Electro Statics on the Black Board

Black board

Electro Statics is the study of interaction b/w stationary charges. Charges are the properties associated with particles. Chalk powder consists of many such particles. Now we need to know how chalk pieces are made, where are they obtained from. This helps us to understand the type of charge associated with it. Sticks of processed chalks are produced especially for use with chalkboards in white and also in various colors. These are not actually made from chalk rock but from calcium sulphate in its dihydrate form, gypsum.
When we use a chalk piece to write on the black board, what we write is visible clearly. When we use the same chalk to write on a well polished granite stone, nothing is visible. Why does this happen? Going back to the lessons on Friction, we know that friction is developed on the surface with greater irregularities which are not visible through naked eye. By comparing black board with a polished granite stone, we find greater irregularities on black board than on polished granite stone. So when we write using chalk on the black board a friction is developed. Due to this the phenomenon of  charging a body by friction takes place. Thus the minute chalk particles get charged and stick to the board due to force of attraction.
We also witness some small quantities of chalk particles spilling down while we write. This is because some particles may get charged or the magnitude of charge may relatively be small. When we rub the black board with a force greater than that of the electrostatic force, what we had written gets erased as the particles again due to friction stick on the sponge of the duster. Some times when a repeatedly used duster is used to erase the board, we notice that the board is not erased clearly because the sponge of the duster is very densely occupied by the chalk particles. Thus we have to clean the duster after repeated use. This can be achieved by simply dusting it out by repeated beating on the surface of the wall. This concludes electrostatics on the Black board.