Experiment 15: Ohm’s Law
Figure 15.1: Simple Series Circuit
EQUIPMENT
Universal Circuit Board
Power Supply
(2) DMM’s
150 ⌦ Resistor (R1 )
330 ⌦ Resistor (R2 )
560 ⌦ Resistor (R3 )
Miniature Light Bulb and Socket (R4 )
(1) Jumper
(6) Wire Leads
Figure 15.2: Schematic: Simple Series Circuit
79
80 Experiment 15: Ohm’s Law
Advance Reading
Text: Ohm’s Law, voltage, resistance, current.
Lab Manual: Appendix B, Appendix C - DMM
Objective
The objective of this lab is to determine the resistance
of several resistors by applying Ohm’s Law. Students
will also be introduced to the resistor color code and
refresh their graphing skills.
Theory
Figure 15.3: Color Code Schematic
Ohm’s Law states that the current, I, that flows in a
circuit is directly proportional to the voltage, V , across
the circuit and inversely proportional to the resistance,
R, of the circuit: Color Number Multiplier
V
I= (15.1) Black 0 100
R
In this experiment, the current flowing through a resis- Brown 1 101
tor will be measured as the voltage across the resistor Red 2 102
is varied. From the graph of this data, the resistance is
determined for Ohmic resistors (Ri , i = 1, 2, 3). Non- Orange 3 103
Ohmic resistors (R4 , light bulb) do not obey Ohm’s Yellow 4 104
Law.
Green 5 105
Ammeters are connected in series so that the cur-
Blue 6 106
rent flows through them. The ideal ammeter has a re-
sistance of zero so that it has no e↵ect on the circuit. Violet 7 107
Real ammeters have some internal resistance.
Grey 8 108
Voltmeters are connected in parallel to resistive White 9 109
elements in the circuit so that they measure the poten-
tial di↵erence across (on each side of) the element.
The ideal voltmeter has infinite internal resistance. Tolerance
Our voltmeters have approximately 10 M⌦ (10⇥106 ⌦)
internal resistance so that only a minuscule amount of Gold 5%
current can flow through the voltmeter. This keeps the Silver 10%
voltmeter from becoming a significant path for current
around the element being measured. (No Band) 20%
Resistors are labeled with color-coded bands that indi- Table 15.1: Resistor Color Code Values
cate resistance and tolerance. The first two color bands
give the first two digits of the value (Fig. 15.3). The
third band gives the multiplier for the first two, in pow-
ers of 10. The last band is the tolerance (Fig. 15.3),
meaning the true value should be ±x% of the color
code value. Refer to Table 15.1 for standard color val-
ues.
There is no need to memorize the color codes for lab.
For example, a resistor that has two red bands and a
black multiplier band has a resistance of 22 ⌦.
Prelab 15: Ohm’s Law 81
Name:
1. Write the equation and a qualitative statement for Ohm’s Law. (20 pts)
2. What are “ohmic” and “non-ohmic” devices? (20 pts)
3. Complete the following statement: An ideal ammeter has an internal resistance of , while an
ideal voltmeter has an internal resistance of . Explain why these are desirable attributes for
the respective measuring instruments. (20 pts)
4. If I vs. V is plotted, what value is obtained from the slope? Note that we are investigating the function I = V /R
and fitting our data to the slope-intercept equation of a line. (40 pts)
82 Experiment 15: Ohm’s Law
PROCEDURE
PART 1: Measures of Resistance
1. Determine the nominal resistance for the three re-
sistors: interpret the color codes according to the
color code chart in Table 15.1.
2. Measure the actual resistance, R, of the three re-
sistors using the ohmmeter and record them in the
table provided.
3. An ideal ammeter has no resistance; this ammeter
does have a small resistance. Measure the resistance
of the ammeter (200 mA DCA).
PART 2: Ohm’s Law Applied
4. Build a simple series circuit using R1 , an ohmmeter,
an ammeter, and a jumper (This will look similar Figure 15.4: Spade Connection to Circuit Board
to Fig. 15.1, but without the power supply).
5. Measure the equivalent resistance of the circuit us-
ing the ohmmeter and record this value in the table
PART 4: Graphing
provided. Include units and uncertainty.
13. Open Graphical Analysis. Enter all of your voltage
6. Remove the ohmmeter and connect the unplugged
and current data as four separate data sets (one for
power supply to the circuit. Connect a voltmeter to
each resistor). Include the point (0,0) in each set.
the circuit, across the power supply leads (in paral-
[Other graphing software may be used, provided the
lel).
graphs include all requisite elements.]
7. Have your TA check your circuit. Plug in the power 14. Plot I vs. V for the three Ohmic resistors on one
supply and turn it on. graph. Apply a linear fit to each one.
8. Test Ohm’s Law (V = IR) by verifying that the 15. Calculate the resistance of each circuit using the
current increases linearly with applied voltage. Ap- slope of your I vs. V graphs. Compare these Rgraph
ply 1 V, 2 V, 3 V, and 4 V to the circuit. Measure values to the measured Req values using the percent
current and voltage and record them in the table di↵erence formula (Eq. A.2, Page 155).
provided. Include units and uncertainty.
16. Plot a separate I vs. V graph for the light bulb.
9. Repeat the Part 2 procedure for R2 and R3 .
17. Print a copy of both graphs.
PART 3: Non-Ohmic Device
10. Build a series circuit using R4 , the light bulb
(Fig. 15.4).
11. Measure the current and voltage as you increase the
applied voltage in 0.2 V increments up to 2.0 V,
then continue in 1.0 V increments up to 4.0 V. Ad-
just the voltmeter scale to obtain the most signifi-
cant figures possible.
12. Turn o↵ and unplug the power supply; turn o↵ the
DMM’s.
Experiment 15: Ohm’s Law 83
QUESTIONS
1. Read the information in the next column. How
much current would it take to cause pain? What
was the maximum current you measured for this
experiment?
2. Why was there no danger to you while you per-
formed this experiment? The current required for
this experiment is as high as 30 mA. Some experi- Can voltage kill you?
ments will require current as high as 5.0 A. Explain It’s actually current that kills. So why are “Hazardous
why there will be no danger to you. Read the in- Voltage” signs so prevalent? Paul Hewitt1 explains it
formation in the next column again, more carefully very nicely:
if necessary.
Consider Ohm’s Law: V = IR. What is
the resistance of your skin? That depends
3. Is the graph of I vs. V for the light bulb linear?
on the state of your skin: dry or wet. If
What does this tell you about the resistance of a
it’s wet, is it water or sweat? Sweat, of
light bulb as the filament gets hotter?
course, contains salt; salt water is a good
conductor.
4. Compare the experimental (DMM, graph) values for
each ohmic resistor. The resistance will be dramatically di↵er-
ent for di↵erent situations! Very dry skin
has a resistance of about 500,000 ⌦, while
5. Do the experimental values fall within the tolerance
skin wet with salt water has a resistance
of the resistors? What might cause the values to ex-
of about 100 ⌦. Once the voltage of a de-
ceed the tolerance?
vice and your skin’s resistance are known,
we can estimate the current that will flow
6. The power output of a circuit is given by: through your body.
Current E↵ect
V2
P = I 2R = = IV (15.2)
R 0.001 A Can be felt
0.005 A Is painful
The resistors used in this experiment are 2-watt 0.010 A Causes involuntary
resistors. What is the maximum power output of
R1 when 9.0 V is applied across it (use your graph muscle contractions (spasms)
value)? 0.015 A Causes loss of muscle control
0.070 A If through the heart, causes serious
7. Calculate the power output of each ohmic resistor
(use your graph value) when a potential of 7.00 V disruption; probably fatal if
is applied. current lasts for more than 1 second
8. Verify, using only the units provided in Table 14.1, Table 15.2: E↵ect of Electric Current on the Body
that each part of Eq. 15.2 is equal to J/s. What is
the unit of power output? Note that the e↵ect caused by these currents are ap-
proximate values. It is quite difficult to get volunteers
for this area of research!
1 Hewitt, Paul G., 2006. Conceptual Physics. Pearson Addison Wesley, San Francisco.
