Tuesday, July 15, 2008

How Does a Pencil Eraser Work?

The pencil eraser works based on the friction developed between the eraser and the paper. Friction is what causes your hands to heat up when you rub them together. When you rub two objects the roughness of their surfaces contact each other and rub against each other causing friction. A simple pencil is made of a combination of wood and finely ground graphite and clay, mixed with water and pressed together at high temperatures into thin sticks or rods. Graphite is a mineral composed of an element called carbon, and it is black in color. There are also mechanical pencils that need rods of graphite to function like a pencil. You may have heard that pencils are made of lead, but that is actually a misunderstanding that is based on the initial thoughts of those that first discovered graphite - they believed it to be lead, which was not the case at all. However, many still refer to graphite in pencils as lead. Graphite particles are arranged in layers or sheets. A pencil mark consists of graphite particles that have peeled off from the pencil point onto the paper. These particles have an angular, gritty look to them when viewed under a microscope. When the pencil is used on a sheet of paper, the graphite particles lie slightly below the surface of the paper, interlocked between its fibers. A single rub using an eraser sufficiently soft to reach between the fibers will pick up most of the graphite particles. Looking at the eraser you can see undamaged graphite pieces sticking to the surface. An effective erasing material scratches the paper surface, producing the familiar small spindles of rubber or eraser material, which wrap up the graphite particles. When you look at these under an optical microscope at 200 times magnification (200x), these look like roly-poly puddings studded with graphite raisins.

Little Lion Experiment:

Erasers come in a variety of colors: white, pink and gray are some of them. Sometimes the color difference is because of a dye or because the eraser is made of a different type of rubber. Go around your house and see how many types of different erasers and pencils you have. For example, compare number 2 pencils with a number 3 pencil. Also, if someone in your home has a mechanical pencil, you can purchase different types of pencil leads (like hard black or soft) or they might have different leads you can use. The bright-colored erasers (like purple and yellow) are usually white erasers in disguise! You can also use erasable pen as a pencil type. See which one of these works best with different types of pencils and ink. Can you erase the ink with a pink or white eraser? Is there one eraser that works for all lead types? Knowing what you know about how erasers work, why do you think certain erasers do not work with other types of pencil lead and ink?