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February Wednesday, 20, Announcements Homework 5 due today Homework 6 (pickup assignment) due March 5 Pickup graded homework and quizzes Today and next Monday Midterm Exam next week on Wednesday (2/27) Chapters 1 9 Bring one page (2-sides) for reference to exam Numerical values of constants are provided Can bring calculator Sample Midterm (hand out today) Solutions (on web next Monday, but try first!) Midterm Review part of Monday's class Discussion Sessions (7 9 PM, YKK220-001) on Monday and Tuesday evenings Other help by arrangement (email: TAs, me) Electricity and Electrostatics Today's lesson Finish Heat Flow Intro Electricity and Magnetism Electrostatic Phenomena Electric Charge Insulators and Conductors Electrostatic Force Electric Field Electric Potential Review Heat Flow Heat flows from higher T to lower T (warmer to cooler) Heat rises (hot air balloon) Three types of heat transport: Conduction flow of heat/energy through materials in contact thermal conductivity - property of material (metals, woods, plastics, hand, house insulation) travels through a medium [T average K.E. of atoms/molecules] Convection flow of heat or heat transfer via motion of a fluid contains thermal energy (air, water, steam) carried by a medium (energy) Radiation flow of energy by electromagnetic waves (chapter 15) Heat infra-red wavelengths (shorter than radio waves longer than visible light) thermos bottle (vacuum with silvered walls reflects waves) needs no medium Thought Question When sunlight warms the land near a body of water, a breeze starts to blow. Which direction usually and why? Concept Question: Why does firewood burn better when it is raised above the bottom of a fireplace grate? A. convection B. conduction C. radiation D. none of the above 68% 12% 9% 12% A . B . C . D . E . Concept Question: How does the space shuttle get rid of the thermal energy (heat) of its engines in an airless environment (space)? A. convection B. conduction C. radiation D. none of the above 97% 0% A . B . 3% C . 0% D . E . Concept Question: Why do meats and vegetables cook much more quickly when there are metal skewers sticking through them? A. convection B. conduction C. radiation D. none of the above 97% 0% A . B . 0% C . 3% D . E . Intro Electricity & Magnetism "Tell me what electricity is, and I'll tell you everything else" British physicist William Thomson (1824 - 1907) also known as Lord Kelvin Electricity provides the force that holds together atoms, molecules,...... matter Electrum is Greek for amber (a precious fossil resin) Amber rubbed with fur attracted bits of wood & fabric William Gilbert (1540-1603), English scientist, Attributed the word electric (from electrum) to amber Electricity & Magnetism linked together as different manifestations of the same force Magnetism Magnesia is part of ancient Greece (now in Turkey) rich with minerals Pieces of iron ore from Magnesia attracted bits of iron Euripedes (Greek playwright) called these lodestones magnets Magnets - ancient thought Greeks cured diseases, caused melancholy, test for chastity in women, magnet power destroyed if rubbed with garlic, ..... After Greeks - technological period........more on magnetism later! Electricity The basic understanding of electricity and magnetism developed in the late 1700s and early 1800s. Volta (battery) & Oersted (Elec. mag.) Ampere, Faraday, Ohm, Maxwell Electricity - all around us example - lightning control of electricity led to extensive modern day use batteries, lights, homes, computers, everything... Electrostatic phenomena - electricity at rest Electric circuits - motion of electric charges (next lesson) Chap. 12 Reading Quiz Static Electricity Comb near hair on dry day sparks in dark Rubber comb after combing hair (charged negative) Likewise, rubber rod with fur, silk on plastic (positive), shoes on carpet, .... Demo - charging of objects (rods) What's going on? Rubbing generates charge (how?) All involve electric charge and its transfer Electrostatic force Electrostatic force what's happening here? Demo Electric Charge Electric charge fundamental quantity basis for all electric phenomena matter - all matter is made up of atoms & molecules atom + atoms have electrons around a nucleus electrons - point-like particles that carry a negative charge charge can be positive or negative (Ben Franklin, 1750) "Like charges repel each other, opposites attract." conserved quantity cannot be created or destroyed quantized it comes in small packets of charge e "All matter held together by the electric force" What does this mean? Electrostatic Force force between electric charges Electroscope is used to measure the type and strength of electric charge Demo - electroscope Insulators and Conductors Conductors - (conduct or permit electric charge to flow) "loose" electrons, move quasi-freely metals (copper, silver, aluminum) rods, our bodies, .. Insulators - (restrict flow of electric charge) "tightly-bound" electrons, don't flow rubber, plastic, glass,... Semiconductors - (control the flow) between conductors and insulators carbon, silicon,... transistors, integrated circuits Superconductors - (charge flows without any resistance) until 1987 only at temperatures near absolute zero now up to 100 kelvins What's next? What are the implications? Charging by friction and contact as discussed previously, in demo, etc. by induction charging without touching an object 1 2 A B + + A + + B 3 4 A B +A + + + B Charging Mobility of electric charges Lightning storms Charging Transport of electric charges Demo - electrostatic "Van de Graaff" generator "Van de Graaff" Generator Attraction of Insulators to Charged Objects Pith ball (insulator) Charges in insulators cannot migrate Paper, styrofoam (insulators) can be attracted to a charged object ++- +++Insulators - when charged object is brought near atoms become polarized, form electric dipoles [electrostatic precipitators remove (charged plates) particulate matter from smoke stacks]

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