222/122 Ohm's Law
Activities Lab
2/20/12
Name ____________________
Partner ___________________
Date_________ Class _______
Ohm's Law
Equipment: Resistors, multi-meters, VOM, alligator clips, wires, breadboard, batteries, 1/4 or
1/2 Amp fuse, low voltage power supply.
Object: The object of this exercise is to learn how to set up simple circuits, use electrical
meters, calculate the effective resistance of resistors connected in series and/or parallel, and
examine the electrical properties of some devices and to obtain Ohm's law.
Ammeter
Light Bulb
Ohmmeter
–
+
V
Power Supply
Figure 1
Resistor
Voltmeter
Symbols for Electrical Components.
Kilo (K)
1000 (103)
1000000 (106)
thousand
thousandth
micro (µ)
0.001 (10-3)
0.000001 (10-6)
nano (n)
0.000000001 (10-9)
billionth
Mega (M)
mlli (m)
million
millionth
Table 1. Common prefixes for the metric system and electronics
Exercise one: Measuring resistance
Find a 220 ohm resistor. The resistors get there name because they resist or impede the flow of
electricity (electric current) (See appendix of this lab for color code and resistor information
and http://www.dannyg.com/javascript/res2/resistor.htm for a visual resistance calculator.
Using a digital multimeter select the proper resistance scale (ohms or Ω). Make sure that two
leads are properly plugged into the meter. One should be connected to the point which on most
meters is marked with common or (-) or is colored black, the other should be connected to the
meter where it is called volt -ohms, (V-Ω), (+) or sometimes with a red color. Measure the
resistance of the resistor by placing the other sides of the leads on opposite sides of the resistor.
It is OK to measure the resistance of this resistor using your fingers to hold the leads. This is
because its value is relatively low.
Record ___________ .
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Now reverse the direction of the leads on the resistor and remeasure.
Record ___________ .
Do resistors have about the same resistance in each direction? ______
What is the percent difference between the color code value and the measured resistance?
___________ Is this within the tolerance limits of the resistor (four color)?____________
Repeat the measurement using an analog VOM meter, (one with a needle). VOM stands for
Volt-Ohm-Milliamp Meter. You must first zero the meter by placing the two leads
together,(short-circuit) and zero with the zero adjust control. You must repeat this procedure
whenever you change resistance scales but not for any other types of measurements. Make sure
that you use a resistance scale which places the needle closest to the center of the scale for the
most accuracy. Record _________
Measure a 1 Megohm ( 1x106 ohm) resistor by holding the two leads of the VOM to the two
leads of the resistor with your fingers.
Record _______________ You will probably find the measured resistance to be much lower
than the value indicated on the resistor.
You may have found a large error in measuring this way. Repeat but this time hold at least one
of the leads using an alligator clip (looks like tiny alligator jaws) so that your fingers are not
touching any metal. Record ____________
Now hold the ohmmeter leads between your thumb and index fingers so that you may measure
your resistance (the analog ohmmeter works best for this).
Your resistance _________________
Why do you think that the measurement was so much in error when you held both ends of the
resistor? Hint: people conduct electricity. ________________________________________
__________________________________________________________________________
You may use either the digital or the analog meters in the rest of this lab.
Exercise two: The bread board
The breadboard is a device with lots of holes, some connected together, which is used by
experimenters for making temporary circuits. You "plug things" into the holes to connect them,
but first it's important to know how the holes are connected. If two holes are connected there
will be nearly zero resistance between them, if they aren't connected there will be nearly an
infinite amount of resistance between them. Holes that are connected act as a single point
electrically thus items plugged into connected holes are connected together.
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Using an ohmmeter determine which holes are connected and which are not in your
breadboard. Make a sketch of the layout showing how the holes are connected.
BreadBoard 
Sketch
Exercise three: Resistors in series
Place a 220 ohm and a 390 ohm resistor in the breadboard so that they are joined on one end as
shown in figure 1. The upper part of the picture shows how they look on the breadboard, the
bottom one shows the schematic diagram of the situation. We will use schematic diagrams
from now on.
figure 1
Prediction: What do you predict for the combined resistance of these two resistors? ________
Measure the total resistance of the two resistors in series. Record ____________
Did your prediction agree with the measurement?___________
Connect three 100 Ω resistors in series as shown below.
a
b
100 Ω
100 Ω
100 Ω
Prediction: What do you predict for the combined resistance of these three resistors? _______
Measure the total resistance of the three resistors in series. Record ____________
Did your prediction agree with the measurement?___________
Question: What is the rule for finding the equivalent resistance of resistors in series?
__________________________________________________________________________
__________________________________________________________________________
Is this a true statement? The equivalent resistance of resistors in series is always greater than
the largest value for any resistor in that group. YES / NO
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Exercise four: Measuring potential difference
Caution: It is very important that any meter used to measure potential difference (voltage) is
set to the correct scale and the wires plugged into the right holes. If the meter is incorrectly set
on current (amps) It will almost certainly blow a fuse or become damaged if you try to measure
voltage with it.
If you didn't read the caution above do it now!
Measure the potential difference of one of the 1.5 Volt flashlight batteries to four significant
digits using a digital multimeter (DMM). Place the positive probe on the positive end of the
battery and the negative one on the negative (non-pointy) side __________
Reverse the leads and re measure ________ what do you observe? Is there a difference when
you reverse the leads in magnitude? ________ Sign? _______ .
Carefully hook up two 1.5 V batteries in series (+ connected to -) as shown in figure 2, the
longer, vertical, line represents the positive side of the battery. You should end up with a wire
with one free end(A) and the other end connected to the negative of one battery. A wire which
goes between the batteries from the positive of the first to the negative of the second. Finally a
wire from the positive end of the second battery to a free end(B) Measure the Voltage from A
to B by placing the free ends into a voltmeter. Record ___________
Figure 2
What is the rule regarding the voltage of batteries in series? ____________________________
____________________________________________________________________________
Now carefully connect the batteries in parallel (+ to + and - to -) as shown in figure 3. Measure
the potential difference between A and B. Record __________
Figure 3
What is the rule regarding the voltage of batteries in parallel? __________________________
____________________________________________________________________________
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Exercise five: Ohm's Law
Ohm's law is a relationship between resistance, voltage, and electric current in a circuit. The
electric current is a measure of the rate of charge flow in a circuit. It is analogous to the rate of
water flow in a pipe. The water flow might be measured in gallons per minute, the electric
current is measured in coulombs per second or amperes (Amps).
You will be using a power supply to produce a known direct (steady) current (DC) voltage on
the circuit in figure 4. Make sure that the voltmeter and ammeter are set to the correct DC
settings, start with the ammeter at its highest scale and reduce until you get a reading. Begin
with the power supply DC voltage control at its lowest CCW (counter clockwise) setting and
the current control at its highest CW (clockwise) setting. ALWAYS adjust the voltage control
and leave the current control (limiter) at its highest position. The fuse should have a value of a
quarter amp. Do not turn the power supply on until the circuit is complete and the meters are
adjusted to the correct scale. Note: some ammeters have a separate place to plug in the probe,
usually labeled with an "A". Do not use any holes labeled "10 A" here.
100 ž
Figure 4
Here we use the symbol for a battery to represent the power supply. The fuse is a piece of fine
wire protected by glass which melts if the current gets too high, 1/4 or 1/2 amp in our case.
Check the fuse by eye to see if it is good before starting. See figure 6. Condition of fuse
__________
Figure 5
Caution: It is very important that any meter used to measure potential difference (voltage) is
set to the correct scale and the wires plugged into the right holes. If the meter is incorrectly set
on current (amps) It will almost certainly blow a fuse or become damaged if you try to measure
voltage with it.
If you didn't read the caution above do it now! Yes I know you've read this before!
Once you're satisfied that everything is wired correctly (It doesn't hurt to check). Turn on the
power supply and gradually increase the voltage from 0 to 6 Volts in about one Volt
increments, checking and recording the current as you do so in the data table below. Record
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both to the maximum accuracy of the meters that you use ( Does changing a scale perhaps give
you more significant digits?)
Power Supply - Volts
Current (Amps)
1
2
3
4
5
6
7
Now, using a computer graph the voltage vs. the current. Voltage on the vertical axis and
current on the horizontal axis. Is it a straight line? Are the voltage and current proportional to
each other? _____ NOTE: Use the x-y plot for the graph or things may not turn out well.
Carefully find the slope of the graph. Show your work on the graph. The slope of the graph is
________ .
Is the slope of the graph equal to the resistance of the resistor? _____
Find the per cent difference. _________
CAREFULLY LABEL THIS GRAPH AND STAPLE TO THE BACK OF THIS REPORT.
Is the slope constant? _____ What happens to the resistance of the resistor as the voltage is
increased? __________________________________________________________________
If V is the voltage of the power supply, I is the current through the resistor, and R the resistance
of the resistor write an equation involving these three variables which satisfies the data that you
obtained. The straight line (linear) relationship between current and applied voltage is known
as Ohm's law.
Question: If a voltage of 25 V is applied to a 45 ohm (Ω) resistor what current will result?
Show your work.
If you place a 90Ω resistor in place of the 45 Ω resistor above, what will the current be?
If you place a 22.5Ω resistor in place of the 45 Ω resistor above, what will the current be?
If you place a 50 V battery in place of the 25 V battery (keeping 45 Ω resistor), what will the
current be?
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Activities Lab
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Exercise six: Resistors in parallel.
If all went well in the above section you should have experimentally derived the relationship
between the voltage drop across a resistor (V in volts), the resistance R (in ohms Ω ), and the
current I (amps):
V=IR
It can be used for any part of the circuit or the circuit as a whole.
Using the breadboard connect two resistors in parallel as shown in figure 2.
Figure 6
If a 12 volt battery is connected between A and B:
what is the expected current through the 220 Ω resistor? _______________
what is the expected current through the 390Ω resistor? _______________
Because of the conservation of electric charge, the total current flowing from A to B is the sum
of these two individual currents. Itot=I220+I390
I220
Itot
Itot
I390
what is the expected total current from A to B? _______________
What is the expected total effective resistance (remember V=12 volts) Rtot= V/ Itot
___________________
Measure the resistance between points A and B. Record _______________ .
If two resistors R1 and R2 are connected in parallel (as the 220 Ω and 390 Ω resistors are
above) and a voltage V is applied across them. Write and equation (using V and Rs) for
the expected current through R1.
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the expected current through R2.
The expected total current flowing from A to B.
If we want to be able to write Itot = V/Rtot , what must (1/Rtot) be?
Compare this derived value with that given in your text.
Question: Is this a true statement: The equivalent resistance of resistors in parallel is always
lower than the value of the lowest resistor? _____
Exercise 7: A light bulb
Remove the resistor from the previous circuit and replace it with a 5 to 7 volt light bulb.
100 ž
Again increase the applied voltage in about one volt increments until you reach 6.0 volts. Enter
the data in the table below and make a graph of Voltage vs. current as you did before. NOTE:
Use the x-y plot for the graph or things may not turn out well. Use V=IR to compute the
resistance of the light bulb at each voltage.
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Power Supply - Volts
Activities Lab
2/20/12
Current (Amps)
Resistance (Ohms)
1
2
3
4
5
6
7
What happens to the slope as the temperature goes up? ______
Is the resistance constant in this case? _____
Resistance can change with temperature changes. From your results and graph what can you
conclude about the dependence of resistance in light bulbs vs. temperature. In other words does
the resistance increase or decrease with temperature?
Big hint: the temperature of the bulb when bright is higher than when it is dim!
____________________ STAPLE THIS GRAPH TO THE BACK OF THIS REPORT.
Based on the experimental evidence thus far, discuss whether Ohm's law is true in all cases, that
is, does the current always increase linearly in proportion to voltage? Explain based on what
you have observed.
_____________________________________________________________________________
_____________________________________________________________________________
Exercise 8: Current
Use the same light bulb circuit that you used above with the power supply set at 6.0 volts. Notice
that the ammeter measures the current going INTO the light bulb from the battery. Now break
the circuit between the light bulb and negative battery terminal and insert the same ammeter in
the circuit here so that you can measure the current coming OUT OF the light bulb.
Current in = _______________ ,
Current out = _________________
How does the current coming out of the light bulb compare to that going into the light bulb?
_____________________________________________________________________
Answer the following: Be careful! Many people miss one or more of these!
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True or False, The light bulb uses up current.
True or False, The light bulb uses up electric charge. Hint: I= Q/t
True or False, The light bulb uses up electrons. Hint: Electrons have charge.
Disconnect the ammeters from the circuit and reconnect the circuit so that the bulb lights. Switch
the range switch on both meters to DC Volts and remove the wire in the "A" or amps terminal.
(This makes it less likely that someone will hook the meter up incorrectly.
Exercise 9 Potential Difference and Power
Now using one of the meters set to measure voltage (use a scale of at least 20 VDC)
measure the potential difference (voltage) between the negative terminal of the power supply and
the terminal on the light bulb closest to the positive terminal on the power supply. (The negative
(com) on the meter goes to the negative on the power supply).
Result ________. This is the potential of this side of the light bulb
Now measure the potential of the other side of the light bulb by leaving the meter attached to the
negative on the power supply but moving the positive lead to the side of the bulb closest to the
negative of the power supply.
Result ________. This is the potential of this side of the light bulb
Answer this question: What is different between the two sides of the bulb and what is the
difference?
_____________________________________________________________________________
This is called the potential difference across the bulb or the voltage drop across the bulb.
Usually to find the potential difference one places the voltmeter leads on either side of the device
being measured (with the common or negative closest to the negative of the power supply) and
makes the measurement.
Do it and compare with the result above.
P.D.= ∆V=_________ Volts
The power (in Watts) consumed by a resistor is given by the product of the current (I) and the
voltage (V) P=IV. Calculate the power of the light bulb at 6 volts the circuit above. Show
your work.
Power = ___________
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Home work
Resistors in parallel
Assume that you have 3 resistors in parallel as shown in figure 9. The power supply is set to
6.0 Volts.
Figure 9
Calculate the equivalent resistance of the network of three resistors. Show your work.
Re = ____________
Use Ohm's law to calculate the current supplied by the power supply. Show your work.
I (supply calculated) = _________
What will the voltage across each resistor be? Hint: They are all connected directly to the
power supply.
V(220 Ω top) = ___________
V(220 Ω middle) = __________
V(390 Ω) = _________
What general rule can you make about the voltage of resistors connected in parallel?
___________________________________________________________________
Compute the current (using Ohm's law) through each resistor. Show your work.
I (220 Ω) _________
I (220 Ω) _________
I (390 Ω) _________
Fill in the blanks:
In a series circuit all resistors have the same _______current or voltage_________
In a parallel circuit all resistors have the same _____ current or voltage _________
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Appendix: Color Codes
Color
0
1
Black
Brown
Example I
Orange-Yellow-Blue-Gold
2
Red
34,000,000 + 5%
3
Orange
4
Yellow
5
Green
6
Blue
7
Violet
8
Gray
9
White
5%
Gold or 10-1
10% Silver or 10-2
or 34 M Ω + 5%
Example II
Red-Black-Gold-Silver
2 ohms + 10%
Example III
Yellow-Black-Black-Gold
40 Ω + 5%
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