Electric Charge & Stuff

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Electric Charge & Stuff
Welcome to PHY2054 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 Electricity

Probably never happened, but good story!
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.)
Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

Experiment #1 Rods will attract each other Rods will repel each other
Pivot motion Rubber rod Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

Experiment #2 Rods will attract each other Rods will repel each other
Pivot Rubber rubbed with skin of dead rabbit Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

The charges on the two rods are ..
Since we treated both rods in the same way, they should be of the same type ……. different types I have no idea what you are asking for. 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
Stronger Weaker The same

If the two rods are brought closer together, the force acting between them will get …
Stronger Weaker 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 Charge

Experiment #3 Rods will attract each other Rods will repel each other
Pivot Glass rubbed with wool Glass rubbed with wool Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

Experiment #4 Rods will attract each other Rods will repel each other
Pivot Rubber rubbed with skin of dead rabbit Glass rubbed with wool Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

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
Positive Negative An object without either a (+) or (-) charge is referred to as being NEUTRAL.

Example - Tape

Separation

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

Signal propagation in neurons
Signal propagation in neurons. Neurons are components of the nervous system of the body that transmit signals as electrical impulses travel along their length. These impulses propagate when charge suddenly rushes into and then out of a part of the neutron called an axon. Measurements have shown that, during the inflow part of this cycle, approximately enter the axon. How many coulombs of charge enter a 1.5 cm length of the axon during this process? (sodium ions) per meter, each with charge +e

Materials Two kinds of materials: Insulators Conductors
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??
Rods will attract each other Rods will repel each other Nothing will happen Something not listed above will happen

Neutral (Conducting) Object - POLARIZATION
Positive charge attracts negative charges. Rod becomes “polarized. Negative end is closer to positive charge Distance effect causes attraction.

Contact Transfer Q Q/ Q/2

A Question 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.

Ways to charge an object
Rubbing or bond breaking (same thing) Transfer Direct transfer Polarization Induction

Quickie: How many kinds of charge are there?
Two: +,- Three: +,-.neutral

Induction

Polarize

Ground

Remove Ground

Positive !

Balloon Physics

Same as before: Polarization

What happens when two surfaces touch or rub?
Bonding! What happens when two surfaces touch or rub?

The Triboelectric Series
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! 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.

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.

Coulomb’s Law

The Unit of Charge is called
Coulomb’s Law Force is a Vector ! The Unit of Charge is called THE COULOMB Smallest Charge: e ( a positive number) 1.6 x Coul. electron charge = -e Proton charge = +e

Force is a VECTOR!!

EXAMPLE Q = +12 nC. What is the magnitude of the force F on charge Q?

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

The Gravitational Field That We Live In.
M m mg 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

to know the Electric Field
Doing it Q A Charge r F q The spot where we want to know the Electric Field

General-

Force  Field

The two S’s Superposition Symmetry

What is the electric field at the center of the square array?
Superposition and Symmetry

The FIELD DIAGRAM

Field Lines

NEW CONCEPT FLUX How Much Stuff Is There?

What is so important about FLUX??
OUTWARD Pointing Normal CLOSED Surface DF

What is the TOTAL FLUX leaving a closed surface??

Gauss’s Law Gaussian Surface

Gauss’s Law

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 in
the interior of a conductor. Gauss’ law on surface shown Also says that the enclosed Charge must be ZERO. Again, all charge on a Conductor must reside on The SURFACE.

Charged Conductors s E=0 E Charge Must reside on the SURFACE - - - - -
Very SMALL Gaussian Surface

Isolated (Charged) Conductor with a HOLE in it.
E=0 everywhere inside the conductor. So Q (total) =0 inside the hole Including the surface.

A Spherical Conducting Shell with A Charge Inside.
A Thinker!

So much for Mr. Coulomb!