end of Activity 4, Do Not Change
the position of the CURRENT knob.
If you do accidently change the CURRENT knob position, tell your instructor.
8. Check that the circuit operates correctly.
a) Make sure the Voltage knob is turned fully counterclockwise (CCW).
b) Switch the power source ON.
Physics 1201
III-9
c) Slowly turn the VOLTAGE knob forward (CW) and back (CCW). Watch as the
voltage display on the power source (VOLT) changes as you rotate the knob: you
are controlling the voltage applied to the circuit. As you increase the voltage
(CW), the current reading (AMP.) should increase, but it should not exceed the
maximum limit set earlier – if the current does not behave as expected as you
adjust the voltage, consult your instructor.
d) When you are satisfied the circuit is behaving properly, set the voltage to 0.0
(fully CCW) and turn the power source OFF.
Activity 4
Note:
Take measurements.
The VOLT and AMP. meters on the Power Supply are not as accurate as the
multimeters. When you record your measurements of voltage and current,
record the readings from the multimeters.
1. Measure the current through the resistor for different voltages; increase the voltage
by equal increments (if you can, the increments do not have to be exactly the same).
Take measurements up to about 20 V. Record the values in the Table 1.
Table 1
Enter the units of the measured
current below.
voltage
(V)
current
(
)
0.0
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Physics 1201
2. When you are finished taking measurements, turn off the power source, and
disconnect all leads.
voltage V
(
) units
3. Use your measurements in Table 1 to plot a graph of V vs. I (voltage on the vertical axis,
current on the horizontal axis). Include an origin, (0,0).
current I (
) units
4. Does your graph represent a linear (straight line) relation? If so, draw the line – use a
ruler! Find the slope of the line you have drawn – what are its units?
Physics 1201
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5. Compare the slope of the line to the resistance you measured in Part 2 of Activity 2.
6. What property of the circuit does the slope represent? What general relation between
current, I, and voltage, V, does the graph represent? Write the relation as an equation.
When you are finished, disconnect all leads and turn off the power supply and meters.
Stop. Have your instructor check your progress and that your worktable is cleaned up
before you leave the lab room.
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Physics 1201