0175 Physics Lecture 1 (June 22, 2009)
Introduction to the Course Electric Charge Coulomb's Law Structure of Matter Examples Electric Field Electric Field Lines
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Lecturer Information
Lecturer: Prof. Brian R. D'Urso Phone: [phone] E-Mail: [e-mail] Monday and Wednesday, 1:00 2:00 pm 315 Allen Hall
Office Hours:
Please give me feedback early and often if you have comments or suggestions! Email is my preferred means of contact. If you need to meet with me outside of office hours: schedule an appointment with me in advance.
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Where to Go for Help
1. Your Recitation Section. 2. Office Hours:
Lecturer: Prof. Brian R. D'Urso E-Mail: [e-mail] 315 Allen Hall Office Hours: Monday and Wednesday, 1:00 2:00 pm Shonali Dhingra, [e-mail] Office Hours: Tuesday and Thursday, 8:30 pm 9:30 pm and Friday, 2:00 pm 4:00 pm Naufer Nusran, [e-mail] Office Hours: Tuesday and Thursday, 3:00 pm 5:00 pm Sui Chi Woo, [e-mail] Office Hours: Tuesday and Thursday, 1:00 pm 3:00 pm
TAs:
3. Appointment with Lecturer or any TA.
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Syllabus
See handout.
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How your grade will be determined:
Exam 1: Exam 2: Comprehensive Final Exam: Quizzes (Recitation Sections): Homework (WileyPlus): Lecture Participation (clickers): 100 points 100 points 150 points 75 points 60 points 15 points
Total: 500 points
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Homework Assignment #1
Reading: Chapters: 21, 22 Problems: See WileyPLUS Try to register within next 24 hours! First homework due Thursday night!
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Course Outline (Approximate)
New force: the electrostatic force. Electric current. (Exam 1) New force?: magnetism. Connection between electricity and magnetism: Maxwell's Equations. (Exam 2) Traveling electromagnetic waves; light. Some properties of light. (Final)
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Discover a new force: the Electrostatic Force
Rub: Fur on Amber Silk on Clear Plexiglass Fur on PVC Amber Amber Plexiglass PVC Repels Plexiglass Attracts Repels PVC Repels Attracts Repels
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Charge and the Electric Force
In analogy with the relationship between mass and gravitational attraction, we associate this new force, called the electrical force, with a property we shall call charge. Experiments show that there are two kinds of electric charge, which we shall call positive (+) and negative (-). The choice of which type of charge is positive is arbitrary. The convention was introduced by Ben Franklin. He made an unfortunate choice. For our experiments, rubbing amber with fur places a negative charge on it.
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Properties of Electric Charge
Units: Coulomb (defined in terms of electric current). Charge is quantized. Elementary Particles: proton, electron, neutron, ... e = 1.60x10-19 C 1 C 6x1018 fundamental charges. Charge is conserved. Coulomb's Law: Force between two point charges.
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Halliday, Resnick and Walker: Problem 21-57 What is the total charge in coulombs of 75.0 kg of electrons?
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Coulomb's Law
F= k q1q 2 r2 1
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N m2 k= = 8.99 10 C2 4 0
0 = 8.85 10 -12
C2 N m2
inverse square law attractive for unlike charges, repulsive for like charges direction: along the line joining the charges Newton's 3rd Law action-reaction pair: Superposition of forces
r r F12 = -F21
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Vector Nature of Coulomb's Law
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What is the direction of the electrostatic force on charge +q? a) b) c) d) e) none of the above
+Q
+q
-Q
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What is the direction of the electrostatic force on charge +q? a) b) c) d) e) none of the above
+Q
+q
+Q
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Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces?
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What is the electric force acting on charge q3?
Q2 a) - 4 0 2a 2 1 b) zero Q2 c) + 4 0 2a 2 1 Q2 d) + 4 0 a 2 1 e) none of the above
q1 = +2Q
q 2 = -Q
q 3 = +Q
a
a
+x
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Halliday, Resnick and Walker: Problem 21-10
Three particles are fixed on an x axis. Particle 1 of charge q1 is at x = -a, and particle 2 of charge q2 is at x = +a. If their net electrostatic force on particle 3 of charge +Q is to be zero, what must be the ratio q1/q2 when particle 3 is at (a) x = +0.500a and (b) x = +1.50a?
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Halliday, Resnick and Walker: Problem 21-8
In Fig. 21-23, four particles form a square. The charges are q1 = q4 = Q and q2 = q3 = q. (a) What is Q/q if the net electrostatic force on particles 1 and 4 is zero? (b) Is there any value of q that makes the net electrostatic force on each of the four particles zero? Explain.
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Classification of Materials Electrical Properties
Conductor: charges can move around relatively freely in material. Typically negative charges (electrons) move. Insulator (or nonconductor): charges are not able to move around within the material. Charge may still be present.
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Charging Objects of Separation of Charges
rubbing insulating objects contact, e.g., conductor to conductor induction (and grounding)
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Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pithballs is charged by touching them with one of these objects. It is found that pithballs 1 and 2 repel each other and that pithballs 2 and 3 repel each other. From this we can conclude that a) 1 and 3 carry charges of opposite sign. b) 1 and 3 carry charges of equal sign. c) all three carry charges of the same sign. d) one of the objects carries no charge. e) we need to do more experiments to determine the sign of the charges.
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Charging by Induction
Use of Grounding
Fig.a
Fig.b
Connection to ground
-
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A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a). After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c). Based on this information, we can conclude that within the conductor a) both positive and negative charges move freely. b) only negative charges move freely. c) only positive charges move freely. d) We can't really conclude anything.
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Halliday, Resnick and Walker: Problem 21-13 In Fig. 21-26a, particles 1 and 2 have charge 20 C each and are held at separation distance d = 1.50 m. (a) What is the magnitude of the electrostatic force on particle 1 due to particle 2? In Fig. 21-26b, particle 3 of charge 20.0 C is positioned so as to complete an equilateral triangle. (b) What is the magnitude of the net electrostatic force on particle 1 due to particles 2 and 3?
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Importance of the Coulomb Force
Combined with quantum mechanics, the Coulomb force is responsible for: chemical binding stability of matter electrical neutrality of matter
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Structure of Matter
Fundamental Charge: e = 1.6010-19 C Electron: me = 9.10910-31 kg, q = -e Proton: mp = 1.67310-27 kg, q = +e Neutron: mn = 1.67510-27 kg, q = 0
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Comparison of Coulomb's Law with Newton's Law of Universal Gravitation
Coulomb' s Law : F = k q1q 2 r
2
m1m 2 Newton' s Law of Universal Gravitation : F = G 2 r
Both are inverse square laws, with the force acting along the line joining the point particles. There are two kinds of charges, but only one type of mass. Gravity is always attractive, while the electric force can be attractive or repulsive The electrostatic force, e.g., between an electron and proton, is enormously stronger.
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A hydrogen atom is composed of a nucleus containing a single proton, about which a single electron orbits. The electric force between the two particles is 2.31039 greater than the gravitational force! If we can adjust the distance between the two particles, can we find a separation at which the electric and gravitational forces are equal? a) Yes, we must move the particles farther apart. b) Yes, we must move the particles closer together. c) No, at any distance.
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Shell Theorems
1. A uniform spherical shell of charge exerts no electrostatic force on a charged particle that is located inside the shell. 2. A uniform spherical shell of charge attracts or repels a charged particle that is outside the shell as if all of the shell's charge were concentrated at its center.
Properties of the Shell Theorems: They apply to inverse square law forces in three dimensions. In particular, they apply to both Coulomb's Law and Newton's Law of Universal Gravitation. The proofs are based on calculus (why Newton invented calculus).
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Shell Theorems
m1 F1
m2
r
F1 = G
m1m2 r2
m2
m1
F =0
Q2
r F1
Q1
F1 =
1 4 o
Q1 Q2 r
2
Q2
Q1
F1 = 0
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Question Identical isolated conducting spheres A and B carry equal amounts of charge and are separated by a large distance compared with their diameters. The electrostatic force acting on sphere B due to sphere A is F. A third identical sphere C, having an insulating handle and initially neutral, is touched first to sphere A, then to sphere B, and finally removed. In terms of F, what is the electrostatic force that now acts on sphere B? a) (1/4)F b) (1/2)F c) (5/8)F d) (3/4)F e) none of the above
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A positively charged ball is brought close to a neutral isolated conductor. The conductor is then grounded while the ball is kept close. Is the conductor charged positively or negatively or is it neutral, if the ball is first taken away and then the ground connection is removed? a) Positive b) Negative c) Neutral
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Electric Field
Coulomb's Law: action at a distance Electric Field: local viewpoint
A field is a quantity that is assigned a value at each point in a region of space scalar field: temperature vector field: gravitational field, velocity field of a flowing fluid, electric field
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