PICTURES COMING SOON

Pencil “lead” does conduct electricity. You can show this with a simple LED, some wire, and a 9-volt battery. You can make it more quantitative with different “drawings” and even more so with an inexpensive electronic multimeter.

Pencil “lead” does conduct electricity

Equipment: an ordinary no. 2 pencil (other hardnesses work, too); a sheet of paper; one LED (light-emitting diode; almost any color or size; pennies); a 9-volt battery (even a run-down one may work; free, or maybe $3); wire leads(“leeds”) for the 9V battery (there are snap-on connectors, as shown ($0.50 but hard to purchase singly); you can also twist ordinary wires around the battery terminals and hold them on, say with tape.

Why pencil lead conducts electricity: Pencil lead is, of course, not metallic lead as was sometimes used to make marks centuries ago. It is graphite mixed with clay and fired to make it hard. Graphite is a form of carbon, as is charcoal (rather impure) and diamond. It conducts electricity, with some amazing special properties having been found recently, but those are long stories. Graphite is in the form of single sheets, one atom thick! Bulk graphite is made of perhaps thousands of sheets stacked together. The sheets easily slide apart, which makes graphite easy to write with as the sheets smear across the paper. Fortunately for this demo, the sheets overlap enough, even with that clay spreading around, so that they maintain connections of some of the sheets as far as you write. So, a nice, thick pencil mark acts as an electrical conductor, if a weak one… but good enough to carry a current to light an LED!

Setting up the demo: Wrap the lead from the positive terminal of the 9V battery to the longer lead of the LED. This is the lead that’s intended to carry a positive current into the LED to light it up. See the note at the end about positive and negative. Be sure you have the correct lead on the LED; applying 9V the wrong way might burn out the LED. Make the wrapping good and tight; expose more bar wire on the battery lead if you need to. You can strip off the insulation with a sharp knife held almost parallel to the wire – CAREFULLY. Strip the insulation off on two or three sides and then pull off the remainder. You’re now ready.

PICTUREs, with annotation

Lighting the LED: Place the LED down on the sheet of paper. Press down both the negative lead of the ED (the shorter lead, not wired) and the negative wire from the 9V battery. Put them any handy distance apart. DO NOT let the LED lead and the battery wire touch each other or you will instantly flow out your LED; it is not meant to carry huge currents that would result. Oops, did it? Just get another LED and start over. Now scribble with the pencil to bridge the gap between the LED lead and the 9V battery wire. The LED will start to glow! The heavier you make the pencil mark the more the LED will glow. Heavier marks make more conductive paths for the electricity. Of course, you can erase part or all or your pencil mark and see the effects.

Different pencil marks: Move to a clean section of the paper and try using smaller or larger distances between the LED lead and the battery lead. See how that affects the intensity of the glow. You’re learning about electrical resistance in “series,” just as you effectively learned about electrical resistance in “parallel” by making thicker or wider pencil marks the first time.

If you have some other equipment: You can measure the electrical resistance of the pencil mark or marks that you make with a multimeter (cheap – as low as $5, though fancier ones up to, say, $50, have many more functions, more sensitivity, and greater accuracy). Here’s a picture of one being used to make the measurement:

PICTURE

You can put the LED and battery off to the side and just examine the pencil marks with the multimeter. Set the function dial to ohms (with the symbol Ω, or word “ohms”). Press one lead of the multimeter to one end of the pencil mark and the other lead to the other end. Any modern multimeter will automatically choose the appropriate “range” of resistance. In the case shown, the reading is XXXkΩ. The “k” stands for “kilo,” or thousand. That’s a pretty high resistance but it does pass enough current. The resistance is high because most of the contacts between pieces of graphite have been interrupted by smudges of clay. You can figure out how much electrical current flows through that resistance. The LED “drops” 2 to 3 volts of electrical potential, depending on its color. The rest of the 9V drops across your pencil mark (the drop across wires is negligible). So, if you picked a red LED and a battery that has an actual 9V, then there’s 7V or so to pass across the pencil mark. The current is then those 7V divided by the resistance. That’s 7V/50,000 Ω or about 0.00014 amperes; amperes are the measure of current. We can express the current also as 0.14 thousandths of an ampere, or 0.14 milliamperes (mA). A typical LED at maximal current flow might pass 20 mA, or, for a bigger one, 50 mA… and there are huge LEDs, too, that pass more than a full ampere. Even at small current flows that you got with the pencil marks you can see enough light from the LED.

About positive and negative things in electricity. There’s an unfortunate terminology in electricity. In most electrical conductors, it’s the electrons that move. They have a negative charge. However, by convention, the flow of electricity or current is called positive when it moves in the direction opposite to the flow of electrons. Think of it this way: negative things moving to the left achieve the same transfer of electrical charge as positive things moving to the right. This way of thinking becomes automatic as you work in electricity and electronics. You usually don’t even worry if it’s electrons or ions moving.

 


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