2A Slide Like This Every Day Today we begin chapter 17 – Electric Charge, Coulombs Law and the Electric FieldThere will be NO QUIZ this week.Clicker use will begin on Friday.Bring your i-clicker to class every sessionBring 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
6Allowable Predictions (Use your clicker if you have one.) Rods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
7Experiment #1 Rods will attract each other Rods will repel each other PivotmotionRubber rodRods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
8Experiment #2 Rods will attract each other Rods will repel each other PivotRubber rubbed withskin of dead rabbitRods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
9The charges on the two rods are .. Since we treated both rods in the same way, they should be of the same type……. different typesI have no idea what you are asking for.Leave me alone … I’m napping!
10If you rubbed the rods longer and/or harder, do you think the effect that you see would be StrongerWeakerThe same
11If the two rods are brought closer together, the force acting between them will get … StrongerWeakerThe same
12Definition of sortsWe 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:MassCharge
13Experiment #3 Rods will attract each other Rods will repel each other PivotGlass rubbed with woolGlass rubbed with woolRods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
14Experiment #4 Rods will attract each other Rods will repel each other PivotRubber rubbed withskin of dead rabbitGlass rubbed with woolRods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
15What’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 PROPERTYOther properties are mass, colorWe call this NEW PROPERTY .………. ………CHARGE.There seems to be two types of charge.
16We call these two types of charge PositiveNegativeAn object without either a (+) or (-) charge is referred to as being NEUTRAL.
21We have also observed that there must be TWO kinds of charge. Call these two typespositive (+)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.
27Signal 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, approximatelyenter 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
29Materials Two kinds of materials: Insulators Conductors Electrons and Protons are tightly bound to their positions. Hard to move them around.ConductorsElectrons 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
30What about a charged rod and a piece of wooden dowel?? Rods will attract each otherRods will repel each otherNothing will happenSomething not listed above will happen
31Neutral (Conducting) Object - POLARIZATION Positive charge attracts negative charges.Rod becomes “polarized.Negative end is closer to positive chargeDistance effect causes attraction.
33A QuestionTwo 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.
34Ways to charge an object Rubbing or bond breaking (same thing)TransferDirect transferPolarizationInduction
35Quickie: How many kinds of charge are there? Two: +,-Three: +,-.neutral
43What happens when two surfaces touch or rub? Bonding!What happens when two surfaces touch or rub?
44The 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 NEGATIVENo! No!No! No!
45SUMMARY: So far we have found? There are TWO types of charge.PositiveNegativeLike Charges AttractUn-Like charges repelThe force between charges increases as they are brought closer together.This charge separation results from chemical bonds which are severed.
46Getting down to business: We will discuss the inverse square law that describes how electric charges interact via forces.This is COULOMB’S LAWWe 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
47Forces Between Charges The force between charges is along the line between them.The direction depends on the type of charges-Like Charges RepelUnlike Charges AttractThe Magnitude of the force is proportional to the inverse square of the distance between the charges.
48Coulomb’s LawThe 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 asThis relationship is called Coulomb’s law.
54Fields 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.
55Example – Gravitational Field. Property is MASS (m).Force is mg.Field strength is defined as Force/Property
57Properties 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.
59Electric FieldIf 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.
77Conducting 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!
78More on ConductorsCharge 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.
79Isolated Conductor Electric Field is ZERO in the interior of a conductor.Gauss’ law on surface shownAlso says that the enclosedCharge must be ZERO.Again, all charge on aConductor must reside onThe SURFACE.
80Charged Conductors s E=0 E Charge Must reside on the SURFACE - - - - - Very SMALL Gaussian Surface
81Isolated (Charged) Conductor with a HOLE in it. E=0 everywhereinside the conductor.So Q (total) =0 inside the holeIncluding the surface.
82A Spherical Conducting Shell with A Charge Inside. A Thinker!