Download Chapter 21 Wiki-summary

Chapter 21 Wiki-summary
All matter is made up of atoms, which in tern are made up of protons, neutrons
and electrons. Protons and neutrons are made up of quarks, while the electron is
considered an elementary particle, as it is not made up of anything else. Electrons have a
negative electric charge (-), protons have a positive electric charge (+) while neutrons do
not have a charge. Since atoms are made of these particles, everything is filled with tiny
electrical charges. The balance of the negative and positive charges creates an electrically
neutral object (wood, plastic, etc) when not interacting with another object else. Anything
with charge can interact with anything charged (or uncharged) through electromagnetic
force. Each charged particle emits an electric field, and (if in motion) can create a
magnetic field as well. Both of these forces can act upon other objects to create a
repulsive or attractive force.
When two positively or negatively charged objects (atoms for example) come
within range of each other they will repel, while a positive and a negative charge will
attract to each other. This is simply demonstrated with two magnets, if the positive and
negative sides are placed near each other, they will attract and come together, however if
one of the magnets is turned around then they will repel from each other and it will be
difficult to get them together. In addition to his, a positive (or negative) charge in contact
with a neutrally charged object will have an attractive force as well. This is due to an
induced charge, which means that when a charged object is put near an uncharged
objects, the electrons in the uncharged object will maneuver around to be either closer or
further away to the charged object (depending on its charge). This is depicted in the
diagram below.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
(http://www.ce-mag.com/archive/2000/mayjune/mrstatic.html)
The white block (B) was originally neutrally charged, but when the grey positively
charged object (A) comes close, the electrons try to get towards that object, thus creating
charge separation.
The electrostatic force between charged particles can be calculated through
Coulombs Law. Mathematically, it is defined as:
Fk
q1q2
r
r2
where q is the charge of particle one and two (in coulombs), r is the distance between
them (in meters) and k is a proportionality constant (8.99 x 109 N m2 C-2) which is also

defined as:
k
1
40
where 0 is the permittivity constant.
You may recognize the similarities between Coulombs Law and Newton’s Law of

Gravity:
F G
m1m2
r
r2
where G is the gravitational constant (6.674 x 10-11 m3 kg-1 s-2), m is the mass of the two
objects (in kg) and r is the distance between them (in meters).

Both of these equations are inverse square laws that describe specific interactions
between particles, this explains why they look almost identical and determine the same
thing, force.
To find the magnitude of the force alone, simply take the absolute value of the
charges, then to make the correct vector formation look at the charges and intuitively
determine the direction of the force (if possible). In the case where a particle is being
affected by more than one other particle, the net force on it is the sum of the individual
forces with the other particles.
An important assumption in these types of situations is that a shell of uniform
charge attracts (or repels) a charged particle that is outside the shell as if all the shell’s
charge was at the center. In addition, if a charged particle is inside a shell of uniform
charge, then there is no net electrostatic force on the particle from the shell, as it is under
the same force from the sphere and it all cancels out. This same assumption is made in
the situations of gravity, as all of the concentration is assumed to be at the center of the
object.
Some materials have more free electrons (also called conduction electrons),
defined as the electrons on the outermost shell of the electron configuration; with these
electrons they can conduct a charge much better than others. These materials are called
conductors, copper, aluminum, human body, etc. are all examples of this process.
Insulators however do not carry charge freely, thus they do not have the same effect that
a conductor would. Examples of this type of material are dry wood, glass, etc. Semiconductors are a mix between conductors and insulators, as they can carry an electrical
charge, but not as well as conductors. Superconductors however conduct electricity
without any resistance, which makes it a perfect conductor. The current (written in
amperes; A) is the rate at which charge moves past a point. Mathematically, it is
determined as:
I
dq
dt
or the current (I) is defined as the rate of change of charge with time.

Charges are not actually random numbers, as they are quantized. This means that
the energies can only have certain values, or allowed values. Any charge (q) can be
determined through the equation
q= ne where n =  1,  2,  3….
In this equation e is defined as the elementary charge, which is approximated to
be 1.602 x 10-19 C. This is an important constant as –e is the charge of an electron, and +e
  
is the charge of a proton. Quarks however do not follow this rule, they do not have whole
number quantized energies, instead they have charges of  e/3 or  2e/3 but they cannot
be detected individually.
An example of all of this at work is static electricity. All of us at some point or


another, after walking on the carpet with socks or even taking off a sweater, and then
touching something get a small electrical shock. Although it may seem complex, the
physics behind this is quite simple. During the process of walking, or taking off the
sweater, the contact of the fabrics creates a pathway for electrons to move from on to the
other. This creates charged objects when originally they were neutral, and this charge
gets transported into your body. Now that your body is negatively charged (as you have
gained electrons), when you touch a conductor (door knobs, metal in general, etc.) the
extra electrons jump from you to the material you have touched, thus making the shock
that you feel.