PHY 2049: Physics II

1. PHY 2049: Physics II • Tutoring Center is open in room NPB 1100, M-F 10:00AM -3:00PM. It is free. • Hopefully all webassign problems have been solved. Please see me immediately after the class if there is still an issue.

2. PHY 2049: Physics II • About bicycle riding: If some one shows you a trick, it means that, • It is possible to do that trick • The person who showed you, can do the trick. • It does not mean that you can do the trick. • Try it a lot of times and you can too.

3. Electric fields from different geometrical objects. • wires • Rings and disks • Planes • Another algorithm Al-Khwarizmi the Persian astronomer and mathematician, wrote a treatise in Arabic in 825 AD, On Calculation with Hindu Numerals, which was translated into Latin in the 12th century as Algoritmi de numero Indorum[1], which title was likely intended to mean "Algoritmi on the numbers of the Indians", where "Algoritmi" was the translator's rendition of the author's name; but people misunderstanding the title treated Algoritmi as a Latin plural and this led to the word "algorithm" (Latin algorithmus) coming to mean "calculation method". The intrusive "h" is most likely due to a false cognate with Greek αριθμος arithmos = "number". (wikipedia: http://en.wikipedia.org/wiki/Algorithm#Etymology

4. PHY 2049: Physics II • Flux and Gauss’ theorem

5. PHY 2049: Physics II • Gaussian (imaginary) surfaces • Flux = Φ = ∑ E.dA • E.dA for a cube is easy to visualize. • Let calculus do it for a sphere or any other shape. • Φc = qtotal /εo

6. S1: Φc = q/ εo • S2: Φc = -q/ εo • S3: Φc = 0 • S4: Φc = 0

7. For a cylinder with axial field, no contribution to flux from the side walls. • From the far top, >0 • Near (bottom) , <0

8. PHY 2049: Physics II • No force/field from the charge outside • On inside circle E.4πr2=q/εo Coulomb’s law • Very outside, E = k 5q/r2 P + + + + + +

9. PHY 2049: Physics II • The electric field in a metal is zero • Charge +q on inside surface. • Because it is neutral, outside surface must be -q.

10. PHY 2049: Physics II • Electric field is radial. • E.A = 0 for the top and bottom surfaces. • Sideways: E 2πrh = λh/ε0 E = λ/ 2πrε0

11. PHY 2049: Physics II • Plane • 2EA = σA/εo E = σ/2εo

12. PHY 2049: Physics II • E = σ/εo inside

14. PHY 2049: Physics II • Gauss’ theorem • Electric fields due to a point charge (Coulomb’s), a wire and a plane. • Charge shells don’t act inside. • In an insulator with uniformly distributed charge, only charge enclosed contributes to the field.