PHYS 241 Electricity & Optics

Transcription

1 PHYS 41 Electricity & Optics Physics & Astronomy Department Home page: Course Home page: CHIP Home page: Piazza: Room Undergraduate office Room 11 - Help center Room 90 - Physics Library Course Home page: Schedule: Reading, Homework, Exams and Holidays *Read the assigned Chapter sections prior to attending class. *Note Exam Dates *Homework is usually due on Sundays at 11:59 PM. Homework is done through CHIP. CHIP Home page: 1

2 Syllabus Questions about grades, grade checks, absences, course withdrawal, etc please see Prof. Pyrak-Nolte Syllabus *Register your I-clicker on CHIP. *Always bring your I-clicker to class. *5% of your point total is based on lecture quizzes. CHIP: Homework:

3 Syllabus (see posted Syllabus for complete details) Piazza: Online Forum We will use Piazza for class discussions to assist you in getting help fast and efficiently from your classmates and the teaching assistants. Rather than ing questions to the teaching staff, you are encouraged to post your questions on Piazza: You can sign up for a Piazza account and enroll in our class page at: Additional Resource: Short YouTube Videos on selected topics. 3

4 Emergency Preparedness A Message from Purdue To report an emergency, call 911. To obtain updates regarding an ongoing emergency, sign up for Purdue Alert text messages, view There are nearly 300 Emergency Telephones outdoors across campus and in parking garages that connect directly to the PUPD. If you feel threatened or need help, push the button and you will be connected immediately. If we hear a fire alarm during class we will immediately suspend class, evacuate the building, and proceed outdoors. Do not use the elevator. If we are notified during class of a Shelter in Place requirement for a tornado warning, we will suspend class and shelter in the basement. If we are notified during class of a Shelter in Place requirement for a hazardous materials release, or a civil disturbance, including a shooting or other use of weapons, we will suspend class and shelter in the classroom, shutting the door and turning off the lights. Please review the Emergency Preparedness website for additional information. Forces Strong: short range ~ m Binds the protons and neutrons to form the nuclei Electromagnetic Binds electrons to protons to form nuclei, atoms to form molecules etc. Weak: short range ~ m Neutron decay and natural radioactivity Gravitation Dominant on large scales Electromagnetism Electric force Rubbed amber attracts straw Magnetic force Magnetite attracts iron 180 Hans Christian Oersted ( ) Current of electrons produces magnetic attraction 1830 Michael Faraday ( ) Time-dependent connection between electric and magnetic phenomena 4

5 Electric & Magnetic Fields Manifest their existence through interactions with matter div Described by Maxwell s Equations div ( B ) = B = 0 curl curl Differential Form E ( ) = E = ρ ε 0 E ( ) = E = B B t t ( ) = B E = µ o J +εo Integral Form S " " S " C C E d A = Q enclosed ε o B d A = 0 E d l = B d l = " S B t d A " µ I + µ ε E o o o S Lorentz Force F = q E + q v B t d A James Clerk Maxwell ( ) Lecture 1: Electric Charges & Coulomb s Law reducestatic-cling.html andreacarlisle.files.wordpress. com/01/0/staticcling_dogs.jpg Electric Charge Electric charge an intrinsic characteristic of the fundamental particles that make up objects Positive Charge Negative Charge + Electrically neutral: object contains equal amounts of positive and negative charges 5

6 Electric Charge Net charge of a system Algebraic sum of all the charges Non-zero net charge implies imbalance Imbalances small compared to total + and charges in an object Law of Conservation of Charge The net charge of a closed system never changes One of the fundamental laws of nature Electric Charge Electric charge is quantized q = ne, n = ±1,±,±3,... $ = n Elementary charge: e = (98) x C Business/images/us%0penny.jpg Typo in notes Coulomb (C): 1 C = the amount of charge transferred through the cross section of a wire in 1 second when there is a current of 1 ampere in the wire Charge of Particles Particles Charge Electron e Positron e + Proton e + Anti-Proton e Neutron 0 Photon 0 Typically electrons that transfer charge between objects Nucleus charge= +Ze Atom with Z electrons is neutral Proton charge: Electron charge: e + = 1.60 x C e - = 1.60 x C 6

7 008 by W.H. Freeman and Company Example How many electrons are in a quartz rod with r = 0.95 cm and l = 50.8 cm? And what is their total charge? Quartz is SiO : Z Si = 14, Z O = 8, ρ quartz =.66 g/cm 3 molecular weight = g Mass of rod m = ρv = ρ πr l =.66 π 0.95 cm 3 cm N molecules = 385 g Avogadro s number g molecules = molecules mole N protons = N electrons = (14 + 8) = Q protons = Q electrons = C = C ( ) 50.8 cm = 385 g Huge charge Conductors vs. Insulators Conductor: material in which electric charges can move around freely. Insulator: material in which electric charges are frozen in place. Semi-conductor: material in which electric charges can move around but not as freely as in conductors. Super-conductor: no resistance to the movement of charge. 7

8 Interaction of Charges: Insulators Insulator: material in which electric charges are frozen in place Interaction of Charges: Insulators Force of Repulsion Force of Attraction Charges with the same electrical sign repel each other Charges with opposite electrical signs attract each other. Demo Interaction of Charges Charged objects interact by exerting forces on one another Pith Balls 8

9 Mobility of Charge Conductor: material in which electric charges can move around freely Negatively charged plastic rod will attract either end of the electrically isolated copper rod Reason: charges in copper rod can redistribute themselves. Demo Mobility of Charge Pie Tins The Useful Concept of Ground The Earth is a (relatively poor) conductor Dissolved mineral salts are good conductors The Earth is very large Macroscopic charges can flow into or out of the earth without changing its net charge by any significant degree This property can be quite useful 9

10 Charging by Induction Maximizes distance between same charges 1. Bring a charged rod close to conductor. 3. Break connection to ground, keeping the charged rod in place. Ground the conductor. 4. Remove the rod. The sphere is charged. Demo Charge Induction Chimes Charged Conducting thread Grounded Insulating thread Coulomb s Law of Electro-static Force r q 1 q Charles-Augustin de Coulomb ( ) The electro-static force of attraction/repulsion has a magnitude: F = k q 1 q r Coulomb s Law where: k = 1 4πε o = 8.99 x 10 9 Nm / C and the permittivity constant is ε o = 8.55 x10 1 C / Nm 10

11 Coulomb s Law of Electro-static Force F = 1 Q 1 Q ˆr 4πε o r r 1 = r r 1 ˆr 1 = r 1 / r 1 Vector from q 1 to q Unit vector in r 1 direction r Force repulsive F 1 Force by 1 on + r F 1-1 Force attractive *Each particle exerts a force of this magnitude on the other particle. *The two forces form an action-reaction pair. Coulomb s Law of Electro-static Force Force exerted by q 1 on q at a distance r 1 F 1 = kq q 1 ˆr r 1 1 q 1, q in coulombs (C) r 1 in meters (m) F 1 in newtons (N) F 1 Same sign charges: F 1 is in the direction of r 1 Opposite sign charges: F 1 is in the direction opposite to r 1 Coulomb s Law Analogous to Newton s Equation of Gravitation F = k q q 1 F = G m m 1 r r k electro-static constant Inverse Square Law Charge Analogous Differ G gravitational constant Inverse Square Law Mass Attractive/repulsive depending on sign of charges Two kinds of charges Dominates on small scale Always attractive One kind of mass Dominates on large scales 11

12 Demo Electro-Static Force vs. Newton s Force of Gravitation x 4 Problem Solving Strategies Draw a clear FORCE diagram Use consistent units (meter, Coulomb, Newton) Remember that the force is a vector Look for symmetry Principle of Superposition When several point charges are put together, the total force on any one charge is the vector sum of the each of the separate forces acting on that charge. Exercise: F = F 1y + F 31y = F 1y y F R=1m F 31 Q 1 F 1 F = k Q 1 Q r cos N m (10 6 C) F = C ( 1m ) F = N Q 60 0 Determine force on Q 1 Q 3 Q 1 =Q =Q 3 =1µC x 1

13 Example Three charges on a line q 1 = C q = C q 3 = C R = 10 m What is the net force on q 1? F net F net = F 1 + F 31 F 1 = kq q 1 r 1 F 31 = kq q 3 1 r 31 ˆr 1 ˆr 31 Keeping subscripts straight and figuring out the directions of ˆr 1 and ˆr 31 is tedious. Draw a force diagram F 1 q 1 + F31 x Try to avoid the temptation to plug in numbers immediately. Let Q = C. Then q 1 = +Q, q = +Q, q 3 = Q Also, r 1 = R r 31 = 3 4 R F net = F,1 + F 3,1 = k Q + k Q R 3 4 R = kq = kq 7 = 14kQ R 9 R 9 9R = F net N ( ) ( ) 13