Electricity and Magnetism
Two natural substances, amber and lodestone, have awakened curiosity since ancient times. Amber is sap that oozed long ago from certain softwood trees, such as pine. Over many centuries, it hardened into a semitransparent solid akin to model plastics and ranging in color from yellow to brown. It is a handsome ornamental stone when polished, and sometimes contains the remains of insects that were caught in the sticky sap. Ancient Greeks recognized a strange property of amber. If rubbed vigorously against cloth, it can attract nearby objects, such as bits of straw or grain seeds.
KeywordsElectric Charge Electric Field Strength Electric Force Gravitational Potential Energy Test Body
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