Download LAB:Ohm`s Law

Name __________________________
Period ______ Date _______________
Lab Partners _____________________
_____________________
_____________________
_____________________
V
Ohm’s Law Lab
I=V
R
Purpose: To verify Ohm’s Law
by showing that current is directly proportional to voltage
and inversely proportional to resistance.
Materials:
 Variable Power supply that can be set between 1.5V and 6.0 volts (no higher than 6 V)or two
batteries
 Three different value 0.5 watt resistors resistors ranging from 33 ohms to 500 ohms
 Three wire leads
 Multimeter
Procedure:
Part I
 Read and record the values of the three different resistors you chose, including the tolerance
(%error) factor for each reading.
 Set the multimeter to read ohms and measure the resistor value for each of the three resistors
you chose, as shown in the diagram below.
Using the
Multimeter
To measure
Resistance
Part II
 Set up a simple series circuit with the power supply connected to the first of your resistors
 Set the voltage to 1.5 volts (or use a single dry cell) and turn the power supply on
 Set the multimeter to read DC Volts at its lowest reading, then turn on the multimeter
 Measure and record the voltage across the resistor as demonstrated in class and in the picture
below by:
o touching the positive probe to the side of the resistor leading to the positive connection
at the power source and simultaneously
o touching the negative probe to the side of the resistor leading to the negative
connection at the power source (see diagram on following page)
Using the
Multimeter as
a Voltmeter

Power
source
Remove the probes from the resistor and turn off the multimeter.
Part III
 To measure and record the current through the resistor:
o Disconnect the negative end of the resistor from the power source and attach it instead
to the positive probe on the multimeter.
o Attach the negative probe of the multimeter to the negative power source outlet.
o Turn the multimeter dial to measure the smallest amount of DC current (DC Amps)
o Turn on the multimeter and record the current measurement, being careful to note the
prefix (milli-, micro-) before the amperage reading.
Using the
Multimeter
to measure
Current
Part IV
 Repeat the above steps after setting the voltage to double (3V instead of 1.5V)
 Repeat the above steps with each of the other two different value resistors.
Part V
 Calculate the current for each different trial using Ohm’s Law and compare your calculated
value of current to your measured value for the current using a % difference calculation
Questions:
1. Draw a circuit diagram below showing the setup for this experiment by using the appropriate
circuit symbols. Be sure to include the power source, the resistor, the ammeter, and the
voltmeter in your diagram. Label each device with the appropriate letter.
2. According to your lab results, what relationship exists between the voltage and the current
through a particular resistor?
3. According to your lab results, what relationship exists between the resistance and the current
at a certain voltage?
4. How accurate was the resistor code on your resistors?
What percent error did you find between your highest value resistor code reading and actual
resistance you measured for that resistor using the ohmmeter?
Was the % error within what was claimed by the gold or silver resistor tolerance bands?
5. Did the lab prove Ohm’s Law to be true? How do you know?
Name ________________________
Resistors and Resistor Codes
Built into any appliance are devices called resistors designed to encourage the electron current
to give off its energy (called a voltage drop). These resistors are manufactured to have a
certain value, measured in “ohms” (or “”), and are often made of ceramic.
The value of the resistor is indicated along its length by a series of color bands, the first two
of which represent two digits from 0 to 9, and the third of which represents the multiplier, or
number of zeros to follow the two digits.
The color band code and is read from left to right with the resistor’s gold or silver bands on
the right.
The code is easy to remember… it follows the colors of the rainbow:
Color
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
Digit
(1st two
bands)
0
1
2
3
4
5
6
7
8
9
Multiplier
(3rd band)
1
10
100
1000
x 104
x 105
x 106
x 107
x 108
x 109
The gold and silver bands at the other end of the resistor give the “tolerances” or error factor.
Gold is 5%, silver is 10%, gold and silver would be 15%.
Practice reading the resistor codes on the following page, then on real resistors. Have a partner
check your readings.
For each of the following resistors, the color bands are stated in the order in which they would
appear from left to right. State the value for each resistor on the blank to the right of the
resistor. For the last few resistor drawings, use real resistors and indicate the colors of the
bands and the value of the resistor in the blanks provided.
Red, green, blue, silver
Orange, yellow, black, gold
Blue, gray, white, gold, silver
Green, black, brown, gold
Brown, red, black, silver, gold
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Your choice of resistors:
Colors: _________________________________
Colors: _________________________________
Colors: _________________________________
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