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INTRODUCTION During the nineteenth century so many advan ces were m ade
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understanding the electrical
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an investigator named Georg Si mon O hm. O hm was interested in examining the relative
conductivity of me tals a nd in investi gating the relationship betwee n the e lectromoti ve forc e
(potential difference) and the current in a conductor .
By taking w ires made from differe nt materials but having the same thickness, passi ng a current
th ro ugh thes e wires and meas ur ing the e lectromotive forc e (i .e., th e potential difference between
the ends of the conducting wire). he was able to e 'perimenta lly determ ine the r lati ve
conductivity of certain metals such as silver, copper, and gold.
In another experiment using a p iece of apparatus that he built himsel[ Ohm investigat d the
effect of current in a conductor on the voltage drop across the conductor. He found th at G r a
given conductor the voltage drop was directly propo rtional to the curre nt in the wire . When
voltage is plotted ag ainst th e cll rr nt in a given conductor, the data can be fit ted to a stra ight line ,
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the slope of which is the l'eSistal lce of the conductor. T his result was p ublished in 1826. In
recognition of Ohm 's work, thi s emp irica l relationship bears his name.
DISCUSSION OF PRINCIPLES
Ohm's Law can be
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ritten algebraicall y as:
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v=
JR
(1)
V
J =­
(2)
R
V
R=­
(3)
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Lab #1- Ohm's Law
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JI.....
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f">....... i ',,<pJ.
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where V represents the p otc Ilt i a ~drop ac ross the conductor (measured in vo lts), I the CUITcnt in
the conductor (measured in amperes), and R the res istance of the conductor measured in units
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called "ohms" (sy mbolized by
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n, upper-case Greek omega).
Re istance and Resistors
Resistance is a propert_ of materials. Res istors are cond ucti ng devices made fro m materials
which satisfy Olun's Law.
If the potential difference across a resistor is set at 1 volt, and if a current of 1 amp is meas ured
in the conduc tor, th e n its re istance is determined to be 111, o r 1 W. Instead of usi ng th in "vires as
Ohm d id in his original experiment, you will replicate his results lIsing small cylindrical ceramic
r sistors . You wi ll notice colored bands on the resistors. These bands D nn a code that indi cates
the resistance of the resistor. Later in thi s experiment you willlearn ho w to read lhis color code .
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Combinations of Resistors
Resistors can be combined in simple circuit arrangements that increase or decrease the overal l
resistance in the circuit. These arrangements are called parallel and series circui ts. Figure 1
shows the resistors in a parall el arrangement and Figure 2 illustrates two reo istors connected in
seri es.
Figure 1 - Parallel Circuit
Figure 2 - Series Circuit
In order for charges to move in a conductor, there must be a potenti al difference across the
conductor. In order for charges to move through a circuit, there m ust be a complete path lead ing
away from and bac k to the source of emf (V AB in F igure 1).
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Lab #1- Oh m's Law
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As you can see, in the paralle l arrange ment shown in Figure 1, the current can divide at the
junction A and recombine at j unction B . T herefore , the
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thro ugh R, and R, mav be
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diffe rent. Notice that in th is case
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T hat is, the pot ntial drop across each resi stor is the same .
In the series arrangement shown in Figure 2 the current in the circuit goes through each res istor.
If we compute the pote ntial drop V J across R, usi ng Ohm ' s Law, it is merely;
.:;
;:.--.T
(5)
Likewi se the drop across R2 is:
(6)
The potential drop across both resistors is:
(7)
One can think of the applied voltage V AB being divided between the two series res istors
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Using some simpl e algebra and your understanding of the potential drops in a simpl e series or
parallel circuit, the relationships for determining equivalent res istance for re i. tor in series
and/or parallel can be derived. These relationships are:
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1
1
1
Re quivalent
Rl
R2
Rn
----=-+-+­
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(The reciprocal of the eq uivalent res istance
IS
the sum of the re ciprocals of the indi idual
resi stances. )
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Requivalent
= Rl + R2 + R n
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(The equivalent resistance is simply the sum of the individual resistances.)
As sugges ted by th eq uations above, the sums include a term for each resistor in the circuit.
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PROCEDURE:
Determinillg Resistance
We wi ll use a computer and softwa re to act as our battery and voltmeter, while we wil l monitor
the CUlTent with a hand-held mul timetcr. Do not turn on the power to your c ircuit until your lab
instructor has ch eked your wiring.
1.) T urn on th . computer and monitor, the Interface Device, and the Power Amplifier.
a. Plug the rear output of the Power Amplifier into Analog Channel C of the fnte rface .
b. Plug a vo ltage probe into
nalog Channel A of the Signal Inte rface.
2) Open the fol der "LABS" by double clicking on the folder if it is not already open. On ce tbe
Labs folder is open . open the " Ohm' s Law" file. The file should open to 10 k like th at shown
in F igure 3 below. The window shows both a digital and analo g voltmeter, both measuring
the voltage fro m the voltage probe.
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Figure 3 - Start Up Screen
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3.) Connect the circuit shown in Figure 4, using the resistor labeled as " 100 W" on th circuit
board . We will use a small multimcter as an ammeter. Set the multimeter to the range marked
2 DCA.
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Volt3.ge probes
to interface
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V
Voltmeter
(co mp uter interface)
Figure 4 - Circuit Dia gram for Proc edure A
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Our "voltmeter" is actually our computer connected to the Signal Int rface. Vo ltmeters have
a relatively hi gh resi stance and are placed in parallel with the resistor. Ammeters. wi th their
relati vely low resistance, are placed in seri es with the resistor.
4.) Select the Signal Gen rato r window by clicking in that window. The Signal Generator
wi ndow is shown in Figure 5. Turn on the signal generator by clicking on the 0
button .
Cl ick the DC button if it is not already highlighted. Set the DC voltage to 1 V by cli cking on
the small up arro w next to the voltage setting.
\"" CIDSe Button
\ (C lDse Window by clicking he re) 0 NI OFF
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Sig nal Generat or
~- Volt3.ge Adjustme nt
Figu re 5 - Signal Generator Window
La b #1- Ohm's Law Your Name
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5.) Pre '5 Command M to start taking data. With R J in the circuit, obtain six current and oltage
measurements by increasing the voltage in I-volt steps . Record t his information in the data
table below.
Ohm's Law Data
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Voltage (volts)
III
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Current (amps)
II
II
II
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2
3
4
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Reading the resistor code
The resistance of most ceramic res isto rs can be determined from the col ored bands pri nted on the
resistor. Each color re presents a digit from 0 to 9 (see Figure 6).
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Figure 6 - Resistor Color Code
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Th first two bands give the mantissa of a number in scientific notation: the third gi ves the power
often. The fourth band gi ves the tole rance (uncertainty expressed as a percentage) in tbe value of
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the resistance (Gold
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= ~ 5%; Silver :J ;; 10%, no 4th band ~~O%). Th fifth band, if present,
gives the power rating of the resist r. We will not be concerned with the 5th band. however in
order to know whic h end of a resistor to "start" from when reading the color code, it may be
useful to remember that the 4th banel , if present, is metallic in color (gold or silver).
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Results
In this section, all experimental results are presented. Theo retical predictio ns may be presented
here to provide a com pari on with the experimental data. Be sure that all figures and tables are
properly introduce I and labeled. B sure to provide sufficient discussion of all data presented, as
well as a compaJis n of analyti cal , theoretical, and experimental res ults. Make every eff011 to
account for any disparities between the theory and exp rimental results as best you can.
ote that
all figures and tables must be mentioned and described in the body of the t xt. Don't inc lude a
figure and say nothing of it in the body of the text. Refer to each by number (e.g., Figure 1). \Iso
make sure to adequate ly answer all questions posed in the lab hand ut. Use subsection. as
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Discus ion or Analysis
sually you will be requi red to analyze your results and draw conclusions from the
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exp~ri men t.
Conclusion
Th is section you make a declarative statement whether the experiment succeeded and how you
cmne to that determination.
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http://homepages.u d ayton.edu/~hardierc/ECE401/ECE401L/Formal Report Guidlines.pdf
Lab ttl - Ohm's Law
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the resistance (Gold =
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Si lver
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± 10%, no 4th band :~O%). The fi fth band, if present,
gives the power rating of the resistor. We will not be concerned with tht: 5th band. however in
order to know which end o f a resistor to "start" from when read ing the co lor code, it may be
useful to remem ber that the 4th band, if present, is metallic in color (go ld or silver).
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Results
In this section, all experimental results are presented. Theoreti caJ predicti ons may b presented
here to provide a com parison with the experimental data. Be sure that all figures and tables are
properly introd uced and labeled. Be sure to provide sufficient discllss ion of all data presented. as
well as a compariso n of ana lyti cal, theoretical, and experimental resu lts. Make every c1Tort to
account fo r any disparities between the theory and experi mental resu lts as best yo u can . No te that
all fi gures and tables must be mentioned and described in the body of the text. Don't incl ude a
figure and say noth ing of it in the body of the text. Refer to each by n umber (e .g., Figur 1). Also
make sure to adequat Jy a nswer all questions posed in the lab handout.
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Discussion or Analysis
sually you will be requ ired to analyze your results and draw con lusions from the exp riment.
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Conclusion
Thjs section you make a declarative statement whether the experiment succeeded and how you
came to that dd erm ination.
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Lab #1- Ohm's Law
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