Question about circuit and alligator clips on graphite

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Question about circuit and alligator clips on graphite

Postby amyStudent » Tue May 08, 2012 7:32 am

I am doing the Dimmer Switch project:

I did my testing yesterday, three trials measuring the reading of the light for each setting on my pencil using a light meter. I have a question.

I noticed when i clipped the alligator clip to the graphite, the number that appeared on the lux meter was much higher than just touching the graphite with the alligator clip at the same spot. Can you help me understand why?
Thank you,

Re: Question about circuit and alligator clips on graphite

Postby jmehta14 » Tue May 08, 2012 6:07 pm

Dear Matthew,

The results you have observed can be explained by the fact of the limited area of contact created when just touching the graphite. This limited area doesn't allow for the electrons to "efficiently" flow through the wire. Mathematically, this can be explained by the formula: R = (p*L)/(A), where "R" is resistance, "p" is a special property of the wire/graphite called resistivity, "L" is the length of the wire, and "A" the cross-sectional area of the wire. As you can see a bigger cross-sectional area leads to lower resistance whereas a lesser cross-sectional area gives a higher resistance. Basically, the increased area allows the electrons flowing through the current to enter the wire at more places, and thus allows the electrons to roam more freely through the wire.

I hope this helps.

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Re: Question about circuit and alligator clips on graphite

Postby amyStudent » Wed May 09, 2012 7:40 am

Jay - Thank you for trying to help me understand.

I am in fifth grade...

I'm still not sure i understand... does the pressure when clamping it change the amount of electricity compared to just touching it? (Parent note: the pencil is halfway shaved down, so graphite is exposed on the top only. When he clamps the alligator clip, he is therefore clamping down on graphite on one side and wood on the other.)

Does clamping the alligator clip around both the wood and the graphite lessen the amount of electricity than just clamping it around just the graphite? (I haven't tested using only graphite.)

Also, I noticed that it sparks sometimes when you try to move the alligator clip. Why is that?

And, when I did my readings with the lux meter, the meter NEVER stopped on a single number. Why not? It did show higher numbers and lower numbers for the different spots on the graphite, which I understand. But I wondered why it never seemed to "stop" and give me a single reading.

Thank You,


Re: Question about circuit and alligator clips on graphite

Postby rmarz » Wed May 09, 2012 11:42 am

Matthew - As Jay explained, the alligator clip makes a much more effective electrical contact with the graphite rod than just a light surface contact. I suggest to reduce this contact variable and assure a more even contact pressure that you use another alligator clip to clamp the bulb connector at the various test distances on your pencil. That way you have taken a bit of variability out of your results. Often I actually go out to my garage to test the project and see if I understand the difficulty some students are having. I did that in this case and produced the same setup with a #2 pencil and a 9 volt power supply. I used a Christmas tree bulb from a 100 light strand. These bulbs are rated at about 2.5 volts each for optimum light output and lifetime. At that voltage the bulb conducted about 150 milliamps. Using the #2 pencil and a 9 volt battery, I found the current to vary between 140 to 300 milliamps over the length of the pencil. That probably means the bulb won't last too long at the higher output.

I don't believe the 9 volt battery will have a very long lifetime at these high currents either. In any case these suggestions should make it easier for you to get the data you need to record reliably.

You observed that there were noticeable bright sparks when the wire made contact with the pencil. If you think about the current that is flowing (about 250 milliamps) and the very small area of contact (perhaps just one ten-thousandth of a square inch) the current density at the point and moment of contact would be about 2500 amperes per square inch. A very high current density. That is a level beyond the largest industrial arc welders, but in this case it is occurring in an insignificant physical area. The spark you saw was likely the burning of a small piece of carbon from the pencil lead. Interesting phenomena.

You probably also noticed that the pencil lead got warm to the touch. If you calculate the power loss in the pencil lead with this bulb it is about 1 watt (0.25 amperes, 4 volts drop). Over the length of a 5 inch pencil, the heat can certainly be felt to the touch. Good luck with your results.

Rick Marz
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Re: Question about circuit and alligator clips on graphite

Postby amyStudent » Thu May 10, 2012 7:20 am

Dear Rick-

Thank you for the help!

In my setup, I taped the lightbulb and the LUX meter to the ground and walls of a cardboard box lid so the bulb and the LUX meter would always be in the same place. I also used the same color tree lightbulb in this experiment for all three trials. (I did burn out a few, too!)

I think you answered my question about why I was seeing better results "clipping" the alligator clip rather than just touching it to the surface.


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