It was the 6th century B.C. Greek philosopher and scientist Thales of Miletus who discovered an interesting phenomenon when one rubbed amber (a fossilized secretion of ancient trees) with fur: the amber could attract light objects such as straw1. In modern parlance, we would say that the both the amber and the straw were electrically charged. No one knows what electric charge actually is or why it exerts force on other charges around it2. We can only study its behavior.
Scientists in the 18th century sought to explain why electrically charged objects not only attracted but also repelled one another; they posited two types of electric charge — positive and negative. Two objects with like charge (positive and positive, or negative and negative) repelled each other. Two objects with unlike charge attracted each other3.
Much later it was found that electric charge could be traced to the atom itself. Protons are all positively charged, electrons are negatively charged, and neutrons have no charge. All protons and electrons have the same absolute amount of charge, so an atom with the same number of protons and neutrons is electrically neutral4.
When objects having electric charge are at rest, they radiate simple electric fields, which act on other electric charges resembling the way gravity acts on mass. Like the gravitational pull between two masses, the electrical force between two electric charges at rest is inversely proportional to the square of the distance between them. If you move two charges closer together so that the distance between them becomes only a third of what it was, the force between them will increase nine-fold5.
Interesting things happen when charges move. Our civilization depends on electric current, which usually consists of free electrons traveling through electric wires forming rivers of electricity6. Electric currents generate a second force called the magnetic force, a force that attracts or repels other electric currents7. Whenever moving charges speed up, slow down, or change direction, they generate waves in the electric and magnetic fields around them. These waves, known as electromagnetic radiation, form such familiar phenomena as radio waves, visible light, and X-rays8. More about electromagnetic radiation in a later post.
Footnotes
- Watkins, Thayer. The Economic History of Amber, San Jose State University website. To view, click here.
- Shpenkov, George P., What the Electric Charge Is. To view, click here.
- This began with the work of the French chemist Charles François du Fey (1689 – 1739) and the American scientist Benjamin Franklin (1706 – 1790). The work of these two men are neatly summarized by the Center for Integrating Research + Learning on the National High Magnetic Field Laboratory website Magnet Lab. Click here to view their summary of Du Fey’s work (starting from the fifth paragraph) and here to view their summary of Franklin.
- Ohio State University Department of Chemistry and Biochemistry website. Atomic Structure, section “Protons, neutron & electrons”. To view, click here.
- Hyperphysics website, Department of Physics and Astronomy, Georgia State University, section “Coulomb’s Law”. To view, click here.
- Hyperphysics website, Department of Physics and Astronomy, Georgia State University, section “Electric Current”. To view, click here.
- Rensselaer Polytechnic Institute website. Introduction to Magnetism and Induced Currents. To view, click here.
- Caltech university website. Electromagnetic field of an accelerated charge. To view, click here.
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