Electrostatics

the study of electrical charges at rest

A charged object is created by the separation of charges:

• an atom is electrically neutral; it has the same number of protons (positive charges) as it does electrons (negative charges)
• objects are charged by adding or removing electrons
• a positive charge occurs when there are fewer electrons than protons; its classical definition is the charge accumulated by a glass rod rubbed with silk or wool
• a negative charge occurs when there are more electrons than protons; its classical definition is the charge accumulated by a hard, rubber rod rubbed with fur

The choice of which name went with which charge was arbitrary. We follow the convention today that was set by Benjamin Franklin. Franklin called the charge accumulated by the rubbed glass rod to be positive and that on the rubber rod negative. Franklin also argued that whenever a certain amount of charge was produced on one body in a process, an equal amount of the opposite charge was produced on another body. In any process, the net change in the amount of charge produced is zero.

Law of Conservation of Electric Charge The net amount of electric charge produced in any process is zero. If one region or object acquires a positive charge, then an equal amount of negative charge will be found in neighboring regions or objects.

During the last century, it became clear that electricity begins inside the atom itself. In a simplified view, the postivively charged nucleus (containing postively charged protons) of the atom is surrounded by negatively charged electrons. Neutral atoms contain equal amounts of protons and electrons. If an electron is gained or lost, the atom is now either negatively or positively charged and is called an ion.

When objects are charged by rubbing, the electrons (which are free to move) are transferred from one object to another. For example, if a plastic ruler is rubbed with a paper towel, the plastic ruler becomes negatively charged because electrons are transferred from the towel to the plastic ruler. Eventually, this charged is lost, leaking off onto water molecules in the air. Water molecules are polar (though neutral, their charge is not distributed uniformly).

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