Transcript Lecture 3

PHY 2049: Physics II
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PHY 2049: Physics II
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About bicycle riding: If some one shows
you a trick, it means that,
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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.
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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
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Flux and Gauss’
theorem
PHY 2049: Physics II
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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
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S1:
S2:
S3:
S4:
Φc
Φc
Φc
Φc
=
=
=
=
q/ εo
-q/ εo
0
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
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No force/field from the
charge outside
On inside circle
E.4πr2=q/εo Coulomb’s
law
Very outside, E = k
5q/r2
+
+
++P
+
+
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The electric field in
a metal is zero
Charge +q on
inside surface.
Because it is
neutral, outside
surface must be -q.
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Electric field is radial.
E.A = 0 for the top and
bottom surfaces.
Sideways:
E 2πrh = λh/ε0
E = λ/ 2πrε0
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Plane
2EA = σA/εo
E = σ/2εo
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E = σ/εo inside
PHY 2049: Physics II
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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.