Why do we use a pencil
Why do pencil leads conduct electricity?
As an introduction to the topic of "electricity", everyday objects are often examined and divided into electrical conductors and non-conductors. In addition to various metal objects, there is often a pencil under the electrical conductors. Based on the name, that seems logical.
However, the pencil owes its name to a mistake. Graphite has always been used for the mine. Graphite is one of the natural manifestations of the element carbon. But graphite was initially mistaken for the lead ore galena (lead gloss). This is how the pen was given the name “pencil”, which is still used today. Instead of pure graphite, the leads are now made from a mixture of graphite and clay. How hard a lead is depends on the mix ratio. The higher the graphite content, the softer the lead.
In order for a substance to be able to conduct electricity, it needs freely moving charged particles. In metals, these are the outer electrons of the metal atoms, which, as so-called “electron gas”, can move freely throughout the metal crystal and, so to speak, are shared by all atoms.
Graphite consists of crystalline, flat layers. Within these layers, the carbon atoms are tightly bound to one another in hexagons. Each carbon atom uses three of its outer electrons to form these bonds. The remaining fourth outer electrons can move freely in the entire layer. As a result, graphite has a metallic sheen and can conduct electricity like a metal, but only along the layers. Graphite is a non-conductor (insulator) perpendicular to the layers.
In order to examine the conductivity of a pencil lead, all you need in addition to a pencil that is sharpened on both sides is a battery, a lamp with a socket and 3 cables.
Due to the different graphite content, not all pencils are equally suitable. We have had the best experience with a 6B pen. Interestingly, the leads of the very soft pencils (8B, 9B) did not conduct any electricity. You cannot avoid buying and testing several different pens.
Didn't find the right pencil? The → experiment kit “Electricity & Magnetism II” by → EMS Kraus contains the material and understandable instruction cards for this and other illustrative experiments on electric current.
http://www.cumschmidt.de/s_leitf_el01.htm (as of January 6, 2018)
https://de.wikipedia.org/wiki/Bleistift (as of January 6, 2018)
http://www.chemieunterricht.de/dc2/kristalle/c-mod.htm (as of January 6, 2018)
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