Lecture Power Points Chapter 16 Physics Principles with

Lecture Power. Points Chapter 16 Physics: Principles with Applications, 6 th edition Giancoli © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials.

Chapter 16 Electric Charge and Electric Field

Units of Chapter 16 • Static Electricity; Electric Charge and Its Conservation • Electric Charge in the Atom • Insulators and Conductors • Induced Charge; the Electroscope • Coulomb’s Law • Solving Problems Involving Coulomb’s Law and Vectors • The Electric Field

Units of Chapter 16 • Field Lines • Electric Fields and Conductors • Gauss’s Law • Electric Forces in Molecular Biology: DNA Structure and Replication • Photocopy Machines and Computer Printers Use Electrostatics

16. 1 Static Electricity; Electric Charge and Its Conservation Objects can be charged by rubbing

16. 1 Static Electricity; Electric Charge and Its Conservation Charge comes in two types, positive and negative; like charges repel and opposite charges attract

16. 1 Static Electricity; Electric Charge and Its Conservation Electric charge is conserved – the arithmetic sum of the total charge cannot change in any interaction.

16. 2 Electric Charge in the Atom: Nucleus (small, massive, positive charge) Electron cloud (large, very low density, negative charge)

16. 2 Electric Charge in the Atom is electrically neutral. Rubbing charges objects by moving electrons from one to the other.

16. 2 Electric Charge in the Atom Polar molecule: neutral overall, but charge not evenly distributed

16. 3 Insulators and Conductors Conductor: Insulator: Charge flows freely Almost no charge flows Metals Most other materials Some materials are semiconductors.

16. 4 Induced Charge; the Electroscope Metal objects can be charged by conduction:

Charging by conduction means that contact was made in the process. Free electrons from the neutral object can pass over to the positively charged object. The two objects end up with the same net charge.

Induction does not allow the neutral object to be touched by the charged object. The free electrons do not leave the neutral object, but instead move toward the positive charge. Creates a charge at either ends of the metal rods, so that if the rod was divided in half each half would have a charge. No net charge has been created, just a separation.

16. 4 Induced Charge; the Electroscope They can also be charged by induction: -by use of a ground connecting it to the Earth. When the charge is brought close, the electrons are repelled and will leave through the ground wire. Then if the ground wire is cut the rod is left positively charged. However, if the negatively charged rod is removed before the ground wire is cut; then the electrons will move back to the rod.

16. 4 Induced Charge; the Electroscope Nonconductors won’t become charged by conduction or induction, but will experience charge separation:

16. 4 Induced Charge; the Electroscope The electroscope can be used for detecting charge: The greater the amount of charge, the greater the leaves separate.

16. 4 Induced Charge; the Electroscope The electroscope can be charged either by conduction or by induction.

Electroscopes can be used to determine the sign of a charge, if it is first charged by conduction with a negative charge. Then if a negatively charged object is brought close (induction) more electrons move down the to the leaves and they move further apart. And if a positively charged object is brought close (induction) more electrons move away from the leaves and the leaves come closer together.

16. 4 Induced Charge; the Electroscope The charged electroscope can then be used to determine the sign of an unknown charge.

16. 5 Coulomb’s Law Experiment shows that the electric force between two charges is proportional to the product of the charges and inversely proportional to the distance between them. -double the charge on one object, then the force doubles -increase the distance, and the force decreases with the square of the distance.

16. 5 Coulomb’s Law Coulomb’s law: (16 -1) This equation gives the magnitude of the electric force that one object exerts on a second.

The Gravitational Constant G can be determined by experiment. Its value was first found by Henry Cavendish in 1798 with a device called a Cavendish balance. Two identical masses are suspended from an extremely thin and delicate quartz fiber. The two large masses can be moved close to the smaller masses and the attraction between M and m will cause the fiber to twist. After calibrating the system so the amount of force needed to produce a given twist is known and the attraction force between m and M can be calculated and then solve for G. F = G m 1 m 2 r 2

16. 5 Coulomb’s Law The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if they are the same. Newton’s 3 rd Law

16. 5 Coulomb’s Law Unit of charge: coulomb, C The proportionality constant in Coulomb’s law is then: (9. 0 X 10^9) Charges produced by rubbing are typically around a microcoulomb:

16. 5 Coulomb’s Law Charge on the electron: Electric charge is quantized in units of the electron charge on an electron –e / charge on a proton +e Cannot lose a fraction of an electron, so the net charge on any object must be a integral multiple

16. 5 Coulomb’s Law The proportionality constant k can also be written in terms of , the permittivity of free space: (16 -2) applies to objects whose size is much smaller than the distance between them

16. 5 Coulomb’s Law Coulomb’s law strictly applies only to point charges. -applies to charges when they are at rest which is known as Electrostatics. Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.

16. 6 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.

16. 6 Solving Problems Involving Coulomb’s Law and Vectors Vector addition review:

1. Calculate the magnitude of the force between two 3. 6 microcoulombs point charges 9. 3 cm apart. F= K Q 1 Q 2 r 2 = (9 x 109)(3. 6 x 10 -6)2 =13. 5 N (. 093)2 2. Find the force on the center charge. + Q 1 = 4 u. C 2 m Q 2= -5 u. C 4 m + Q 3= 6 u. C Calculate: F 21= force on charge 2 by charge 1 F 23= force on charge 2 by charge 3

3. Find the force on the +10 u. C charge in the figure below. + 6 u. C 30 cm + 10 u. C 20 cm _ -8 u. C

16. 7 The Electric Field The electric field is the force charge, divided by the charge: (16 -3)

16. 7 The Electric Field For a point charge: (16 -4 a) (16 -4 b)

16. 7 The Electric Field Force on a point charge in an electric field: (16 -5) Superposition principle for electric fields:

16. 7 The Electric Field Problem solving in electrostatics: electric forces and electric fields 1. Draw a diagram; show all charges, with signs, and electric fields and forces with directions 2. Calculate forces using Coulomb’s law 3. Add forces vectorially to get result

16. 8 Field Lines The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.

The direction of the electric field is defined as the direction of the electric force on a tiny, positive test charge, A test charge is a fictitious charge endowed with a special quality: the test charge exerts no forces on nearby charges and therefore does not disturb the charges in the vicinity. Electric Lines of Force: -lines of force originate on and come out of positive charges whereas, they are directed toward and end on negative charges.

16. 8 Field Lines The 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.

Which charge below has the greater magnitude? How can you tell? (b)

16. 8 Field Lines Electric dipole: two equal charges, opposite in sign: - field lines are curved and directed from the positive to the negative -the direction of the electric field at any point is tangent to the field line at that point What would happen to the field lines if the charges were equal and the same charge?

16. 8 Field Lines The electric field between two closely spaced, oppositely charged parallel plates is constant. The fringing of the plate near the edges can often be ignored, especially if the separation of the plates is small

16. 8 Field Lines Summary of field lines: 1. Field lines indicate the direction of the field; the field is tangent to the line. 2. The magnitude of the field is proportional to the density of the lines. 3. Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge.

16. 9 Electric Fields and Conductors The static electric field inside a conductor is zero – if it were not, the charges would move. The net charge on a conductor is on its surface.

16. 9 Electric Fields and Conductors The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move. Electric field II to the surface would exert a force on free electrons causing them to move In static condition, the Ell must be zero.

Inside a nonconductor, which does not have free electrons, a static field can exist. If a metal box between two parallel charged plates is hollow then the external field on the box is not changed since the electrons are free to move to the surface. The inside of the box is zero. This is effective for shielding something delicate on the inside of the box. + - That is why it is safe inside a car during a lightning storm. For a view of lightning hitting a cage containing a person: http: //www. youtube. com/watch? v=PT_MJotk. Md 8

16. 10 Gauss’s Law Relates electric charge and electric field Involves Electric Flux which refers to the electric field passing thru an area Electric flux: (16 -7) Electric flux through an area is proportional to the total number of field lines crossing the area. N∞ EAl

16. 10 Gauss’s Law Flux through a closed surface:

16. 10 Gauss’s Law The net number of field lines through the surface is proportional to the charge enclosed, and also to the flux, giving Gauss’s law: (16 -9) This can be used to find the electric field in situations with a high degree of symmetry.

16. 11 Electric Forces in Molecular Biology: DNA Structure and Replication Molecular biology is the study of the structure and functioning of the living cell at the molecular level. The DNA molecule is a double helix:

16. 11 Electric Forces in Molecular Biology: DNA Structure and Replication The A-T and G-C nucleotide bases attract each other through electrostatic forces.

16. 11 Electric Forces in Molecular Biology: DNA Structure and Replication: DNA is in a “soup” of A, C, G, and T in the cell. During random collisions, A and T will be attracted to each other, as will G and C; other combinations will not.

16. 12 Photocopy Machines and Computer Printers Use Electrostatics Photocopy machine: • drum is charged positively • image is focused on drum • only black areas stay charged and therefore attract toner particles • image is transferred to paper and sealed by heat

16. 12 Photocopy Machines and Computer Printers Use Electrostatics

16. 12 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

Summary of Chapter 16 • Two kinds of electric charge – positive and negative • Charge is conserved • Charge on electron: • Conductors: electrons free to move • Insulators: nonconductors

Summary of Chapter 16 • Charge is quantized in units of e • Objects can be charged by conduction or induction • Coulomb’s law: • Electric field is force per unit charge:

Summary of Chapter 16 • Electric field of a point charge: • Electric field can be represented by electric field lines • Static electric field inside conductor is zero; surface field is perpendicular to surface • Electric flux: • Gauss’s law: