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- 1. The Electrostatic Force • Charge of objects • And separation distance – Influence the force between them.
- 2. As q increases, F increases.
- 3. As D increases, F decreases. Distance Force
- 4. Coulomb’s Law • Notice that d is squared!!!!!! This is another example of an inverse-square law. 1 2 2 c elec k q q F d = q charge (Coulombs or C) Distance (meter of m) Electric Force (Newtons or N) kc (8.99 x 109 N-m2 /C2 )
- 5. Symmetries • Recall Newton’s Law of Gravitation: • And Coulomb’s Law: • Notice the similarities?? 1 2 2 c elec k q q F d = 1 2 2grav Gm m F d =
- 6. Proportionality Constants • Notice their relative size!!! 1020 difference! 1 2 2 c elec k q q F d = 1 2 2grav Gm m F d = G (6.67 x 10-11 N-m2 /kg2 ) was a proportionality constant Newton’s Law of Universal Gravitation kc (8.99 x 109 N-m2 /C2 ) is the proportionality constant Coulomb’s Law
- 7. Using Coulomb’s Law • Why do they use such small numbers for ordinary objects? Wouldn’t it make more sense to call them 1 C instead?? • The answer lies in the famous “Oil Drop Experiment.”
- 8. Using Coulomb’s Law • So what sort of charges are we talking about? q is 1-10 C VERY LARGE q is more like 10-6 or 10-9 C
- 9. The Elementary Charge • http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/c d_errata_fixes/section4_5.html
- 10. The Elementary Charge • Robert Millikan -1909 – electric charge is quantized. • Quantized - things can only have certain values, not just any value. • Ex. Is it possible to own 2 ½ bicycles??? • Animation 1 (Quicktime) • Animation 2 (Java)
- 11. The Elementary Charge • Millikan demonstrated that all the oil droplets had charges which were multiples of each other! • All the droplets carried some multiple of what is called the Elementary Charge, 1.6 x 10-19 Coulombs. • This is the charge of a single proton or electron. • Ordinary objects have charges like 10-9 C, this is still about 10 billion excess charges!!!
- 12. Using Coulomb’s Law • Why is Coulomb’s Law called an “inverse- square law?” • Because of the d2 in the equation. • Doubling the distance between objects cuts the force by ¼ , just like Newton’s Universal Law of Gravitation. • Ex. Sound intensity, Spray paint, the “Butter Gun”
- 13. The Inverse-Square Butter Gun
- 14. Bohr’s Model of Hydrogen • You’ve seen the similarities between Newon’s U.L.o.G. and Coulomb’s Law. Niels Bohr used this similarity to create a model of the Hydrogen atom. • Just like the Earth orbiting the Sun, Bohr pictured the electron orbiting the proton. • Animation

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