PHY 2049: Physics II

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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.

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

PHY 2049: Physics II

Flux and Gauss’ theorem

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

S1: Φc = q/ εo

S2: Φc = -q/ εo

S3: Φc = 0 S4: Φc = 0

•For a cylinder with axial field, no contribution to flux from the side walls.

•From the far top, >0

•Near (bottom) , <0

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

++

+

+ +

+

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.

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

PHY 2049: Physics II

Plane 2EA = σA/εo

E = σ/2εo

PHY 2049: Physics II

E = σ/εo inside

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.