Transcript Electrostatics - Bristol

Is For Electricity
fundamentals that drive our world
Charge It!!!
 What is evidence of charge?
 What are the two kinds of charge?
 What are the three ways for and object or material to
become charged?
HW: Learn 194-201  At least 6 talking to the text
points.
The Mighty Atom
Since there are
an equal number
of electrons and
protons, the
net charge on
this atom is 0.
The Mighty Atom
The only way for it to
become charged is
to gain or lose
electrons.
Two Types of Charge
NEGATIVE (-) : More electrons than protons or an
excess of electrons.
POSITIVE (+): Fewer electrons than protons or a deficit
of electrons.
NEUTRAL – Is a state of charge, NOT A TYPE OF
CHARGE!
Neutral is equal amounts + and -.
Quick Check
What is the charge on each atom?
Charging Can Be Done By..
FRICTION:
Rubbing two objects together to remove or deposit
electrons.
CONTACT:
Touching one charged object to a neutral to remove or
deposit charge.
INDUCTION:
Bringing one charged object near a neutral so charges
polarize or separate.
The Fundamental Concept
No matter what – the net charge in any process is always
zero.
Charge is always conserved.
One object might lose charge to another, but the pair of
objects still has a net charge of zero.
Field of Dreams isn’t about Baseball
Sign Convention
How Charges Interact
How Objects Interact
Proof Is In The Pudding
Proof Is In The Pudding
Quick Check
Insulators
 Rubbing wool over the
R, tape reacted near the
R but not near the N.
 Charge was localized
(stuck) near the R and
couldn’t move over to
the N.
R
N
Conductors
The Pie Plates
The key to getting a net
charge on the pie plate
is to touch it so
electrons rush from
the plate leaving it
with a net + charge.
Conductors & Insulators
CONDUCTORS:
Allow charges to pass through them.
Induction causes charges to separate.
Charges will become evenly distributed on the surface of
a conductor.
Conductors & Insulators
INSULATORS:
Do not allow charges to pass through them.
Induction causes atoms/molecules to polarize.
Charges will become stuck in certain regions of the
material.
Quick Check
Polarization of Insulators
Polarization of Conductors
Neutrals Will Be Attracted!
Quick Check
Charged rubber rods are placed near a neutral
conducting sphere, causing a redistribution of
charge on the spheres. Which of the diagrams
below show the proper distribution of charge on
the spheres? List all that apply.
Charge It!
As we have seen that charges behave differently in
different materials due to the nature of the bonds
involved.
Metallic bonds allow electrons to flow freely whereas
covalent bonds do not.
The dividing line is the metalloids on the Periodic Table.
(Thought you were done with chem, eh?!?!)
Excuse Me Mr. Coulomb, But
Would You Like To Charge That?
Like gravity, charges exert forces over a distance. So
perhaps we can use gravity as an analog to forces
created by charges.
Coulomb’s Force Pendulum
A positively charged metallic sphere
is hung from a non conducting
thread.
An equally charged negative sphere
is brought near by.
Coulomb’s Force Pendulum
A positively charged metallic sphere
is hung from a non conducting
thread.
An equally charged negative sphere
is brought near by.
The other sphere shifts towards it.
The angle is proportional to the
relationship of Fe , mg and
separation distance r.
F
θ
d
e
+
mg
Coulomb’s Force Pendulum
We know the equation of gravity
m1m2
F=G
d2
θ
r
Fe
d
mg
L
Coulomb’s Force Pendulum
Which brings us to this for small
angles:
We can use similar triangles to
relate the two kinds of forces.
Fe/d =mg/L
Fe/mg = d/L
Which says that gravity and the
electric force are directly related.
r
Fe
θ
d
But what about r?
mg
L
Coulomb’s Force Pendulum
Some Simulations
http://webphysics.davidson.edu/physlet_resources/bu_s
emester2/index.html
Coulomb’s Force Pendulum
Since they are directly related we
conclude both follow the inverse
square law.
Fe = k
q1 q2
r2
L
θ
r
Fe
mg
Coulomb’s Law
q1 q2
Fe = k 2
r
Where q is charge measured in coulombs which must be
noted as + or - charges
r is separation distance in meters
k is a constant 9 x 109 N·m2/C2
How Big Is A Coulomb?
The charge on an electron (e-)
−1.602176487×10−19 C
And a proton (p+)
+1.602176487×10−19 C
It takes 6.2414 ×1018 electrons to = 1C
For singly ionized water that’s .000187 grams
Imagine the amount of charge a few grams would
hold and you understand a lightning bolt!
A mole of e- = 96,484 C. Holy @#&*$!
Field of Dreams, Part I
Charges and charged objects generate electric fields.
These fields can be thought of as vectors passing
through empty space.
How the field vectors interact is how forces of attraction
and repulsion are transferred.
We can’t see these fields, only observe their interaction
with other objects.
Perhaps a look at gravity would help.
Electric Field Strength
q1 q2
Fe = k
d2
Coulombs Law
m
m
1
2
F =G
g
d2
Gravity
Electric Field Strength
q1 q2
Fe = k
d2
Coulombs Law
m
m
1
2
F =G
g
d2
When these terms are collected we get g the
acceleration due to gravity, 9.81 m/s2 leaving us with
the familiar F = mg.
Electric Field Strength
q1 q2
Fe = k 2
d
These terms collect to form a notation of field strength
called the Electric Field Intensity, E, which is a vector
field, + or -.
F = Eq
Where E is measured in N/C.
Electric Field Notation
To note a field we typically show a line (or plate) with a
vectors as field lines.
+
Projectile Motion Link
Shoot a charge through the field and it will curve.
+
-
Fields Created By Charges
http://www.falstad.com/emstatic/
http://www.falstad.com/vector2de/
http://www.falstad.com/vector3de/
Force Related to Distance
Lift an object and do work against gravity…..
Hey that sounds familiar!!!!!!!!!!!
It’s work against gravity, or PE.
Wonder if it works for moving a charge in an Electric
field?
Work Done On Charges
PE = mgh
Bowling Ball
E is similar to g
h is similar to r or d
m is similar to q
Work Done On Charges
Bowling Ball
I can say that I did so many
joules of work, total.
PE =mgh W = Fd
Or I could say I did so many
joules of work on the ball.
PE = Joules/ BB
Work Done On Charges
Bowling Ball
Each ball has the same PE.
The more balls the greater the
potential to wreak havoc on
the floor.
Total PE = 3 x (J/BB)
Or 3 J/BB
Work Done On Charges
Bowling Ball
If I imagine that each BB is
equal to one coulomb of
charge we can write this:
PE = J/C
Volt = J/C
Work Done On Charges
Bowling Ball
Volt = J/C
Which is called POTENTIAL.
A Return to Gravity
q1 q2
Fe = k
d2
F = Eq
Coulombs Law
m
m
1
2
F=G
d2
F = mg
Coulombs Law In New Way
q1 q2
Fe = k
d2
F = Eq
E = kq/r2
(I)
E = F/q
(III)
(II) subbing II into I
Work Done To Move A Charge
http://webphysics.davidson.edu/physlet_resources/bu_s
emester2/index.html
Work On A Charge
W = Fd …………. W = Fed
Fe = Ed
E = Fe/q
(I)
E = k q/r2
(II)
Working a number of substitutions of the these 5 expressions brings some
more
W = qV
V = k q/r