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A Review of Basic Atomic Structure & Terms
Atom – smallest piece of any element that still has the
properties of that element
 electrically neutral: # of protons = # of electrons
 Atomic number - # of protons in an atom
Determines what element it is.
 Atomic mass essentially tells how many protons and
neutrons an atom has, but it’s rarely a whole number
because most elements have isotopes
Isotope – versions of the same element containing
different numbers of neutrons in the nucleus
Ions – a charged atom
 Positive ion – more p+ than e-, so lost electrons
 Negative ion – more e- than p+, so gained electrons
A Review of Basic Atomic Structure & Terms
Basic atomic structure :
particle
symbol
location
mass (kg)
Charge (C)
proton
p+
nucleus
1.673 x 10-27 e = +1.602 x 10-19
electron
e-
electron cloud
9.11 x 10-31 e = - 1.602 x 10-19
neutron
n0
nucleus
1.675 x 10-27
0
YUCK!!
Almost beyond comprehension, the nucleus is incredibly
small & dense, while the electrons are incredibly rare &
far out!
The protons are tightly held in the nucleus by the Strong
Nuclear force (one of the 4 fundamental forces) and so
it takes a nuclear change (aka nuclear reaction –
examples: fission or fusion) to change the number of
protons in an atom!
But the electrons are WAY out there… often easily
swiped away by even just the brush of a hand…
Electrostatics and Basic Behavior
Electrostatics is the study of static electricity
► electro refers to electric charge
the charge subatomic particles have… (e-, p+)
► static means ‘at rest’
as opposed to moving,
which is called current electricity (Ch 18…)
Like charges repel; opposite charges attract…
with a force determined by Coulomb’s Law.
Law of Conservation of Electric Charge:
While charge can be moved from one place/object to
another, the total amount of electric charge will
remain the same… charge can’t be created or
destroyed.
Insulators vs Conductors
Conductors:
Insulators:
electrons flows freely
almost no electrons flow
due to low electron affinity
due to high electron affinity
Ex: metals, water
Ex: plastics, glass, wood
3 Ways to Charge an Object
► Charging
by Friction: rub 2 neutral objects together.
If they’re different electron affinities, one will give up
e’s becoming positive and the other will grab the extra
e’s becoming negative.
Ex: static cling in laundry or with clothes/hair
plastic slides at a park playground
water particles/clouds in a thunderstorm
► Chg by Contact: touch charged object to a neutral one
The excess charge will be looking to repel farther away
and so the neutral one’s surface becomes a great place
to do that – it ends up charged as well, with the same
charge as the originally charged object.
Ex: metal leaves on an electroscope
if you’re charged (by friction typically), then
you touch (shock) something
3 Ways to Charge an Object
► Charging
by Induction (aka induced charge
separation): bring a charged object near (don’t touch
them) to a neutral one and the charges on the neutral
one will separate.
The “like” charges move away from the originally
charged object, leaving the neutral object with a
positive side and a negative side
Ex: electroscope again
rod sways stream of water due
polar molecules in H2O
during a thunderstorm… the set
up for lightning to strike
the ground
1 Way to Discharge an Object
► Grounding:
Connect it, either directly, or by
means of a conducting material, to the massive
Earth which is capable of accepting a lot of
excess charge, but not becoming charged itself.
Ex: lightning rod
a shock from a anything (doorknob,
person, etc) on a dry day
dangling wire from car/wheelchair
3rd prong or polarized plugs
Examples of Electrostatics: Photocopy Machines
Photocopy machine:
• drum is charged positively
• image is focused on drum
• only black areas stay charged and therefore
attract toner particles
• image is transferred to paper and sealed by heat
Example of Electrostatics: Computer Printers
Laser printer is similar, except a computer controls the
laser intensity to form the image on the drum
Coulomb’s Law
Name after French Charles Coulomb; published in 1785:
Coulomb’s Law is the mathematical relationship that is
used to determine the strength of the electric force
(Fe) between 2 charged particles (q) or objects.
Extremely similar to Newton’s Universal Law of G! (p507)
NUL of G
► Any 2 masses attract
CL for Electric Charges
► Any 2 charges attract
or repel!
► FG = Gm1m2/r2
► Fe = k q1q2/r2
► Remember inverse square? ► So inverse square as well
► G = 6.67 x 10-11 Nm2/kg2 ► k = 9.0 x 109 Nm2/C2
That’s a huge difference between constant values!!
k ≈ 1020 bigger than G
That’s why…
► hair can stick up with balloon or sweater
► socks hang onto each other out of the dryer
► refrigerator magnets stay up on the frig door
Correct interpretation of these phenomena:
The Fg of the entire Earth is weaker than the Fe of the
tiny charged particles (p’s & e’s) on these objects!
We don’t hear much about Fe, even though it’s so much
stronger than Fg, because most objects are electrically
neutral, so Fe = 0, but objects always have mass, so
there’s always Fg.
But at the atomic/molecular level, it’s responsible for all
the forces we’ve studied so far, except gravity!
all contact forces (pushes & pulls)
normal force
friction
elastic
A few additional notes on Fe = kq1q2/r2
charges (q’s) must be small relative to the distance
between them – called point charges
► q is the quantity of charge present on an object
 units are Coulombs (C)
 Charge is quantized – smallest amt possible called
elementary charge – the amt of a single e- or p+:
► qe = -1.6 x 10-19 C & qp = +1.6 x 10-19 C
► So then 1 Coulomb of charge has 6.25 x 1018
individual charges in it!
 If q’s are the oppositely charged,
then Fe is an attractive force
 If q’s are similarly charged,
then Fe is a repulsive force
►
Superposition for Electric Charges
Principle of Superposition: for multiple point charges, the
forces on each charge, from every other charge, can be
calculated and then added as vectors.
► The
net force on a charge is the vector sum of all the
forces acting on it. Fon q = 1Fq + 2Fq +3Fq + …
1st: find F between each set of charges,
using Coulomb’s law.
2nd: add up those F’s using vector addition
techniques.
Review of Vector Addition
(a) original vectors to be added
(b) adding them tip to tail
(c) adding them by parallelogram
(d) adding them by components
Electric Fields
Force fields – the space around an object where that
object is able to apply a “force-at-a-distance”.
► Used to explain the problem of forces being applied
without contact – started with Newton’s UL of G.
► Idea from Michael Faraday in mid 1800’s
► Examples: gravity, electric, magnetic
► Understood now as not real, just an easy way to
picture Einstein’s curved space idea.
► This space reaches out infinitely far in theory,
► but in practice, it’s strength falls by ISL.
► Electric Fields (all fields) are vector quantities
Understanding Electric Fields
The electric field is defined as the force acting on a small
charge, divided by the amount of that charge:
E = Fe/q(2) Units: Newtons / Coulomb
Where q2 represents the charge of a test charge…
which refers to a small amount of (positive) charge,
relative to q1 (the charge creating the field) that it
won’t disturb/affect the field it’s located in.
But really, a test charge is a concept,
we don’t actually need to have a q placed there…
And E doesn’t depend on the mag of q2,
because however you change q2,
Fe will change proportional… by Coulomb’s law…
Understanding Electric Fields
Before you think you’re very confused, you’ve actually seen
this before!!
Think back to gravity, the other “field” we’ve covered:
recall
Fg = mg
for weight, right?
then rearrange to g = Fg/m …
If you doubled m, would that change g??
NO! Because Fg would change proportionately, so g is
the constant, created by the “main” mass creating
the field, at a particular location.
So really, g can be thought of as “gravitation field
strength”, instead of just acceleration due to
gravity, where g can be found by taking the force
of gravity, Fg, per amount of mass, m, that’s
located in the field, at that location.
Well, that’s what E is for charges….
Understanding Electric Fields
Let’s go back to the two masses in Universal Gravitation:
Remember how Fg = FG on the surface of a planet
and m?g = Gm1m2/r2
so
g = Gm1/r2 ??
This means m2 doesn’t affect the value of g, but m1 does!
It’s the same for the two charges in Coulomb’s Law:
Fe = Fe (just for continuity with above)
and q?E = kq1q2/r2 near a point charge
so
E = kq1/r2 !!
So E is a measure of the effect of the charge creating the
field at that location, unaffected by the test charge
placed at that location.
Math of Electric Fields
So now we have two equations that deal with E…
1st, the definition of all electric fields, no matter the shape
of the charge creating them:
E = Fe/q
(comparable to g = Fg/m)
where this q is the one in the field, and it won’t alter
the value of E, but can be used to determine E, if we
know the amount of force acting on q.
2nd, the equation for E created by a point charge:
E = kq/r2 (comparable to g = Gm/r2 )
where here we see E is a measure of the effect of the
charge creating the field at that location, unaffected by
the test charge placed at that location.
So it’s really important to remember which q is which in
each equation!!
Both of them find E in units of Newtons/Coulomb or N/C.
Math of Electric Fields
Based on E = Fe/q:
► if q(2) is positive,
E & F are in the
same direction,
► if q is negative,
E & F are in
opposite directions.
But this is confusing…
If we used our “mechanics” way to show
electric force around a point charge, it
would look like this:
And we haven’t even represented the
electric field yet. This could get very
confusing, so instead we use…
Electric Fields Lines
Recall vectors can be represented by arrows of a
particular length, pointing in a particular direction.
That has worked great up until now, but physicists came
up with a more practical and useful way to indicate
the strength and direction of fields:
 Direction: always points to where the positive test
charge would go if released at that point in the
field. So in a field created by
►a negative charge, that’s into the charge,
►a positive charge, that’s away from the charge.
 Instead of length to represent strength of field, we
use concentration of lines – the number in a given
area. Their length, then, can be left as infinitely
long to indicate the size of the field.
Electric Field Lines about a Point Charge
► The lines are infinite
► Arrows placed on them show the direction of the test
charge if placed in the field at that point.
► Fewer lines in the same amount of space in the left
picture indicates less field strength than the picture
on the right with a greater concentration of lines.
► There’s still field between the lines, empty space is
just to represent relative strength.
► They’re also 3D! But that’s hard to draw on flat paper.
Electric Field Lines about Point Charges
► Notice the curvature of the field lines
of the electric dipole. That’s from the
vector addition of two fields acting at
the same location.
► This means E acts tangent to the line,
if in fact the lines are curved.
► If one of these charges disappeared,
the lines around the other would be
straight again.
► These curves still map out the path of
a test charge if placed in this field.
► They will never cross: E has only one
single direction at any particular
location… If they crossed, that would
indicate two possible directions.
Electric Field between Parallel Plates
There are actually many uses for oppositely
charged parallel plates in everyday life –
The most common of which is in a
capacitor, which we’ll learn more about
in Ch 17.
Note, as long as the size of the plates is large
compared to the separation distance between them,
then the value of the E is a constant, as shown by
the equally spaced, parallel electric field lines.
The equation to determine it is: E = 4kq/A
A is area of one plate
q is the magnitude of charge on each plate
Electric Shielding
Using an encasement (box, cage, cover wrap) made of a
conductor (metal) to protect (shield) anything
(delicate electronics, you…) from the effects of charge.
Because the repulsive forces
between charges cause them to
spread out over the surface of a
conductor in such a way as
to cancel each other’s field
inside the conductor.
The electric field inside an electric
shielding device is always zero –
naturally – the charges don’t come to rest until it is.
Often referred to as a Faraday’s Cage…
Electric Shielding
Ex: coaxial cable
TV/DVD/cable
electric guitars
your car in an electrical storm
an airplane too
To obtain this zero field strength, charges
congregate on points and corners of various
shapes.
A Different, More Fundamental, Constant than k…
►
A Glimpse to the Future… Maxwell’s Equations
When/If you study E&M in college, you’ll get right into Maxwell’s
equations, named after James Clark Maxwell, who published
them in 1861. (He is also credited with 1st suggesting that light
is an electromagnetic phenomenon.)
They are a set of partial differential (calculus) equations that,
together with the Lorenz Force law, form the foundation of
classical electrodynamics, classical optics, and electrical circuits.
These areas of physics are the basis for all electric, optical and
radio technologies like power generation, electric motors,
wireless communication, cameras, televisions, computers etc.
Maxwell's equations describe how electric and magnetic fields are
generated by moving charges, and changes of each other.
One important consequence of the equations is that fluctuating
electric and magnetic fields can propagate at the speed of light,
and this electromagnet radiation manifests itself in many ways
from radio waves to light and X- or -rays.
A Glimpse to the Future… Maxwell’s Equations
Coulomb’s Law
but reconfigured by
Gauss, early 1800’s
Gauss’s Law
Carl, early 1800’s
Faraday’s Law
Michael, mid 1800
Ampere’s Law, with
Maxwell’s addition
Andre’-Marie, early 1800’s
Where E is electric field, B is magnetic field;
 is flux – refers to the amount of something passing thru an area;
0 is permeability of free space;
0 is permittivity of free space;
Note: 00 = (c2)-1 All 3 “fundamental constants”… see front of text.
A Glimpse to the Future… Maxwell’s Equations
The equations have many “standard” forms…
and even…
Where  is called the Del operator and
refers to a gradient or a change in space