1. Thursday(02/18): Intro. to Ch.20 - Electric Charge and Forces.

HW: Read and Study pages 541-50, then solve problems 42, 43, 44,

47, and 49 on page 559.

2. Friday(02/19): Coulomb's Law and Applications. HW: Read

and Study pages 551-57, then solve problems 52, 53, 55, and 57 on

page 559.

3. Monday(02/22): LAB on Static Electricity. HW: Process lab data.

4. Tuesday(02/23): Post-Lab discussion. HW: Write lab report, due

Wednesday.

5. Wednesday(02/24): Intro. to Ch.21, Electric Fields, Applications.

HW: Read and Study pages 563-69, then solve problems 66, 67, and 68 on

page 585 and also solve problems 74, 77, 78, and 80 on page 586.

6. Thursday(02/25): Sharing Charge, Electric Field Near a Conductor,

Capacitors, Electric Potential, Millikan's Oil-Drop Experiment. HW: Read and

Study pages 575-83, then solve problems 82, 83, 85, and 87 on page 586.

7. Friday(02/26): Applications of Electric Forces. HW: Complete

Electric Force Handout.

8. Monday(03/01): Applications of Electric Fields. HW: Complete

Electric Field Handout.

INTRODUCTION: In ancient Greece amber became widely valued around

1600 BC. Greeks were fascinated by it. The ancient Greek word for amber

is "elektron", meaning - originating from the Sun. The Greeks were also

the first to describe the electrostatic properties of amber. Ancient Romans

loved amber as well. From the writings of Thales of Miletus it appears that

Westerners knew as long ago as 600 B.C. that amber becomes charged by

rubbing. There was little real progress until the English scientist William

Gilbert in 1600 described the electrification of many substances and coined

the term electricity from the Greek word for amber. As a result, Gilbert is

called the father of modern electricity.

One of nature's most spectacular display of electricity is the lightning

observed during a thunder storm. Benjamin Franklin (1706-1790) determined

that electricity originates from charges, positive or negative. We know now

that all material bodies possess electric charges. Electrons carry negative

charges while protons carry positive charges in the nucleus of an atom.

1. The electric force that stationary objects exert on each other is called the

electrostatic force. This force depends upon the distance between the two

point charges and the amount of charge on each. Experiments have

demonstrated that the greater the charge and the closer they are to each

other, the greater the force.

2. If charges have unlike signs, each charge is attracted to one other,

whereas like charges repel each other. These attractive forces and repulsive

forces act along the line between the charges, and are equal in magnitude

but opposite in direction (in accordance with Newton's 3rd law).

3. The French physicist Charles-Augustin Coulomb (1736-1806) experimented

with electric force between two point charges (the unit of charge is the

Coulomb, C). His work resulted in a law. Coulombs Law is defined: The

magnitude of the electrostatic force (F), exerted by one point charge on

another point charge is directly proportional to the magnitudes of the two

point charges, and inversely proportional to the square of the distance (r)

between the charges.

4. For a pair of charges q₁ and q₂, separated by a distance r, Coulomb's Law

may be stated as follows: F = k(q₁q₂/r².

5. The constant of proportionality, k = 8.99x10⁹ Nm²/C². Such a force is

transmitted by the presence of an electric field. The electric field E due to a

point charge q is, E = k(q/r².

6. Electric force and electric field are vectors. Hence, they have magnitudes

and directions. The electric force F and electric field E are related as follows:

F=qE, where the force is on charge q due to the presence of an electric field

at the position of q.

7. When an electric field is confined between two parallel metal plates, the

field is given by E = σ/ε_o, with σ, being the surface charge density, and ε_o

is the Permittivity of Free Space, or ε_o = 8.85x10^-12 C²/Nm².

8. The Principle of superposition also applies to the electric fields produced

by multiple charges. That is, the net electric field at a point due to several

charges is the vector sum of the electric fields due to individual charges.

9. For example, when more than two charges are present, the net force on

any one charge is equal to the vector sum of each of the forces produced by

other charges.

10. In other words, the force on charge q₁ due to the presence of charges q₂

and q₃, is the superposition of the forces exerted by q₂ and q₃. That is, the

net force F on charge q₁ is, F_net = F₁₂ + F₁₃.

where, F₁₂ is the force on q₁ due to the presence of charge q₂ and F₁₃ is the

force on q₁ due to charge q₃.

11. A capacitor is a device that stores charge. Capacitors are

formed by a pair of conductors (usually metal plates) separated by an

insulator. One of the many uses for capacitors is in computer memories.

A typical computer memory chip might contain 16,777,216 capacitors;

each capacitor is charged to approximately 5 volts to store the binary

digit 1, or 0 volts to store the binary digit 0. Another use of capacitors

is to store energy for relatively brief times; for example, the overhead

calculator that I use in class is powered by light energy instead of a

battery, and it has a capacitor to provide power during brief intervals

in which a shadow passes across its photocell. Additional applications

of capacitors include flash cameras, surge protectors, medical

defibrillators, touch pads, keyboards, car ignition systems, and radio

frequency tuners.

12. The electricity equations that we will have derived in class can also be

applied to capacitors since an Electric Field is maintained between its

plates. These are: E = F/q , E = kQ/d² , W = qEd , W/q = Ed , V = Ed ,

and W = qV .

13. The type of capacitor we are most interested in will have a charge Q and

-Q on each conductor. There will also be a resultant potential difference

(voltage), V, between the two conductors.

14. This voltage is linearly dependent on the charge. If we triple the charge,

we triple the voltage. Because of this relationship, the ratio of Q / V is a

constant for that capacitor.

15. The value of Q / V for a given capacitor is known as its capacitance. This

gives the simple equation, C = Q / V . The unit of capacitance is the Farad,

named after Michael Faraday (1791-1867). It is equivalent to one coulomb

per volt.

16. One Farad is an extremely large capacitance; most capacitors come in

units of micro (μ), nano (n), or pico (p) farads.

17. The capacitance of a capacitor is determined by two factors: (i) the

geometry of the capacitor, and (ii) the material between the conductors.

This material is known as a dielectric.

18. In a parallel plate capacitor, capacitance can be calculated by using the

equation, C = ε_oA / d , where C is capacitance, ε_o is the permittivity of free

space, A is the area of a plate, and d is the distance between the plates.

19. And still, we need these steps to solve any problem in Physics:

(i) read the problem and identify the given variables

(ii) determine what you are asked to solve for

(iii) find the correct motion formula to use

(iv) use algebra to isolate the unknown

(v) substitute-in the given information and simplify.

View the Slide Presentation.

Answers to Homework:

Page 559: #42. (a) 2f, (b) ¼F, (c) 1/9 F, (d) 4F, (e) 3/4 F

#43. 1.6x10²⁰ e^-, #44. 1.0x10^-8 N, #47. 3.2x10^-19 C

#49. q_A = 5.2x10^-7 C, q_B = 1.5x10^-6 C, #52. 14 N, #53. 8.2x10^-8 N

#55. 6.7x10^-7 C, #57. 8.1x10^-10 m

Page 585: #66. 2.8x10^-5 C, #67. 3.0x10⁴ N/C, #68. 6.7x10^-7 C

Page 586: #74. 1.8x10⁵ N/C, #77. 1.4 J, #78. -7.2x10^-17 J, #80. 90 V

#82. 2.00 μF, #83. 150 V, #85. 6.75x10^-10 C, #87. .45 J

THE HONORS PHYSICS ARCHIVES Ch.1: Physics Intro. Ch.2&3: Linear Motion. Ch.4&5: Forces. Ch.6: 2-Dim Motion. Ch.7: Gravitation. Ch.8: Rotary Motion. Ch.9: Momentum. Ch.10&11: Work&Energy. Ch.12: Thermal Energy. Ch.13: States of Matter. Semester Review. Ch.14&15: Waves&Sound. Ch.16: Study of Light. Ch.17&18: Reflect/Refract. Ch.19: Int/Diffraction.