Physics 107

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Ideas of Modern Physics

Physics for Future Presidents

Logistics

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See course website at www.physics.wisc.edu 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)

Fundamental concepts

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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

Newtonian mechanics

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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!

Gravity and weight

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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.

Energy

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Kinetic Energy or energy of motion – K= (1/2) mv2 1 Joule = 1 kg-m2/s2

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-m2/s2 1 calorie = 4.2 joule 1 Calorie (food calorie) = 1000 calorie = 1 kcal = 4,200 joule

Energy conversion

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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.

But where did it come from?

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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.

But where did it come from?

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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.  …

But where did it come from?

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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

Time for some therapy

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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.

Powers of Ten

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The universe encompasses many length scales. Let’s look from a Chicago-centric viewpoint:

See http://www.powersof10.com/index.php?mod=watch_powersof10 and http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/