CHAPTER 17 Electric Charge & Stuff A Slide Like This Every Day Today we begin chapter 17 –...

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

qq

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!