We began our discussion of electromagnetism. The "source" of electricity is the electric charge, which comes in two varieties -- positive and negative, a convention that Benjamin Franklin had a hand in establishing. One can get charges on various objects usually by having two surfaces come in contact -- say dragging your feet across the carpet in winter. (In winter the air is drier which allows one's charge separation to be maintained, that is, until the shock occurs.) It was found that opposite charges attract and like charges repel. This nature of this force was quantified by Coulomb. Coulomb's law allows one to calculate the force between two charges.

F_Coulomb = + k q₁ q₂ / r²

The same result had been found by Cavendish earlier -- but he did not publish. The structure of the force law is reminiscent of Newton's Law of Universal Gravitation -- especially in its inverse square dependence on the distance between the charges.

F_Newton = - G m₁ m₂ / r²

The minus sign in Newton's Law denotes that two masses attract each other. In Coulomb's Law there is a plus sign since like charges repel each other. The gravitational and electromgnetic forces are two of the four Fundamental forces. The other two fundamental forces are very short distance forces that apply within the nucleus of an atom: the strong and weak nuclear forces.

Polarization (two senses).

Magnetic dipoles

Static electricity versus Current

Why people today say Benjamin Franklin got it backwards

Volta and frog legs

Side line: What did Watt do?