Physics 107. Ideas of Modern Physics. Physics for Future Presidents. 1/22/10 Carlsmith, Physics 107

Transcription

1 Physics 107 Ideas of Modern Physics Physics for Future Presidents 1

2 Logistics See course website at for details. M,W Lecture: one chapter per week, attendance required A weekly paper (one page, two references) is required on your choice of discussion or internet research topics from text or alternate topic approved by the TA. Due in electronic drop box at LEARN@UW Thursday 10 AM. F work session: attendance required 1) work multiple choice questions in text and hand in bubble sheet answers 2) presentations of best two student papers of the week M,T Discussion: attendance required review of concepts and formula, presentation and discussion of preassigned discussion topics (graded) 2

3 Fundamental concepts Distance 1 meter = 100 cm =1/1000 km= about a yard Time 1 second = 1000 ms interval between heart beats Velocity (directed speed) 1 m/s Acceleration (rate of change of velocity, + or -) 1 m/s/s Mass (a measure of resistance to change in motion) 1 kg = 1000 gm 3

4 Newtonian mechanics Mechanics concerns the motion of objects in space and time Newton s Laws of mechanics: 1) Masses naturally move in straight lines with constant velocity. 2) Acceleration results from impressed force in inverse proportion to Mass: a = F/m or F=ma. (Acceleration can change the magnitude or direction of velocity.) 3) Two objects always exert equal and opposite forces upon each other Simple sounding. Given an understanding of the forces between objects, Newton s laws permit prediction of the motions of a mechanical universe! 4

5 Gravity and weight A magical universal law: All masses attract one another in proportion to both their masses. The earth attracts you and you attract the Earth equally. Your acceleration is g = F/m(you). The Earth s acceleration is F/m(Earth) and negligible. The Moon falls towards the Earth but its inertia carries it forward at the same time. The result is an orbital motion. Weight is the force F of Earth s gravitational pull on a mass m: F= mg with g=9.8 m/s/s acceleration of gravity) 1 Newton = 1 kg-m/s/s Weight on the moon is much smaller. The moon pulls less on you. 5

6 Energy Kinetic Energy or energy of motion K= (1/2) mv 2 1 Joule = 1 kg-m 2 /s 2 Potential Energy - invisible energy stored in the force fields between interacting particles. Example: lifting a mass off the ground requires work and creates potential energy. Releasing object permits the stored energy to appear as visible kinetic energy. Radiation freely moving wave light force fields such as electromagnetic infrared, light, UV and gravitational waves Heat a mixture of microscopic incoherent kinetic and potential and radiation fields at the atomic scale 1 Joule = 1 kg-m 2 /s 2 1 calorie = 4.2 joule 1 Calorie (food calorie) = 1000 calorie = 1 kcal = 4,200 joule 6

7 Energy conversion Energy is transformed but conserved!!! Slam some putty into a wall. It heats up. KE converted to heat. Gasoline engine: chemical potential energy converted to thermal microscopic KE of gaseous atoms which push a piston linked mechanically to wheels to create KE of motion of a car. Hydroelectric power station: Sun heats the ocean lifting water to the tops of mountains. Gravitational potential energy of water converted to rotation of a turbine converted to electrical potential energy that can push electrons along wires. Solar power: nuclear potential energy released in fusion in the Sun converts to thermal radiation intercepted on Earth and converted to heat or directly to electrical energy. 7

8 But where did it come from? Dear Professor Duncan Carlsmith, Hello, my name is Marnie. I'm fi;een years old and I go to the Hudson High School in Wisconsin. I asked a quesbon to my high school physics teacher who didn't know the answer, next I asked my older sister who goes to UW Madison who referred me to you. I asked that since the law of conservabon of energy says that energy cannot be destroyed or created, how did we get energy in the first place? Where did the energy come from to be transferred in the first place? Thank you for your Bme. Sincerely, Marnie Marnie is not her real name. 8

9 But where did it come from? That's a very deep quesbon of course, and I applaud you for asking. According to our present understanding of cosmology, primordial energy derives from the explosion called the Big Bang which created and threw mamer and dark mamer in all direcbons. The source of that inibal energy is unknown. The material energy, the stuff of mamer, wants to fall back together. It is just like when you throw a ball up in the air, why, it wants to return to Earth. The path towards recombinabon is circuitous. The fundamental parbcles of nature interact via electroweak, gravitabonal, and color forces and the amracbve components of these led to bound states which temporarily lock of energy at various length scales - atomic nuclei, atoms at microscopic length scales and, as the universe evolved, aggregates like stars which cooked up heavy nuclei and exploded spewing forth the elements of the periodic table. 9

10 But where did it come from? What drives the burning Sun is fusion of hydrogen nuclei to form deuterium, in the simplest terms. The energy released as these nuclei fall into one another appears in part as electromagnebc radiabon and as neutrinos and the radiabon powers all things on Earth. Solar radiabon powers chemical reacbons in plants which people eat for fuel. It throws water into the sky from where it falls back to Earth. Such solar energy is captured to do mechanical and electrical things via hydroelectric plants. At atomic scales, various combinabons of elemental atoms want to fall together releasing "chemical energy." So we find energy locked up in various ways and can arrange for its release. There is energy to be had combining oil in the ground and with oxygen in the air for example. Hope this helps! Duncan 10

11 Time for some therapy Suppose you are sad or overwrought. Your significant relationship seems to be ending uncontrollably. Something else is driving you nuts. Consider your place from another vantage point. Consider the Universe. 11

12 Powers of Ten The universe encompasses many length scales. Let s look from a Chicago-centric viewpoint: See index.php? mod=watch_powersof10 and micro.magnet.fsu.edu/ primer/java/ scienceopticsu/ powersof10/ 12