General Physics Lab Handbook by D.D.Venable, A.P.Batra, T.Hübsch, D.Walton & M.Kamal
Ohm’s Law
BACKGROUND: A difference of potential is required to produce a current in a circuit.
The relationship between potential difference and current, discovered by George Ohm
(1787-1854), is perhaps one of the most commonly applied relationships in the analysis of
electrical circuits. Ohm’s law is not a fundamental law, but it is applicable to a certain
class of materials called “ohmic” conductors. Materials which do not follow Ohm’s law are
said to be “nonohmic”. Semiconductors exhibit nonohmic behavior.
OBJECT: To demonstrate Ohm’s law and to determine the resistance of a given resistor.
THEORY: Ohm’s law applies to many conductors. For an “ohmic” conductor, it is found
that the potential difference V across a conductor is directly proportional to the
current I flowing through the conductor. Mathematically, Ohm’s law is expressed as
R=
V
.
I
(1)
The constant R is called the resistance of the conductor and is measured in units of
ohms (Ω).
The circuit diagram to verify Ohm’s law is shown below:
R
A
V
+
−
12-volt D.C. Power Supply
Figure 2.1: Ohm’s Law.
In the above figure, R is the resistor, V is the voltmeter which measures the voltage
across the resistor and A is the milliammeter which measures the current through the
resistor.
APPARATUS: OA-3 Op-Amp Designer unit, D.C. voltmeter, D.C. milliammeter, carbon
resistor, graph paper.
OP-AMP DESIGNER UNIT: Top Part: It is a D.C. power supply of variable voltage
(zero to 12 volts as the knob is pushed from left to right). The terminals (two posts)
on it are marked NEG (−) and POS (+).
Bottom Part: The holes within each vertical column are electically connected together. There is no connection between different vertical columns. Therefore the holes
Ohm’s Law:
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General Physics Lab Handbook by D.D.Venable, A.P.Batra, T.Hübsch, D.Walton & M.Kamal
along a horizontal row are not electrically connected. These holes are used to make electrical connection for circuit elements (e.g., resistors) to measuring devices (e.g., ammeter,
voltmeter).
THE CARBON RESISTOR: The most common type of resistor is the carbon resistor.
It is made of compressed graphite coated with an insulating paint. Painted colored
bands are coded to indicate the resistance value of the resistor.
First Band
Tolerance Band
Figure 2.3: The Carbon Resistor.
Each number zero through nine, is represented by a color according to the following
table:
Color
Number
Color
Number
Black
0
Green
5
Brown
1
Blue
6
Red
2
Violet
7
Orange
3
Gray
8
Yellow
4
White
9
The first painted band, the one nearest the end, represents the highest order digit
of the resistance value. The second band represents the second digit. The third band
represents the number of zeros that follow the first two digits. For example, a sequence of
yellow, black, and red bands will represent a resistance of 4,000 ohms.
The fourth band indicates the degree of precision or tolerance to which the resistor
has been manufactured. This band is called the tolerance band and is normally painted
gold or silver. Gold represents a tolerance of 5%. Silver represents a tolerance of 10%.
The absence of fourth band means a tolerance of 20%.
PROCEDURE:
1. Construct the circuit, as shown in the above figure, on the Designer unit.
2. Get the connections checked by the instructor.
3. Set the knob on the power supply to 12 V.
Ohm’s Law:
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General Physics Lab Handbook by D.D.Venable, A.P.Batra, T.Hübsch, D.Walton & M.Kamal
Knob
NEG
−
+
POS
R
A
V
Figure 2.2: The OP-Amp Designer.
4. Choose the maximum voltage range on the voltmeter, and the maximum milliampere
range on the milliammeter.
5. Plug the power cord into an electrical outlet and turn on the unit.
6. Go to successively lower ranges on the voltmeter until the meter needle gives largest
deflection. With 12 V applied across the resistor, now go to successively lower ranges
on the milliammeter until the meter needle gives largest deflection. (DO NOT GO
BELOW THIS RANGE; YOU MAY RUIN THE METER!)
7. Record both the voltage (V ) and the current (I) values in the spreadsheet.
8. Slide the knob on the power supply to the left to decrease the voltage by 1 V. The
current in the circuit will also decrease. Record both the voltage (V ) and current (I)
values in the spreadsheet.
9. Repeat this procedure to obtain records of V and I for successively lower voltages.
10. Calculate the resistance of the resistor.
11. Plot a graph of I as a function of V using computer. Draw a straight line that best fits
the data. Determine the slope of this line, which equals the resistance of the resistor.
12. Compare the values from Procedures 10 and 11 to the value as given by the resistor’s
color code and determine the percentage error.
Ohm’s Law:
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General Physics Lab Handbook by D.D.Venable, A.P.Batra, T.Hübsch, D.Walton & M.Kamal
Data: Ohm’s Law
Reading
Voltage (V)
Current (mA)
1
2
3
4
5
6
Sample Value of R = V /I =
First Band :
Second Band :
Third Band :
Fourth Band :
R from Band Colors = :
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General Physics Lab Handbook by D.D.Venable, A.P.Batra, T.Hübsch, D.Walton & M.Kamal
Additional Laboratory Exercise:
RESISTORS IN SERIES: The equivalent resistance Rs of two resistors, with resistance
R1 and R2 , in series is given by
Rs = R1 + R2 .
R1
(2)
R2
A
V
+
−
12 V D.C. Power Supply
Figure 2.4: Resistors in series.
RESISTORS IN PARALLEL: The equivalent resistance Rp of two resistors, with resis-
tance R1 and R2 , in parallel is given by
1
1
1
=
+
.
Rp
R1
R2
(3)
R1 R 2
.
(R1 + R2 )
(4)
Rp =
R1
A
R2
V
+
−
12 V D.C. Power Supply
Figure 2.5: Resistors in parallel.
Measure the resistances R1 and R2 of the two given resistors individually, as well
as when connected in series (Fig. 2.4) and in parallel (Fig. 2.5) by the method described
earlier. Compare te average value of the resisitance obtained experimentally for the series
and parallel circuits with those determined from Eqs. (2.2) and (2.4), respectively.
Ohm’s Law:
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