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 pencil is made of graphite. Graphite is a mineral composed of an element called carbon and is black in color. A pencil mark consists of graphite particles that peel off from the pencil point by the paper. These particles, which have an angular, gritty look under the microscope, are commonly used in hard black (HB) pencils, typically between 2 and 10 micrometres in diameter. This is about 6 times smaller than the thickness of the human hair!
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 them. Looking at the eraser you can see undamaged graphite piece 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 you house and see how many types of different erasers and pencil leads you have; number two and number three pencils are different types of leads. Remember the bright-colored erasers, like purple and yellow, are usually white erasers in disguise! If someone in your home has a mechanical pencil, you can purchase different types of lead, like HB or soft, they might have different leads you can use. You can also use erasable pen as a lead 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?
Thinking about experiments, scientists look at the different things or factors in the experiment called variables. The variables you have in this experiment are erasers and lead types, assuming you always use the same type of paper. A scientist is always concerned about the number of variables in there experiments because it tells them how many experimental runs they need to do. An experiment run would be a single type of eraser against a single type of lead. The total number of experimental runs you would do in your experiment are the number of erasers times the number of leads. By limiting your variables you limit your number of experimental runs, but you don't want to limit your variables too much otherwise your experiment will not be conclusive or lead to a correct answer that may be misleading. For example, if you use all the lead and eraser types in your house, you can say that you have a conclusive study of the erasers and leads in your house. If you just look at the erasers and leads in your room, you could not say that you know about all the erasers and leads in your house, just about the ones in your room. Sometimes scientists limit there variables, like for instances just the erasers in just your bedroom, and then predict from those experiments to say how the erasers in the house would perform. Not all predictions are good, but sometimes it is the best a scientist can do.
