Chapter 20 - Electricity
Physics 104 – Winter 2005
Electric Charge
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Electric charge – a fundamental quantity of
our universe
Charge is QUANTIZED – the smallest charge
that one sees in nature is the charge of the
electron – 1.602 x 10-19 C
Even in our most modern quantum field
theories, nobody can calculate this number!!!
The charge of the proton is equal in
magnitude, but opposite in sign, to that of the
electron – THERE ARE TWO TYPES OF
CHARGE
Insulators and Conductors
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Most objects are composed of NEUTRALLY CHARGED atoms.
In an ordinary atom, the number of electrons = the number of
protons. The neutrons are uncharged.
In most substances, the nuclei are rigidly positioned, and the
electrons of a particular atoms orbit that atom’s nucleus. They
do not move around from atom to atom. Because the electric
charges do not move around easily, we say that they do not
CONDUCT electricity. These are known as INSULATORS
In some substances, like metals, and water, the atoms “overlap”,
and the electrons in fact become almost free to move around
from atom to atom. When this happens, we have a
CONDUCTOR.
In some very special substances, like SILICON, for example,
the atoms do not quite overlap, but they nearly do. By just
giving the electrons a tiny amount of extra energy, we can turn
the substance from an insulator into a conductor. These
substances are known as SEMICONDUCTORS.
Massive Charged Objects
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Even in insulators, it is possible to “transfer” charge
from one object to another. We can take some
electrons away from one object, and put them onto
another object
We usually accomplish this by “rubbing” the object –
e.g. balloon on your hair
This accumulated extra charge just sits there on the
object – since it is stationary, we call it STATIC
ELECTRICITY
For every electron we put onto one object, we take an
electron away from another object – CHARGE IS
CONSERVED
Electric Forces
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Like charges repel … opposites attract …
Opposites don't attract happiness
Leigh Dayton, Science writer
14feb05
WHEN it comes to love, forget the notion that opposites attract.
University of Iowa psychologists have studied 291 newly married couples
and discovered the opposite to be true. People select romantic partners
who are like them in interests, values and background, say the authors
of the study, Eva Klohnen and graduate student Shanhong Luo.
Hollywood’s Viewpoint
Hollywood’s Viewpoint …
How to tell if you are kissing correctly …
Coulomb’s Law
q1q2
F k 2
r
9
2
2
k  8.99 x10 Nm / C
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Notes: We ALWAYS put in the charges as
POSITIVE, even if they are negative!!!! Then, we
figure out the DIRECTION of the force from the
(likes repel/opposites attract) rule.
Hey, it goes like 1/r2 !!!!!!!!!!! Just like gravity !!!!!!!!!
Q1
Q2
Initially, Q1 = Q2 –> Equal in Magnitude and Sign ….
Then, we reduce Q1 to HALF of it’s original value ….
What happens to the ANGLE ???
Newton’s 3rd Law – Equal and Opposite Forces – Angle will be smaller for BOTH
Electric Force vs. Gravity
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Which is inherently a stronger force,
the electric force, or gravity?
Let’s answer that question by
considering a hydrogen atom …
Electric Force vs. Gravity
Electric Force vs. Gravity
Electric Force vs. Gravity
Electric Force vs. Gravity
Electric Force vs. Gravity