Graphite Experiment

[proscience]

Hi,

I want to do your Pencil Resistor experiment. If I use a 5 mm graphite stick instead of a #2 pencil, will the 9V battery be strong enough? Do I need to use a higher voltage power source? Can I connect two 9V batteries together and use the same circuit described in the procedure?

I need this answer soon, since this is for school (due Nov. 20).

Thanks!

[rmarz]

proscience - This is a very simple experiment that uses a small lightbulb to respond to the changing resistance of the pencil of varying lengths. A few thoughts. You mentioned using a 5 mm graphite stick. That is quite large, did you mean 0.5 mm? I measured the resistance of a 60 mm length of 0.7 mm diameter HB pencil lead. The resistance was quite low, around 2Ω. A 9 volt battery would drive quite a lot of current through this resistance, and if you had selected an appropriate light bulb, you would see a bright light. The resistance of a 6" pencil will exhibit a resistance (in ohms) that is linear. For example, if the resistance of the 6" pencil is 6Ω, at 3" it would be 3Ω and at 1" it would be 1Ω. What this experiment is really doing is demonstrating this Ω/inch constant. A better way to demonstrate it might be to use a multimeter to measure the actual resistance using the ohmmeter function, or measure current through the resistor and voltage drop across it. Then calculate resistance using Ohm's law of R=V/I.

V = I x R (Voltage = Current multiplied by Resistance)
R = V / I (Resistance = Voltage divided by Current)
I = V / R (Current = Voltage Divided by Resistance)

Here is another very detailed discussion of what makes pencil grades hard and soft by the proportions of graphite to clay. These proportions also make the resistance per inch. Hard pencil lead will have higher resistance per unit of length.

http://users.df.uba.ar/sgil/physics_pap ... lleads.pdf

Rick Marz

[proscience]

Hi,

I am using a 5mm graphite stick that is used in art. I will be comparing three different grades of graphite sticks 2B, 4B, and 6B. I measured the resistance using a multimeter and got a reading of 3.2 Ohms on the 6B stick. The stick is 7.3 cm long. I still need to build a circuit so I can measure the voltage and the current. The multimeter uses a 9V battery, so would the 9V battery in the circuit be powerful enough if I use my sticks rather than pencils? The sticks look cooler than the pencils.

I also looked at pencil marks under a light microscope at 200x. The 6B pencil left clumps that were more spread out than 2B. Is it because the graphite flakes off easier with a softer pencil?

Thanks for the article. Do you still think I can use the procedure list on science buddies? I saw another procedure that is set up the same way, but uses a 330 ohm resistor. What do you think is best.

Thanks

[rmarz]

proscience - You are on the right track. One of the benefits of using the larger diameter rod is that it will be mechanically stronger and resist breakage. Your observations about the 2B vs 6B appearance are likely correct. With regards to measuring the resistance using the 9 volt battery, your test setup is critical. The experiment uses the observation of the light bulb (must be chosen properly) to observe relative brightness. This will not provide an accurate measurement, just an indication of which is brighter/dimmer (lower/higher resistance) of the graphite rod. I think the use of the ohmmeter scale on the digital multimeter is the best way to go. As an aside, just because it also uses a 9 volt battery for resistance measurements, it also is designed to use internal current limiting "multiplier" resistors in the circuit to make the measurement with a very low test current. By comparison, a 1Ω-3Ω load directly on a 9 volt battery will be a near "short circuit" and quickly drain the battery and cause potential heating of the graphite rod. Using Ohm's law, those resistance values in combination with a 9 volt source would see current flow of 3 to 9 amperes. Your 9 volt battery probably has an internal resistance of over 50Ω so the maximum current it might source would be a fraction of that, perhaps 0.4 amperes for a few seconds. Your 9 volt battery would drop to 1 volt if measured with these loads.

If you use the multimeter approach you will probably find that shorting the test leads in the lowest range (probably 0-200Ω) you will get a low reading on the display after a few seconds. This is an error reading introduced by the resistance of the test leads, and will cause your actual readings to be in error. On my meter, it was 0.9Ω. If you are reading 100Ω or 1,000Ω, a 0.9Ω error is not significant. But measuring 1Ω or 2Ω value, it is significant. As a test, I shorted the leads and found that value to be 0.9Ω on my meter. I then measured a known resistance of 2Ω (a precision 2Ω resistor) and it measured 2.9Ω. When you subtract the 0.9Ω error you get a more precise value. Another way of getting a more precise reading is to use a circuit called a "Wheatstone Bridge". It uses three additional resistors along with your 'test' graphite rod and multimeter. You can Google that subject for additional information. I think you have the makings of a nice experiment here, especially if you show how you took into consideration the resistance error introduced by the test leads. You can buy a low resistance precision resistor (even 5% would be sufficient) and show your work clearly. Good luck.

Rick Marz

[proscience]

I thought I could add a resistor to my circuit so I could still use a 9V battery. In the end, I wanted to use my resistance results to calculate the conductivity of my graphite rods.

I was going to use my resistence to calculate resistivity by multiplying the resistence by the area, width x height for a rectangular rod and then divide by the length. Then I was going to divide 1 by the resisistivity to get the conductivity.

I thought it would be better to show data instead of just saying the graphite is a conductor.

Do you think this will work?

[rmarz]

proscience - Your idea is very good, and solves the problem of excessive current in the loop (saving battery life, as well). If you choose a resistor with a value of say 100Ω, and if your graphite rod is about 3.2Ω, your total load is 103.2Ω. Using Ohm's law, I=E/R you can calculate the current in the loop as being about 0.088 Amperes or 88 milliamperes. Almost 97% of the voltage from the battery (8.73 volts) will be dropped across the 100Ω resistor and about 0.27 volts across the graphite rod. You can also calculate the power dissapated by the resistor as P=I^2*R, or about 0.77 watts. You should buy a 1 watt rated resistor for the experiment. A multimeter in the 20 volt range will read two places to the right of the decimal. Should be sufficient for this experiment. If your multimeter has a 2 volt range you might consider an even higher value of series resistor, lowering the current and probably reduce the resistor to a 1/4 watt component.

A caution, there is sometimes a confusing understanding between resistivity and what we are measuring, actual resistance. Resistivity deals with the bulk properties of a material to conduct a current. See the attached link so you know how to apply the math.

There are many variants to approaching the problem, and it sounds like you have a pretty good understanding of Ohm's law at this point so you can define a very good experiment yourself. I agree, that the best projects clearly demonstrate that you understand what is happening, and support your hypotheses with a well defined experiment and good measurement and presentation of data.

http://hyperphysics.phy-astr.gsu.edu/hb ... resis.html

Rick Marz

[proscience]

Hi Mr. Marz,

Thank you very much for helping my daughter with her science experiment. It makes us appreciate that as simple as an experiment may seem, the science behind it can be very complicated. I think my daughter has a good grasp on this experiment.

My question: My daughter's teacher wants her to create computer generated graphs. What PC program should she use for this? She is not sure what she needs to graph. The length of the graphite is constant. Many experiments we've seen on the Internet plot voltage vs current. Would my daughter do the same? Or would she plot her calculated conductivity vs her resistance?

Thank you!

[rmarz]

Mom of proscience - There are many programs that will create a graphic plot of data. Probably the most common is Microsoft Excel. You can produce line, bar and other forms easily from a data set. If you don't have this program, you can download a free, similar program, called "Open Office" from www.openoffice.org or other download sites. The original experiment observed that changing the length of the pencil lead changed it's resistance (conductance). This data, if plotted, would appear as a linear, or straight line graphic (length vs resistance). In your experiment, if the rod length is constant, the thing that would be measured or observed are current, as voltage is changed. That would also appear as a straight line as the resistance is fixed. Anything plotted in this experiment, using resistivity, current or voltage is going to be a straight line. I think the value I noted in the prior comments really were that your daughter understood the concepts of resistivity and conductance and the basics of Ohm's law. Perhaps you could make those part of the experiment which was to support the hypothesis. You could always shorten the length to half or 1/4 and make additional measurements. The use of the different series resistors would also demonstrate Ohms law by showing the different voltage drops across the two resistors at different currents. In that voltage is constant, the sum of the two voltage will equal the battery voltage. I have been assuming that you are using a digital multimeter.

Rick Marz

[proscience]

Hi Mr. Marz,

This is Mom again, since my daughter is in school. She ran the experiment yesterday and got some very small numbers for voltage and current. She used a 330 Ohm resistor along with her graphite rod. We went ahead and did the math to calculate resistivity and conductivity with the results confirming her hypothesis. She predicted that the 6b graphite contained more graphite so it would conduct electricity much better than the 2b or 4b grphite. Her math showed that the 6b rod had the lowest resistivity with the highest conductivity.

Would it be okay if she graphed the resistivity and the conductivity, with conductivity on the y axis and resistivity on the x axis. I think this would be much easier for her to graph. Her average voltage for the 6b rod was 0.0582 V and the average current was 0.0001768 A. She is just beginning to learn exponents at school and hasn't learned scientific notation yet.

I tried graphing the resistivity on the x axis and the conductivity on the y axis and got a negative slope. However, I have no idea what I am doing and I don't want to tell my daughter something wrong.

Since she is in fifth grade, the school tends to frown upon projects that are too advanced for the grade level. Therefore, she needs to be able to do the work herself. So far so good, but I'm concerned about the graph.

Any suggestions?

Thanks!

[rmarz]

Mom of proscience - Sorry you are having difficulties. I've attached the simple schematic of the setup you should have. You can insert the multimeter as shown, set to the 200 milliampere scale to read current. Take it out of the circuit to measure voltage across either the 330Ω resistor or the graphite rod. because upwards of 99% of the voltage will be dropped across the 330Ω resistor, set your scale to 20 volts. The drop across the graphite rod will be quite small, so the 2 volt (2000 milliavoltss) or the 200 millivolt scale should be used. Please send me some of these readings if possible and we can try to analyze what the problem might be. Because of the very low resistance of the large graphite rod, we can be seeing some measurement errors at these levels. Having said that, you should still see a relative lower resistance in the 6B material than the similar 2B graphite in the same setup. Here is a detailed discussion of pencil lead resistivity.

http://users.df.uba.ar/sgil/physics_pap ... lleads.pdf

Rick Marz

[theborg]

Would it be okay if she graphed the resistivity and the conductivity...I tried graphing the resistivity on the x axis and the conductivity on the y axis and got a negative slope. However, I have no idea what I am doing and I don't want to tell my daughter something wrong.

Since she is in fifth grade, the school tends to frown upon projects that are too advanced for the grade level. Therefore, she needs to be able to do the work herself. So far so good, but I'm concerned about the graph.

Any suggestions?

Proscience's Mom, In reference to your question on the graph. Graphing is a great way to depict the relationship among variables in an experiment and can could be done within the bounds of the grade level provided the student understands and can explain that relationship. So it's usually less an issue with the presentation of the data being outside the grasp of the student than it tends to be the concepts behind the experiment/data that are confusing. This doesn't seem to be an issue here. There looks to be a fair grasp of the concept of Voltage, Current and Resistance going on here. That being said, on the issue of what to graph...

Resistivity and Conductivity are inverse parameters of each other. Resistance (R) is the amount by which a material "resists" the conductance of current through it. Inversely, Conductance (C) is the amount by which a material allows the flow of current through it. Mathematically: C = 1/R. What you really want to graph is the relationship between the variables you are testing. In this case, as I read from the above posts, is Graphite Lead Type (2B, 4B, 6B) vs. Resistance. Given that the circuit is the same for all Graphite samples (i.e. voltage input) and the samples are all the same size, then the independent variable (X) you are changing is the level of graphite and the dependant variable (Y) is the measured resistance under the conditions of the experiment. Therefore, I would suggest a bar graph with sample type across the bottom vs. Resistance on the Y-axis to show the difference in resistance as you increase/decrease graphite content. Microsoft Excel would work very well for a computer generated graph.

[proscience]

I apologize for my very late response. Thank you very much for your prompt reply. My daughter plotted the graph as you suggested. The conductivity results she got were within the range of amorphous carbon. She concluded (through research) that pencils usually contain amorphous carbon and clay that helps keep the pencil lead's hardness and shape. Also the more graphite the pencil contains, the less resistance it has and therefore the higher the conductivity.

She wanted to use this experiment to demonstrate the conductivity of graphene (a one-atom thick layer of graphite) and how graphene my one day replace silicon in electronics. Her primary interest is in the use of graphene in sensors for use in bionics or sensors that can be applied directly to human tissues.

We appreciate everyone's help on this project.

Thank you and Happy Holidays!

[theborg]

Very glad the experiment worked out. The thoughts on future applications is very interesting. Tell your daughter to keep up the good work and inspired ideas. Happy holidays to you as well.