2 The primary particle that carries charge (and therefore can be lost or gained) in an atom is a/an: ProtonNeutronElectron
3 When a positively charged object comes close to an negatively charged object, the negative object will:Be attractedBe repelledDo nothing
4 When a positively charged object comes close to an neutral object, the neutral object will: Be attractedBe repelledDo nothing
5 Anytime an object at rest starts to move, what must be present to cause it to move? Difference in electric chargeElectricityA forceMatter
6 A Bit of History Ancient Greeks Observed electric and magnetic phenomena as early as 700 BCFound that amber, when rubbed, became “electrified” and attracted pieces of straw or feathers
7 Properties of Electric Charges Two types of charges existThey are called positive and negativeArbitrarily named by Benjamin FranklinLike charges repel and unlike charges attract one another
8 More Properties of Charge Positive charge – protonsNegative charge – electronsGaining or losing electrons is how an object becomes charged; protons remain with the nucleus
9 A little review: What are some conservable quantities in physics? Mass MomentumEnergyCharge
10 Conservation of Charge Electric charge is always conservedCharge is not created, only exchangedObjects become charged because negative charge is transferred from one object to another
11 More review: A force is …? Contact vs. Field forces? Anything that produces acceleration or a change in motion.Contact vs. Field forces?Contact: require matter to be in contact (ex. friction)Field: matter not required (ex. gravitational, electrical, magnetic)
13 Properties of Charge, final Charge is quantizedAll charge is a multiple of a fundamental unit of charge, symbolized by eElectrons have a charge of –eProtons have a charge of +eThe SI unit of charge is the Coulomb (C)1 e = 1.6 x C
15 ConductorsConductors are materials in which the electric charges move freelyCopper, aluminum and silver are good conductors
16 InsulatorsInsulators are materials in which electric charges do not move freelyGlass and rubber are examples of insulatorsWhen insulators are charged by rubbing, only the rubbed area becomes chargedThere is no tendency for the charge to move into other regions of the material as opposed to conductors
17 Three Methods of Charging FrictionConduction (or Contact)Induction
18 Charging by FrictionThe act of rubbing creates friction which removes or adds electrons to the objects involved in the friction.
19 Charging by Conduction A charged object (the rod) is placed in contact with another object (the sphere)Some electrons on the rod can move to the sphereWhen the rod is removed, the sphere is left with a chargeThe object being charged is always left with a charge having the same sign as the object doing the charging
20 Charging by InductionA negatively charged rubber rod is brought near an uncharged sphereThe charges in the sphere are redistributedSome of the electrons in the sphere are repelled from the electrons in the rod
21 Charging by Induction, final The wire to ground is removed, the sphere is left with an excess of induced positive chargeThe positive charge on the sphere is evenly distributed due to the repulsion between the positive chargesCharging by induction requires no contact with the object inducing the charge
22 Not enough information to tell A charged rod is brought close to a neutral electroscope. When touched by the rod, the leaves both become positive and repel. The rod must have been …Positively chargedNegatively chargedNot enough information to tell
23 Not enough information to tell A neutral electroscope is charged by induction. When touched by the rod, the leaves both become negative and repel. The rod must have been …Positively chargedNegatively chargedNot enough information to tell
24 Not enough information to tell A postively charged rod is brought close to a neutral electroscope to charge it by induction. The top of the electroscope must be:Positively chargedNegatively chargedNot enough information to tell
25 Coulomb’s Law Mathematically, kc is called the Coulomb Constant kc = 8.99 x 109 N m2/C2
26 Coulomb’s Law Typical charges can be in the µC range Remember, Coulombs must be used in the equationRemember that force is a vector quantityIt is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is attractive or repulsive after the magnitude – 3 N attractive)
27 How is the magnitude of the charges proportional to the electric force between them? DirectlyInverselyExponentially
28 How is the square of the distance between two charges proportional to the electric force between them?DirectlyInverselyExponentially
29 Coulomb’s LawIt is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is
30 Things that make you go, “Hmmmm…” A) The electric force is significantly stronger than the gravitational force. However, although we are attracted to Earth by gravity, we do not usually feel the effects of the electric force.Explain why.
31 B) An ordinary nickel contains about 1024 electrons, all repelling one another. Why don’t these electrons fly off the nickel?
32 C) When the distance between two negatively charged balloons is doubled, by what factor does the repulsive force between them change?
33 Electrical Force Compared to Gravitational Force Both are inverse square lawsThe mathematical form of both laws is the sameElectrical forces can be either attractive or repulsiveGravitational forces are always attractive
34 Gravitational Force Electric Force Same Unable to tell If all other variables are held constant, is the electric force or the gravitational force greater for two oppositely charged objects?Gravitational ForceElectric ForceSameUnable to tell
35 Compare Gravitational and Electric Force Calculate the gravitational force as well as the electric force for an electron and a proton which are located 1 cm from each other.
36 Electrical FieldAn electric field is said to exist in the region of space around a charged objectWhen another charged object enters this electric field, the field exerts a force on the second charged object
37 Electric FieldsThe concept of a field is used to describe any quantity that has a value for all points in space.You can think of the field as the way forces are transmitted between objects.Charge creates an electric field that creates forces on other charges.
38 Gravitational FieldMass creates a gravitational field that exerts forces on other masses.
39 Gravitational vs. Electric Fields Gravitational forces are far weaker than electric forces.
40 Van de Graaff Generator An electrostatic generator designed and built by Robert J. Van de Graaff in 1929Charge is transferred to the dome by means of a rotating beltEventually an electrostatic discharge takes place
41 Electrical FieldAn electric field is said to exist in the region of space around a charged objectWhen another charged object enters this electric field, the field exerts a force on the second charged object
42 Electric Field, cont.A charged particle, with charge Q, produces an electric field in the region of space around itA small test charge, qo, placed in the field, will experience a force
43 Electric Field Lines Electric field is a vector. There are electric field lines that help visualize this field and were introduced by Michael Faraday
44 Electric Field LinesElectric field lines are drawn to visualize electric field strength and direction.Introduced by Michael Faraday
45 Electric Field Line Rules Electric Field Lines are drawn pointing in the way that a positive point charge would move when placed by the charged object.Field lines can never crossElectric fields exist even in theabsence of a point charge.
46 Electric Field Line Rules The direction of the electric field is in the same direction as the electric force on the point charge.The relative strength of the electric field is proportional to the number of field lines in a given location.Electric field lines accumulate as sharp points more than rounded objects.
47 ProtonA positive test charge would be repelled by the field++
48 ElectronA positive test charge would be attracted by the field+-