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Unformatted text preview: Scott Hughes 3 February 2004 Massachusetts Institute of Technology Department of Physics 8.022 Spring 2004 Lecture 1: Introduction; Couloumb’s law; Superposition; Electric energy 1.1 Fundamental forces The electromagnetic force is one of the four fundamental forces of physics. Let’s compare it to the others to see where it fits in the grand scheme of things: Force Range (cm) Interaction Exchange “Strength” particles particles Gravity ∞ All mass/energy graviton ( m = 0) 1 Weak 10- 15 All elementary weak bosons 10 24 particles except ( m ∼ 100 × m proton ) photon & gluons Electromagnetic ∞ All charges photon ( m = 0) 10 35 Strong 10- 13 quarks and gluons gluons ( m = 0) 10 37 (The “strength” entries in this table reflect the relative magnitudes of the various forces as they act on a pair of protons in an atomic nucleus.) Let’s summarize the major characteristics of the electromagnetic force: • Strong : It is the 2nd strongest of the fundamental forces — only the appropriately named strong force (which binds quarks and nuclei together) beats it (and only at very small distances). • Long Range : As we’ll discuss soon, the electromagnetic force has the form F ∝ 1 /r 2 , just like gravity. This means it has infinite range : it gets weaker with distance, but just slowly enough that things really far away still feel it. Contrast this with the weak force, which has the form F ∝ exp(- r/λ ) /r 2 , where λ ’ 10- 15 cm — it dies away REALLY fast with r . • Has 2 signs : It can attract and repel. Contrast this with gravity which only attracts. • Acts on charge : More on this shortly. The electromagnetic interaction plays an extremely important role in everyday life. The forces that govern interactions between molecules in chemistry and biology (e.g., ionic forces, van der Waals, hydrogen bonding) are all fundamentally electromagnetic in nature. A thor- ough understanding of electromagnetism will carry you far in understanding the properties of things you encounter in the everyday world. (Add quantum mechanics and you’ve got most of the tools needed to completely understand these things!) 1.2 History of electromagnetism Circa B.C. 500: Greeks discover that rubbed amber attracts small pieces of stuff. (Note the Greek word for amber: “ ηλ²κτρ o ν ”, or “electron”.) They also discover that certain iron rich rocks from the region of M αγνησια (Magnesia) attract other pieces of iron. 1730: Charles Francois du Fay noted that electrification seemed to come in two “flavors”: vitreous (now known as positive; example, rubbing glass with silk) and resinous (negative; rubbing resin with fur) 1740: Ben Franklin suggested the “one fluid” hypothesis: “positive” things have more charge than “negative” things. Suggested an experiment to Priestley to indirectly measure the inverse square law......
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