Experiments
With Resistors
Measuring Resistors
The Resistor
Color Code
A Simple
Resistor Experiment
Measuring
Resistor Accuracy
Resistor
Combinations
Series Resistors
Parallel
Resistors
The
TwoLightBulb Experiment
The TwoResistor
Experiment
Measuring
Resistances
Let's start by making one resistance measurement.

Get a resistor.
A one kohm resistor would be good. You can tell it is a one kohm
resistor by the stripes. They should be brownblackred in that order
from the end, as shown at the right.
You are now ready to measure the resistance. Connect your resistor
to an ohmmeter as shown at the right. Don't get uptight about which
lead goes on which end of the resistor. It doesn't matter.
(The resistor is a bilateral element
and should be the same either way!) You should see a reading in the
window, and that reading should be in the vicinity of one thousand ohms.
Click here for the lesson on setting up an ohmmeter. Here are the
connections you make. They're simple enough.
Take the measurement now! Then reverse the leads to check the claim
above by measuring the resistance with the opposite lead connections.
The
Resistor Color Code
You
probably wondered about those stripes on the resistor. There is a
color code that lets you tell what value the resistor has. Here's
what's important.
This resistor is 1000 ohms = 10x10^{2}
Notice how cleverly we put certain parts of the result in bold text
and colored it. The bolded text corresponds to the stripes, and the
colors are shown on the bold text. Here is the color code.
0 
Black 
Black 
1 
Brown 
Brown 
2 
Red 
Red 
3 
Orange 
Orange 
4 
Yellow 
Yellow 
5 
Green 
Green 
6 
Blue 
Blue 
7 
Violet 
Violet 
8 
Gray 
Gray 
9 
White 
White 
Given a resistor, to calculate the value of the resistance you use the
three stripes. (If there are four stripes, just use the first three.
The last stripe tells you how accurate the resistance value is.)
Here is the algorithm.

The first stripe is the most significant digit,
X, in XY x 10^{Z}.

The second stripe is the next digit, Y, in XY
x 10^{Z}.

The third stripe is the exponent in XY x 10^{Z}.
Here is an example/problem.
Problems
P1. For
the resistor at the right answer the following questions. Note, for
this problem, if you do not get a grade of 100 you answered incorrectly.
Now, what is the first digit on the resistor's set of stripes?
P2. What
is the second digit on the resistor's set of stripes?
P3. What
is the exponent?
P4. What
is the value of the resistor?
A
Simple Resistor Experiment
Any conducting material can be used to make a resistor. Any metal
or metallic alloy can be used. Other conducting materials, like carbon,
germanium or silicon can be used, even if the material does not conduct
as well as a metal. In this exercise you are going to construct some
resistors made of carbon.
First, you are going to have to locate some carbon. Do you have any
on you? Were you able to locate any carbon? You may not have
realized that you have some carbon if you are carrying a pencil.
The carbon
you will use is the carbon found in a pencil lead. On a clean sheet
of paper draw a shape like the one below. It can be smaller but drawn
to scale. The two "pads" at the end of the structure are larger to
permit you to to make connection to your resistor with the ohmmeter leads.
Be sure that your resistor is filled in completely, and when you are done
measure the resistance. Then click on the Enter Resistance button
to show this value. Record your measured value. You will need
to use it later in this lesson.
You have just constructed and measured a resistor. There is a special
symbol for
a resistor that you need to become
familiar with. Here it is.
The
diagonal lines are intended to suggest some resistance to the flow of current.
This symbol can be used in any orientation, and you will often see this
symbol rotated as shown at the right.
1. When you measured your resistor the ohmmeter
that you used actually applied a small voltage across your resistor and
current flowed through it. Often symbols are attached to the resistor
symbol to indicate how the voltage is applied across the resistor and to
define a positive direction for current flow. In a case where both
a current and a voltage symbol were used the situation would look like
what we have shown above. (If you need to review voltage
or current, click the hotwords.
These symbols define polarity for voltage. When the voltage across
the resistor, V, is positive as shown, then positive current will flow
from through the resistorin the direction indicated by the current arrow.
V = I R
OK! You have measured your resistance and you've learned a little
about resistors. Next, you are going to add a second resistor in
series with your original resistor. Draw in the second resistor in
a pattern like the one below. One of the pads on the first resistor
is used as part of the second. You are going to make a few measurements,
but first you need to answer a few questions.
Q1.
What will the resistance of the new resistor be?
The second resistor should be the same physical size as the original resistor.
Since it is the same size and made of the same material it should have
the same resistance. There are physical reasons why that is
so, and there is a mathematical expression that relates the resistance
to length, cross sectional area and a property of the material called resistivity.
For resistors that have a constant cross section, A, and a lenght, L, the
resistance is:
R = rL/A
For now, however, you should expect the same physical structure to give
the same resistance.
Keeping in mind that R = rL/A, answer this
question.
Q2.
If the length of a resistor is doubled, how is the value of the resistor
changed?
OK! Now to the next question.
Q3.
What value do you expect for the series combination of two resistors that
you have made?
Q4.
Where ( which points of X, Y and Z ) should you connect your ohmmeter leads
to measure the second resistance?
Q5.
Where ( which points of X, Y and Z ) should you connect your ohmmeter leads
to measure the series resistance?
Now you're ready to measure values for your second resistor and the series
combination. Go ahead and measure your second resistor.
Is your measured value close to what you expected? You should have
written that down so you can check that against your new value..
Make sure, at this point, that everything is going as expected. If
your second resistance measurement was not what you expected, maybe you
need some art lessons. Maybe your connection wasn't firm enough.
Now that you have the two resistors measured, go ahead and measure the
series combination. When you have that measurement taken and you
think you are close enough to the correct measured value you can go on.
OK! Now you're going to measure some resistors that aren't homemade.
Get at least four resistors, all of different values.

Measure all four values
and check that they agree with what the color code says they should be.
Remember, you won't have great accuracy if you are using 10% resistors!
Use the Hydra to measure them.

Connect two of them in
series as shown below. Use your circuit board as shown below.

Click here if you need
to be convinced that this is really a series combination.

Next measure the series
combination and compare to what you expect.

Measure at least five
different combinations and record in your laboratory book. Now is
the time to start establishing good habits. Record both your measurements
and the values you expected to get from the color
code values.

Compare your results and
predictions remembering that you probably have 10% resistors.
Finally, as a test of what you've learned, measure
the same five combinations in parallel. When two resistors are in
parallel, the same voltge appears across both resistors.

Figure out how to get
them in parallel on the circuit board.

Measure and record values
in your lab notebook.
Some Points to Ponder
When you construct your resistor, you should be able to answer the following
questions.

What happens it you make
the center leg longer?

What happens if you make
the center leg wider?

What happens if you make
the pencil markings in the center leg darker?
You can check the answers to these questions
experimentally. Do that now.
Resistor
Accuracy  Lab Problem
Are you a skeptical, maybe cynical, person? Then consider this idea.

Imagine that you are a
resistor manufacturer. As you manufacture resistors, measure them
as the come off the line. Say you're trying to make 1000W resistors.
You know that when you're done you are going to have a whole bunch of resistors
and that they are going to range from 800W to 1200W . That means
that they run from 20% low to 20% high. While that's not bad, you
also realize that you can sell 1% resistors for a lot more money.

So, here's the idea.
As you measure the resistors, select out the ones that are within 1% of
the value you want. Leave the rest in the bin. Maybe while
you're at it you put the ones that don't make 1%, but are still within
5% into another bin. The result of this process is that you are left
with a bunch of resistors that don't include any that are really accurate.
You sell those as ordinary 20% resistors.

Next, you package the
1% resistors and 5% resistors separately and sell them at a higher price.
Now,
the question is "Is the above process what actually happens?". In
this laboratory problem you will test that idea and gain some practice
measuring resistors in the process. (You can find more discussion
of this idea in "Electrical Engineering Uncovered" by White & Doering
(Prentice Hall, 1997) in Chapter 4.)
Here's what you should do.

Go to your resistor supply
and get twenty (20) resistors of the same size.

Choose something from
100W
to 10,000W.

Measure and record the
values of those resistors and from your measured values come to a conclusion
about the claim above.

If you need to, you can
click
here to go to the lesson on measuring resistance with an ohmmeter.

Your report should evaluate
the claim that resistors are sold by selecting out the accurate values
and selling them separately.

Your reasoning should
be based on the experimental values you obtained from your measurements
seasoned with your knowledge of statistics.
Resistor
Accuracy
Here's
what you're trying to get from this lab problem.
Given that you need a resistor of some nonstandard size,
To know what to expect when you use a resistor from a bin of standard resistors
and use them in series and parallel combinations.
Resistors in series should have an equivalent that has a resistance that
is the sum of the values of the resistors in series. In this lab
problem you will check that theoretical claim, and you will get some practice
wiring series elements. Here are the goals for this problem
Given that you need to put two resistors in series or parallel,
Be able to predict the equivalent value with confidence,
Be
able to make the physical connection in series on a circuit board.
Here is what you should do.

Get some resistors  10kW,
22kW,
47kW,
and connect all series
combinations of two resistors (including two the same) and check individual
values and series
values.

To connect two resistors
in series, use
two resistors in a circuit board as shown below. Notice that the
point in common to the two resistors  the node to which they are both
connected  is made by connecting to a group of five points in a
single row. Make sure you understand the correspondence between
the circuit diagram and the actual circuit.
Your report should answer the following questions.

Is the claim true?

Can you predict the
series
resistance more accurately if you take the "stripe value" for the resistance,
or is it better if you actually measure the individual resistors?

Does the order of
the resistances matter? If you interchange the two resistors does
the series resistance
change?
Resistors in parallel should have an equivalent that has a resistance that
is computed using a standard formula. In this lab problem you will
check that theoretical claim, and you will get some practice wiring parallel
elements.
Here is what you should do.

Get some resistors  10kW,
22kW,
47kW,
and connect all parallel
combinations of two resistors (including two the same) and check individual
values and parallel
values.

To connect two resistors
in parallel,
use two resistors in a circuit board as shown below. Notice that
points in common to the two resistors  the nodes to which they are both
connected  is made by connecting to a group of five points in a
single row. (Those five points constitute a node.) Make sure
you understand the correspondence between the circuit diagram and
the actual circuit.
Your report should answer the following questions.

Is the claim true?

Can you predict the
parallel
resistance more accurately if you take the "stripe value" for the resistance,
or is it better if you actually measure the individual resistors?

Does the order of the
resistances matter? If you interchange the two resistors does the
parallel
resistance change?
The
TwoLightBulb Problem  A Lab Problem
This problem requires two dissimilar light bulbs. Example light bulbs
are shown in the photo below. If you look at the light bulbs closely
they are not the same. Here's what you should note.

The two light bulbs are
designed to operate from the same voltage source. Call the value
of that source V_{s}.

When operated from the
same source voltage, the two light bulbs dissipate different amounts of
power. Call the power values P_{1} and P_{2}.

As questions are raised
in this lesson, you should record your answers and any supporting experimental
observations in your laboratory notebook. You will need that data
and those thoughts to write your report. Questions that you should
answer will always be colored dark blue.
There
is one other requirement.

Your analytical/paper
work should be done and handed to your instructor before you begin the
experimental work.

Your analytical work should
be done entirely symbolically. You will not learn values for the
bulbs until you have done the theoretical work.

The first thing you should
do is to answer the following questions.

If the two light bulbs
are wired in parallel, which light bulb will give off the most light?

Since the two light bulbs
dissipate different amounts of power when connected to the same voltage,
which light bulb will give off the most light?

Devise an experimental
scheme to determine which light bulb has the higher power/wattage rating.

Now, consider what happens
when the light bulbs are wired in series.

Predict which light bulb
will give off the most light when the supply voltage is applied to the
series combination. Is it the higher power or the lower power bulb?

Enter your argument into
your lab book. It should be written as a convincing argument  i.e.
one which would convince someone who knows about power, series resistors,
Ohm's Law, etc.

Hand in your report.

Now, wire the two light
bulbs in parallel.

Apply the source voltage.

Determine experimentally
which light bulb has the higher power rating. Your instructor will give
you the value of the supply voltage you should use.

Determine experimentally
which light bulb gives off the most light.

Now, connect the bulbs
in series.

Apply the source voltage.

Determine which bulb gives
off the most light.
The
TwoResistor Problem
This problem requires two resistors.
Use four 1/4 watt resistors. You will
have two resistors of one value, R_{a}, and two resistors of another
value, R_{b}.

You will need a power
supply that can produce a variable voltage  up to 25 or 30v.

As questions are raised
in this lesson, you should record your answers and any supporting experimental
observations in your laboratory notebook. You will need that data
and those thoughts to write your report.

Your analytical/paper
work should be done and handed to your instructor before you begin the
experimental work.

Your analytical work should
be done entirely symbolically. You will not learn values for the
bulbs until you have done the theoretical work.
Now
do the following

Imagine applying a voltage,
V1 to two resistors in parallel. Assume both resistors have the same
wattage rating.

Predict which resistor
will get warmer, and record your reasoning in your lab notebook.

Imagine applying a voltage,
V2 to two resistors in series. Assume both resistors have the same
wattage rating.

Predict which resistor
will get warmer, and record your reasoning in your lab notebook.

Now, hand in your work
to your instructor and begin the experimental portion of this exercise.
Now,
after you measure both resistors you use,

Apply 5v to a 100W
resistor in parallel with a 150W.
Wait for the two resistors to come to thermal equilibrium. Wait at
least two minutes.

Do not touch the resistors
during this period. Then, see if you can determine which resistor
is warmer.

Determine if the resistors
are operating within their rated range.

Now, slowly increase the
voltage above 5v. At this point you are putting a voltage across
the resistors which will eventually take them beyond their rated wattage.
As you carefully and slowly increase the voltage, the resistor which first
emits smoke is the one which is warmest. Turn off the power supply,
and let that resistor cool down to room temperature and measure the resistance
value to determine if it has changed value.
Now, using the other two resistors, measure
both resistors. Then

Apply 12v to a 100W
resistor in series with a 150W.
Wait for the two resistors to come to thermal equilibrium. Wait at
least two minutes. Do not touch the resistors during this period.
Then, experimentally determine which resistor is warmer.

Determine if the resistors
are operating within their rated range.

Now, slowly increase the
voltage above 12v. At this point you are putting a voltage across
the resistors which will eventually take them beyond their rated wattage.
As you carefully and slowly increase the voltage, the resistor which first
emits smoke is the one which is warmest. Turn off the power supply,
and let that resistor cool down to room temperature and measure the resistance
value to determine if it has changed value.