Download Ohm`s Law 1

Ohm’s Law 1
Single Resistors
Ohm’s Law stipulates that the current in a DC circuit is directly proportional to the voltage which drives
the circuit. The constant of proportionality between the two quantities is called resistance and the law is
usually expressed in the form V = IR. One consequence of this law is that the current and resistance are
inversely related in a circuit with a constant electromotive force.
Purpose
The objective of this lab is to experimentally verify Ohm’s Law by analyzing the relationship between I
and R for a constant voltage circuit.
Equipment
AC/DC Electronics Lab Board
D-cell battery
Multimeter
Procedure
1. You are going to test the current passing through a set of resistors of different values. For the
sake of efficiency, keep the resistors sorted and identified as you go along. Mount the resistors
on a piece of paper by pushing the leads through. Decode the resistance value using the color
code, and record that value in column “A”, “Coded Resistance RN ()” on Table 1.
2. Using the multimeter set to “”, touch one lead to each end of the resistor and measure the
actual resistance by and record that value in column “B” under “Measured Resistance R.” Use
your decoded value as a clue as to which scale to set the meter on. Be sure to record the correct
units. Since this value is considered your raw data, record the value as it actually measured. Do
not convert yet if resistance is measured in k or M.
3. If necessary, now convert your measured resistance to ohms and record the measured resistance
in column “C” under “Measured Resistance RM ().”
4. Measure the current passing through the resistor in the following way. Plug the red lead into the
yellow plug on the meter labeled mA. Connect the red lead to the positive end of the battery.
Touch the black lead to one end of the resistor. Connect a wire from the other end of the resistor
to the battery. Notice that the meter is in series with the resistor in a circuit. Set the meter to the
“A” scale. Turn the dial to the setting where you get the reading with the most significant
figures. Since you are on the mA scale, the maximum reading will be either 200 mA, 20 mA, or
200 µA. Record the reading on the chart in column “D”, actual current. This will need to be
converted to Amperes for column E.
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5. Repeat the current measurement for each resistor by moving the lead ends from one resistor to
the next.
Measuring Voltage
6. Set the multimeter to read voltage. Since you are using a 1.5 volt battery, the pointer should be
on the 2 volt setting. Make sure the slide switch is set to “DC”. Plug the red lead into the red
port marked “V”.
7. Using the circuit board, connect wires in the following fashion. Connect the + end of the battery
to one spring. Connect the – end of the battery to the adjacent spring.
8. Connect the red lead to the same spring as the + end of the battery. Connect the black lead to the
same spring as the – end of the battery.
9. Remove the resistors one at a time and put the resistor so it connects the two springs, completing
the circuit and note the reading in column F.
Table 1
A
B
C
D
E
F
G
H
Coded
Resistance
RN ()
Measured
Resistance
from meter,
in ohms,
K, or M
R
Measured
Resistance
converted to
ohms
RM ()
Actual
Current
I mA or µA
Actual
Current
converted to
Amps
I (A)
Voltage
V (V)
Theoretical
Current
IT (A)
Calculate
using ohm’s
law
% Error
See step 2 in
data analysis
IT = V / RM
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Data Analysis
1. Calculate the theoretical current in Table 1 by dividing the voltage in volts by the measured
resistance in ohms.
IT = V / RM
2. The error is determined by comparing the actual to the theoretical current.
% error = (I – IT) x 100%
IT
3. Construct a graph of Actual Current vs. Measured Resistance.
4. From your graph, what is the mathematical relationship between current and resistance.
5. What were possible sources of experimental error in this laboratory exercise?
6. Your lab report consists of the following submitted at the end of the period:
a. Lab group names on title page, each member of the group will receive the same grade, so
make sure you are satisfied with the results before submitting the lab. If you cannot
come to an agreement regarding the report, you can submit separate reports.
b. Table 1 on Excel, with excel doing all the calculations.
c. Graph of Actual Current Vs Measured Resistance
d. Answers to questions 4 and 5 on the graph page.
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