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Is that the finger???
Gauss’s Law
3.2
Happenings


Wednesday – You should be finished with the
Electric Field and today we start Gauss’s Law
Friday – Yup, more Gauss and, oh yes, a QUIZ!
 Watch

for a new WA on Gauss to appear suddenly.
Monday … Gauss should be history so on to new
stuff. (Sorry)
What is the electric field at the center
of the square array?
A rigid electric dipole is free to move in the electric field represented in the
figure.Which one of the following phrases most accurately describes the initial
motion of the dipole if it is released from rest in the position shown?
A) It moves to the left.
B) It moves to the right.
C) It does not move at all.
D) It moves toward to the top of
the page.
E) It moves toward the bottom of
the page.
Consider the drawing, where the solid lines with arrows
represent the electric field due to the charged object. An
electron is placed at the point P and released at rest.
Which of the following vectors represents the direction of
the force, if any, on the electron?
Gauss’s Law
Let’s ask some questions of ourselves

Given a distribution of charges can we figure out
the Electric Field at a point in space?
 Yes

… just add the effects of each charge (kq/d2)
OK. How about the other direction – can we
determine the distribution of charges if we know the
Electric Field at a number of places.
 Sometimes.
This is the topic that Gauss addressed and
his answer, a mathematical theorem, is quite important
in solving a certain class of problems.
Last time

We can use a vector to represent a small flat area:
 It’s
length is proportional to the area.
 Its direction is perpendicular to the area
 The area need not be square, round or anything else. It
must be small. Very small. Teeny Tiny small.

There is an ambiguity in which of two ways the
vector can point for a particular small area.
What would you guess is inside the cube?
What about now?
How about this??
A.
B.
C.
D.
E.
Positive point charge
Negative point charge
Large Sheet of charge
No charge
You can’t tell from this
A Strange Statement
An object entering a solid region is the same thing as a
negative object leaving!
Think about this…..
Sphere
Light Bulb Emitting
N Photons per sec.
Sphere
Light Bulb Emitting
N Photons per sec.
Sphere
Light Bulb Emitting
N Photons per sec.
This was a brief intro to Gauss’s Law.
Now ..
START YOUR UNITS (on G’s Law …)