CHAPTER 17
Electric Charge & Stuff
A Slide Like This Every Day
Today we begin chapter 17 – Electric Charge, Coulombs Law and the Electric Field
There will be NO QUIZ this week. Clicker use will begin on Friday.
Bring your i-clicker to class every session Bring a scientific calculator ($13.00 – COSTCO) If you have your clicker with you, you may use
it today. There WILL be a Quiz next Friday
Probable First Observation ElectricityElectricity
Idiot!
If lightening had actually traveled down the kite string, old Ben Franklin
would have been toast!
Probably never happened, but good story!
A Quick Experiment
Allowable Predictions(Use your clicker if you have one.)
A. Rods will attract each otherB. Rods will repel each otherC. Nothing will happenD. Something not listed above will happen
Experiment #1
Pivot
motion
Rubber rod
Rubber rod
A. Rods will attract each otherB. Rods will repel each otherC. Nothing will happenD. Something not listed above will happen
Experiment #2
PivotRubber rubbed withskin of dead rabbit
Rubber rubbed withskin of dead rabbit
A. Rods will attract each otherB. Rods will repel each otherC. Nothing will happenD. Something not listed above will happen
The charges on the two rods are ..
A. Since we treated both rods in the same way, they should be of the same type
B. ……. different types
C. I have no idea what you are asking for.
D. Leave me alone … I’m napping!
If you rubbed the rods longer and/or harder, do you think the effect that you see would be
A. Stronger
B. Weaker
C. The same
If the two rods are brought closer together, the force acting between them will get …
A. StrongerB. WeakerC. The same
Definition of sorts
We DEFINE the “stuff” that we put on the rods by the rubbing process as CHARGE.
We will try to understand what charge is and how it behaves.
We add to the properties of materials:
Mass
ChargeCharge
Experiment #3
PivotGlass rubbed with wool
A. Rods will attract each otherB. Rods will repel each otherC. Nothing will happenD. Something not listed above will happen
Glass rubbed with wool
Experiment #4
Pivot
1. Rods will attract each other2. Rods will repel each other3. Nothing will happen4. Something not listed above will happen
Glass rubbed with wool
Rubber rubbed withskin of dead rabbit
What’s Going On?
All of these effects involve rubbing two surfaces together.
Or pulling two surfaces apart. Something has “happened “to each of these
objects. These objects have a new PROPERTY
Other properties are mass, color We call this NEW PROPERTY .………. ………
CHARGE. There seems to be two types of charge.
We call these two types of charge
PositiveNegative
An object without either a (+) or (-) charge is
referred to as being NEUTRAL.NEUTRAL.
Example - Tape
Separation
An Example
Effect of Charge
We have also observed that there must be TWO kinds of charge. Call these two types
positive (+) negative(-)
We “define” the charge that winds up on the rubber rod when rubbed by the dead cat to be NEGATIVE.
The charge on the glass rod or the dead cat is consequently defined as POSITIVE.
Old Ben screwed up more than once!!
++++++++++-------------+++---++---+-++-
From whence this charge???
-+
Easily Removed
AXON
11 +5.6 10 Na (sodium ions) per meter, each with charge +e 11 +5.6 10 Na
Materials
Two kinds of materials: Insulators
Electrons and Protons are tightly bound to their positions. Hard to move them around.
Conductors Electrons are easily removed and moved around. Electrons are said to be MOBILE charges.
There are other kinds of materials that we will not discuss: semiconductors, semi-metals
What about a charged rod and a piece of wooden dowel??
A. Rods will attract each otherB. Rods will repel each otherC. Nothing will happenD. Something not listed above will happen
Neutral (Conducting) Object - POLARIZATION
Positive charge attracts negative charges.Rod becomes “polarized.Negative end is closer to positive chargeDistance effect causes attraction.
Contact Transfer
QQ/2 Q/2
Two iron spheres contain excess charge, one positive and the other negative.
(a) Show how the charges are arranged on these spheres if they are very far from each other. (b) If the spheres are now brought close to each other, but do not touch, sketch how the charges will be distributed on their surfaces. (c) In part (b), show how the charges would be distributed if both spheres were negative.
A Question
Ways to charge an object
Rubbing or bond breaking (same thing) Transfer
Direct transfer Polarization Induction
Quickie: How many kinds of charge are there?
A. Two: +,-B. Three: +,-.neutral
Induction
Polarize
Ground
Remove Ground
Positive !
Balloon Physics
Same as before: Polarization
WHAT HAPPENS WHEN TWO SURFACES TOUCH OR RUB?
Bonding!Bonding!
The Triboelectric Series
No! No!
When two of the following materials are rubbed together under ordinary circumstances, the top listed material becomes positively charged and the lower
listed material becomes negatively charged.
MORE POSITIVE rabbit's fur
glass mica nylon wool
cat's fur silk
paper cotton wood
acrylic cellophane tape
polystyrene polyethylene
rubber balloon saran wrap
MORE NEGATIVE
No! No!
SUMMARY: So far we have found?
There are TWO types of charge. Positive Negative
Like Charges Attract Un-Like charges repel The force between charges increases as they
are brought closer together. This charge separation results from chemical
bonds which are severed.
Getting down to business:
We will discuss the inverse square law that describes how electric charges interact via forces. This is COULOMB’S LAW
We will discuss the FACT that Coulomb's law is a VECTOR equation.
We will add forces acting on a charge from an assembly of other charges.
We will discuss the concept of FLUX and use it to “derive” a useful law for continuous distributions of charge with high symmetry. This is Gauss’s LAW
Forces Between Charges
The force between charges is along the line between them.
The direction depends on the type of charges- Like Charges Repel Unlike Charges Attract
The Magnitude of the force is proportional to the inverse square of the distance between the charges.
Coulomb’s Law
The magnitude F of the force that each of two point charges q1 and q2 a distance r apart exerts on the other is directly proportional to the product of charges (q1q2) and inversely proportional to the square of the distance between them (r2). The relationship is expressed symbolically as
This relationship is called Coulomb’s law.
.2
21
r
qqkF
Coulomb’s Law
Coulomb’s Law
229
0
221
0
/1094
1
4
1
CNmxk
r
qqunit
rF
The Unit of Charge is calledTHE COULOMB
Smallest Charge: e ( a positive number) 1.6 x 10-19 Coul.
electron charge = -eProton charge = +e
EXAMPLE
Q = +12 nC. What is the magnitude of the force F on charge Q?
1.6 × 10-4 N
+12 nC
THE ELECTRIC FIELD
Fields Imagine an object is placed at a particular point in
space. When placed there, the object experiences a force
F. We may not know WHY there is a force on the
object, although we usually will. Suppose further that if we double some property of
the object (mass, charge, …) then the force is found to double as well.
Then the object is said to be in a force field. The strength of the field (field strength) is defined
as the ratio of the force to the property that we are dealing with.
Example – Gravitational Field. Property is MASS (m). Force is mg. Field strength is defined
as Force/Property
gg
gF
m
m
mass
m
Force nalGravitatio
Property
Force
Strength Field nalGravitatio
The Gravitational Field That We Live In.
m Mmg
Mg
Properties of a FORCE FIELD
It is a property of the position in space. There is a cause but that cause may
not be known. The force on an object is usually
proportional to some property of an object which is placed into the field.
Mysterious Force
F
Electric Field
If a charge Q is in an electric field E then it will experience a force F.
The Electric Field is defined as the force per unit charge at the point.
Electric fields are caused by charges and consequently we can use Coulombs law to calculate it.
For multiple charges, add the fields as VECTORS.
Two Charges
220
00
1
r
qk
r
qqk
FE
EqF 0
Doing it
Q
r
q
A Charge
The spot where we wantto know the Electric Field
unit
unit
r
Qk
q
r
qQk
rF
E
rF
2
2
F
General-
unitjj
jjj
unit
unit
r
Qk
q
General
r
Qk
q
r
qQk
,2
2
2
rF
EE
rF
E
rF
Force Field
The two S’s
SuperpositionSymmetry
What is the electric field at the center of the square array?
The FIELD DIAGRAM
NEW CONCEPT
What is so important about FLUX??
CLOSED Surface
OUTWARD Pointing Normal
What is the TOTAL FLUX leaving a closed surface??
0
22
0
2
44
)4(
)(
qR
r
q
REEAAE
AE
i
ii
Gauss’s Law
Gaussian Surface
Gauss’s Law
0
Charge Enclosed
VOLUME CLOSED
ALEAVING
FLUX TOTAL
iTotal
Conducting Materials
Conductors Electrons are free to move. In equilibrium, all charges are a rest. If they are at rest, they aren’t moving! If they aren’t moving, there is no net force on them. If there is no net force on them, the electric field
must be zero.
THE ELECTRIC FIELD INSIDE A CONDUCTOR IS ZERO!
More on Conductors
Charge cannot reside in the volume of a conductor because it would repel other charges in the volume which would move and constitute a current. This is not allowed.
Charge can’t “fall out” of a conductor.
Isolated Conductor
Electric Field is ZERO inthe interior of a conductor.
Gauss’ law on surface shownAlso says that the enclosedCharge must be ZERO.
Again, all charge on a Conductor must reside onThe SURFACE.
Charged Conductors
E=0
E
---
-
-
Charge Must reside onthe SURFACE
0
0
E
or
AEA
Very SMALL Gaussian Surface
Isolated (Charged) Conductor with a HOLE in it.
E=0 everywhereinside the conductor.
So Q (total) =0 inside the holeIncluding the surface.
A Spherical Conducting Shell withA Charge Inside.
So much for Mr. Coulomb!
