Chapter 17 (Ch. 18 hons)
There are 2 basic ‘flavors’ of charge ____ive
and ______ive.
 Positive resides in the nucleus and is
generally not mobile. Negative charges
consist of the electrons which surround the
nucleus and these can be transferred from
atom to atom in some circumstances.
 Forces between charges:
 A + and a + _____ , -- and -- _______ , + and - _____ , -- and + ________ .

Some things to know about charge…
Charge is conserved –
 This comes from the idea that matter is
conserved. Total amount of charge never
changed. So if one object gains electrons
becoming ______ive, the other loses exactly
the same # of electrons becoming
______ive.
 Charge is quantized – (Millikan oil drop
experiment)
 Charge only comes in very specific amounts
– you can’t have ½ of an electron! Each
electron has a charge of-1.6 X 10 -19
Coulombs (C).

There are 3 types of materials as
far as charge is concerned
 Insulators:
 Non-metals
and most compounds
 Valence Electrons are basically bound
to each atom and are not mobile
 Examples:
Conductors:
 Metals, solutions of ionic compounds,
molten ionic compounds. Valence electrons
are loosely attached and can simply drift
from atom to atom.
Examples:
 Semiconductors Also known as the metalloids – they are
along the stairs on the P.T.E. . Their
electrons are loosely bound. They share
properties of both conductors and
_________ . Ex. Silicon, Germanium etc.

Three ways to charge objects
(describe with diags. On test)
 Contact:
Works for insulators well.
 Induction: Works for conductors well.
 Polarization: Works for insulators.
 See board for explanation and
diagrams of each. Your book also
explains each way well. (P631-633)
 The physics classroom - from my
website under static electricity
Charging by contact
Charging by induction
Conductors
Only!!!!!

Charging by polarization
No charge is
Transferred !!!!!


Insulators only.
Homework
P633. Q1 - 6
 Hons. Page 566 1-6, 8.

Van de Graff generator demos.
Answer the questions in red on a separate
sheet of paper. Turn in as your exit slip.
 Please talk to your neighbor and
collaborate on these answers. Don’t forget
explanations. Your recent notes are
useful. Include your collaborator(s)
name(s) on the exit slip but I need a slip
from each person

The VDG is a charge separation device. It
takes electrons from the dome and
transfers them to the base.
 1. The dome is therefore ______ly
charged when operating. Explain.

2. What “jumps” across from the wire to the
dome? Why?
 3. If you place a bunch of conducting metal
plates on the charged dome. What can we now
say about the charge on each plate? Explain.
 4. What should happen to the lightweight plates
that now have the same charges on them?
(Think forces of repulsion or attraction)
 5. If your hypothesis was true and your
explanation was good, GREAT! If not, explain
what happened and account for why it did.

6. “The flying cat”. What happens charges in
the VDG when I place the wire connected to the
base?
 7. Explain in terms of + and – charges. Why did
the fur behave in this amusing way?
 8. Getting all charged up. A lot:little charge can
accumulate on a small object. How could we
accumulate more charge than is on the VDG
dome itself? How about even more still?
 9. Justify the use of the textbooks under the
feet of the students in the last demo in terms of
what we learned in the recent notes.

Coulomb’s Law
Describes how much force there is between
charges.
 The force depends on the ______ of both
charges and the ______ _______ them.

F
= k q1q2
------------r2
q
is each charge in coulombs.
 r is the distance between them in m
Try these concept Q’s….




If there are 2 charges, and I double the size of one
charge, what happens the force between them?
If there are 2 charges, and I double the size of both
charges, what happens the force between them?
If there are 2 charges, and I double the distance
between the charges, what happens the force
between them?
If there are 2 charges, and I halve the distance
between the charges, what happens the force
between them? If there are 2 charges, and I double
the distance between the charges but double the
charge size for both, what happens the force
between them?
Remember, the net force on an object is
the sum of all forces on it. Note carefully
whether the force is due to repulsion or
attraction to figure its direction.
 To do these problems you should draw a
picture showing directions of forces when
there are multiple charges. Remember
how to calculate net forces. (subtract when
in opposite directions)

Superposition principle

The force on a charge due to more than
one other charges is the vector sum of the
forces from all of the charges added
together.
Homework:
 P636 Q1 – 4 , P 639 Q1
 (Hons.) P567 Q1,2,4 P568 Q7,8,9,12

Electric Fields
Chapter 17-3
18.6, 18.7 Honors
 Electric
Field: The area around a
charge in which another charge would
experience a force.
E
= F / q0
where F is the force experienced by a
test charge, qo placed in the field
 Or, if we are not given a force nor a
test charge….
E
=kq/
2
r
, Units are N/C
 q is the charge producing the field.
 Electric
field lines show the
direction of the force on a sample
positive charge placed in the field.
 Rules
for drawing them:
 Begin at + or infinity and end at –
or infinity.
 # of lines drawn is proportional
to size of charge.
 They never cross

If electric field lines are closer, the electric field
is stronger.
Superposition principle

The electric field at a point due to more
than one charge is the vector sum of the
electric fields from all of the charges
added together.
Conductors in electrostatic equilibrium

Charges in CONDUCTORS will redistribute
themselves until there is no net force on each
charge. In this case…….
The electric field inside a conductor in
equilibrium is zero.
 The excess charges lie entirely on the
outside.
 Electric field just outside the surface is
perpendicular to the surface
 Charge accumulates on surfaces with the
smallest radius of curvature (pointy parts!).
The electric field at the point can be so huge
that charges jump off.

Lets plot some electric fields on the board.
 In addition to the concept packet questions,
answer the following problems from the book:
 P639 Q1 (like the one on the board), P647
Q1(substitute words “x-axis for y-axis”),2,3
 Hons H/W : P569 Q25, 26, 29, 31,33,34

Lightning
 First
lets talk about how a thunderstorm
works.. This has been quite a mystery with
many hypotheses that were difficult to
prove.
 Lightning happens in a similar manner to
the spark that occurs when you drag your
feet over the carpet…. Charges are
separated from (similar:different)
materials.
The key to thunderstorm and therefore
lightning formation is strong convection
updrafts of air.
 Recently, particle movement in
thunderstorms has been well studied with
doppler radar. The primary mechanism
seems to be rising rain and falling ice in
strong convection currents.
 In this case, falling hail (ice) rubs off rising
water (rain drops) as they touch, electrons
leave the rain onto the hail.

Lightning protection
The last “point” led to the invention of lightning
rods by Ben Franklin in the late 1700’s.
 Key “point” – lightning rods do not attract
lightning – they prevent it by leaking off excess
charge.
 Lightning rods allow accumulation of charge at
their points where the e-field gets very high.
Charges leak off the point and drift back to
where they came from originally. They prevent
lightning from occurring.

 Protecting yourself from lightning
 In case of a thunderstorm:
 Go inside a car or building (the field inside a
conductor is zero – no field = no current)
 If you can’t go inside, stay low.
Do not…….
Go under a tree.
 Be on water in a boat or swimming
 Be out in the open on higher ground.
 Touch or hold any metal objects (golf clubs,
umbrellas etc.
 Call on a landline phone
 Call from a cell phone from outside
 Use plumbing fixtures

Characteristics of lightning
Sun surface temperature approx. 6000K
 Lightning bolt temp. approx. 30,000K
 Thunder caused by rapid expansion
(explosion) of air by heating.
 Sound travels about 340m/s (1 mi in 5
seconds). Light about 1mi in 2.6 X10-6 s.
 Counting after lightning until thunder heard
can locate lightning strike.
 Lightning bolts have exceeded 5mi long.

Answers to recent h/w

P636 2) 230N attractive, 4)a. 2.2 x 10^5N attractive, b. 9.0 X 10^-7 repelling

P647 Q1 (modified) 4.8 X 10^5 N/C right,
2) 5.1 X 10^11 away from proton
Electric Potential Energy
Ch18.1,2(hons Ch19.1,2)
 There
is energy stored up between any
charges (think of a spring between the
charges)
 In a uniform electric field, Energy change
…..E.P.E. = -qEΔd

Looks like Δ GPE = mg Δ h

For a pair of charges, E.P.E. = k q1q2
r
Electric potential
 A.K.A.
voltage,
 It is the energy per unit charge.
 V = PE / q
If an electric field is uniform…
 The potential difference between a point at
infinity and some place near a point charge
is…….

Δ

V = - E Δ d = k q / r,
r is distance from the point charge.
So energy is the product of Voltage and
charge quantity.
 Sparks from the VDG generator have very
high voltage (400,000V) but low energy as
there are not many charges.
 Current from the outlet is far more deadly as
although there is “only” 120V available, there
are millions of times the quantity of charge
available – enough energy to kill you! (Think
of sparkler analogy)
 Lightning has a deadly combination of high
voltage (millions of V) and lots of charge.

The measurable quantity directly correlating
with electricity and mortality is the current or
charges per second that flow through the
heart muscle. It is not necessarily fatal to be
exposed to high V if current is limited.
 For tonight P669 1,2 (-6.9X10^-19J)
 P673 (use kq/r) 1,2 (110V), P675 Q1,5,6
(3.5X10^9V) use -EΔd


Honors – P598 2, P599 4 (Use work
equation. EPE = Fd),5 (find EPE and use
power equation) ,7 (make EPE = KE, solve
for v),8 (EPE = heat energy added, see heat