Presentation on theme: "Electric Charge and Electric Field Electric Charge and Electric Field"— Presentation transcript:
1 Electric Charge and Electric Field Electric Charge and Electric Field Chapter 17Electric Charge and Electric FieldChapter 17Electric Charge and Electric Field
2 Units of Chapter 17Static Electricity; Electric Charge and Its ConservationElectric Charge in the AtomInsulators and ConductorsCoulomb’s LawSolving Problems Involving Coulomb’s Law and VectorsThe Electric Field
3 Units of Chapter 17Field LinesElectric Fields and Conductors
4 17.1 Static Electricity; Electric Charge and Its Conservation Objects can be charged by rubbing
5 17.1 Static Electricity; Electric Charge and Its Conservation Charge comes in two types, positive and negative; like charges repel and opposite charges attract
6 17.1 Static Electricity; Electric Charge and Its Conservation Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.
7 17.1 Electric Charge in the Atom Nucleus (small, massive, positive charge)Electron cloud (large, very low density, negative charge)Q is used for charge; the unit is coulomb (c)
8 17.1 Coulomb’s Law Charge on the electron: Electric charge is quantized in units of the electron charge.
9 17.1 Electric Charge in the Atom Atom is electrically neutral.Rubbing charges objects by moving electrons from one to the other.
10 17.3 Insulators and Conductors Some materials are semiconductors. Charge flows freelyMetalsInsulator:Almost no charge flowsMost other materialsSome materials are semiconductors.
11 17.2 Induced ChargeMetal objects can be charged by contact (conduction):
12 17.2 Induced Charge - Induction They can also be charged by induction: Charging using a ground connection. The earth is large and a great conductor so charges want to go toward the ground.
13 17.2 Induced ChargeNonconductors won’t become charged by conduction or induction, but will experience charge separation:
14 17.3 Coulomb’s LawExperiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them.
15 17.3 Coulomb’s Law Coulomb’s law: (17-2)This equation gives the magnitude of the force.k is a constant = 8.988x109 Nm2/CQ is the charge in coulombr is the distance between the point charges
16 17.3 Coulomb’s LawThe force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same.
17 17.3 Coulomb’s Law Unit of charge: coulomb, C The proportionality constant in Coulomb’s law is then:Charges produced by rubbing are typically around a microcoulomb:
18 17.4 The Electric FieldThe electric field is the force on a small charge, divided by the charge:(17-3)
19 17.4 The Electric FieldFor a point charge:(17-4a)(17-4b)
20 ExampleCalculate the electric field E due to a point charge of nC at a distance of 5.0mm from the charge.
21 SolutionQ = 2.00 x 10-9 CR = 5.00 x 10-3 mE = k Q = (9.0 x 109Nm2/C2) x (2.00x10-9C)r (5.00x10-3m)= 7.2 x 105 N/C
22 17.4 Coulomb’s LawCoulomb’s law strictly applies only to point charges.Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.
23 17.4 Solving Problems Involving Coulomb’s Law and Vectors The net force on a charge is the vector sum of all the forces acting on it.
24 17.5 The Electric Field Force on a point charge in an electric field: (17-3)F is the electrostatic force exerted on a positive test charge q and E is the electric fieldSuperposition principle for electric fields:
25 17.5 The Electric FieldProblem solving in electrostatics: electric forces and electric fieldsDraw a diagram; show all charges, with signs, and electric fields and forces with directionsCalculate forces using Coulomb’s lawAdd forces vectorially to get result
26 17.5 Field LinesThe electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.
27 17.5 Field LinesThe number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.The electric field is stronger where the field lines are closer together.
28 17.5 Field Lines Electric dipole: two equal charges, opposite in sign: The direction at any point on the field is tangent to the lines. Notice that they go from positive to negative. What would these lines look like if they are both negative or positive
30 17.5 Field LinesThe electric field between two closely spaced, oppositely charged parallel plates is constant.
31 17.5 Field Lines Summary of field lines: Field lines indicate the direction of the field; the field is tangent to the line.The magnitude of the field is proportional to the density of the lines.Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge.
32 17.7 Photocopy Machines and Computer Printers Use Electrostatics drum is charged positivelyimage is focused on drumonly black areas stay charged and therefore attract toner particlesimage is transferred to paper and sealed by heat
33 17.7 Photocopy Machines and Computer Printers Use Electrostatics
34 17.7 Photocopy Machines and Computer Printers Use Electrostatics Laser printer is similar, except a computer controls the laser intensity to form the image on the drum
35 Summary of Chapter 17Two kinds of electric charge – positive and negativeCharge is conservedCharge on electron:Conductors: electrons free to moveInsulators: nonconductors
36 Summary of Chapter 17 Charge is quantized in units of e Objects can be charged by conduction or inductionCoulomb’s law:Electric field is force per unit charge:
37 Summary of Chapter 17 Electric field of a point charge: Electric field can be represented by electric field lines