Einstein’s Special Relativity in VCE Physics

  • Published on
    10-Feb-2016

  • View
    33

  • Download
    1

Embed Size (px)

DESCRIPTION

Einsteins Special Relativity in VCE Physics. PHYSCON February 2012 Keith Burrows AIP Education Committee. Some reasons for including the Relativity DS in Unit 3: - PowerPoint PPT Presentation

Transcript

<p>Slide 1</p> <p>Einsteins Special Relativity</p> <p>in VCE PhysicsPHYSCON February 2012Keith BurrowsAIP Education Committee</p> <p>12012: This ppt is revised from the Feb 2011 teachers conference version. It is based on the 2007 &amp; 2010 versions given at PhysCon that year, but amended to take in the FAQs and the thought experiments in the light of the difficulties in that area. The International Year of Physics was when we started teaching Rel! **Find out who... has taught it last year?intends to teach it this year? ....might teach it this year? ....has a good understanding of relativity? Please feel free to add or question! Spend some time discussing why we should teach it... and how we can teach it. Resources and activitiesWe cant cover the whole lot in one hour! We will skip through the content without much explanation pausing on some of the key sections.</p> <p>Some reasons for including the Relativity DS in Unit 3:Students fascinated by the idea of Einstein and Relativity. Expands their thinking and opens their minds and not only in physics. Its what they want from Physics! We need to attract non-physics students who will end up as journalists, teachers, business people, and even politicians. (Especially politicians!)Relativity is an excellent example of the way science works: Apparent inconsistencies creative thinking experiments new theories Science as creative and imaginative.Fundamental physics: Newtons laws, frames of reference, mass and gravity, relative motion, waves and light, nature of space and time, electromagnetism.Education should be expanding minds not closing them! Non-scientists need to understand how science works especially politicians! (CC science!) Inconsistencies which attracted Einstein were in electromagnetism (not relative motion). Science is not mechanical but creative. This presentation is copyright Keith Burrows but physics teachers are free to use it in their classrooms. It is not to be used for commercial purposes. Some diagrams from Heinemann Physics 12 are used with permission. Suggestions for improvement are welcome! Email me at keithphysics at optusnet dot com dot au (with the appropriate ats and dots)</p> <p>2Some reasons for including the Relativity DS in Unit 3:</p> <p>Classic physics experiments: Michelson-Morley, speed of light, Eddingtons eclipse, travelling clocks, muon lifetimes, etc.Much modern technology can not be understood without relativity: magnetism, nuclear energy, GPS, the synchrotron.And it is (almost certainly) to be included in the Australian National Curriculum.3Why Relativity?What is physics really all about?</p> <p>4Aristotle the world can be understood by careful observation, thought and reason. Copernicus the Earth moves around the Sun. Galileo there is nothing special about zero velocity. Newton too many to list! </p> <p>What physics really is all about:</p> <p>5Oersted found link between electricity and magnetism.Faraday electromagnetic induction, motors, generators, field picture. Maxwell light is an electromagnetic wave. Marie Curie radioactivity </p> <p>6Relativity brings all these fundamental aspects of physics together in a new way!Einstein was Times Person of the century for good reasonRepresents not just the ideal scientist, but also an ideal public person:He questioned assumptions, he looked carefully at what was known, he used his imagination creatively, but intelligently to put forward new and radical ideas. He had a real concern for the social implications of science, as well as being a warm and compassionate human.Newton realised that we assume that time and space are straight...and that time and space are unrelated.</p> <p>7We normally assume no relationship between space and time! But most of us dont even realise that it is an ASSUMPTION. Einsteins relativity is about looking at that assumption more critically and discovering the real relationship between space and time.Einstein said that we should not assume this:He said that travel through time and space were intimately related.Special Relativity is about that relationship.</p> <p>These illustrations from Hawking: Universe in a NutshellThese illustrations from Hawking: Universe in a Nutshell8Travel through space and travel through time are related. One is at the expense of the other (sort of!)Why study relativity?Relativity represents a giant step in the story of physics. Why leave out the climax of the story?It is an excellent illustration of the process and nature of physics. Through it we can get a feel for real physics.</p> <p>Imagination is more important than knowledgeAlbert Einstein9To stop at the end of the nineteenth century seems to leave out the climax of the story! Imagination is as great a part of good science as it is of art...Think of the imagination needed to see that the Earth circled the Sun...or the imagination to see that time and space were not absolute!</p> <p>Einsteins LegacyIn many ways Einstein represents a new way of thinking, not just about space and time, but about everything.</p> <p>Will this new way of thinking have an impact on the way we see our world as profound as that of the physicists of the Enlightenment?</p> <p>(Not yet apparently )</p> <p>10The new ways of thinking introduced by Galileo and Newton had profound implications in areas well beyond physics.It was a turning away from declared wisdom to a new way of discovering truth through experiment and reason.One consequence was the mechanical universe, a universe which operates according to set rules and without any need for divine intervention.Still endless arguments about whether this universe needs a creator, or whether there is any room in that universe for something beyond the physical.The (unimaginative) Lego block universe has become well entrenched in popular culture.Led to a very materialistic culture in which things seem to matter more than people. Look at the mechanical view of education that thinks you can improve it by publishing scores for tests on websites, or the lack of imagination in understanding the effect of what we do on our environment etc..etc..In fact, in many ways politics and public discourse dont seem to have caught up to the enlightenment!Relativity could change the way we think!When the ideas involved in relativity have become familiar, as they will do when they are taught in schools, certain changes in our habits of thought are likely to result, and to have great importance in the long run.Bertrand Russell ABC of Relativity</p> <p>11What are these changes in our habits of thought? Consider the changes that Einsteins theory brought from the approach of 1900: From the mechanical universe where everything had a simple mechanical connection including human interaction to a universe of much greater subtlety and openness. We even find it hard to imagine a world different from the one we inhabit one where we are not tied to endless consumption and false and unsustainable growth. Or one where there are consequences to our endless fouling of our environment apart from the obvious ones that we can see immediately.Albert Michelson (1898)While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established </p> <p>12 and that further advances are to be sought chiefly in the rigorous application of the principles to all the phenomena which come under our notice.Physics at the end of the nineteenth century had become quite convinced of its correctness. Physics in the twentieth century shows the truth of the first part of this statement! And yet most of our current course is tied to those old grand underlying principles. Even in the Synchrotron DS we refuse to talk about relativity!</p> <p>The view in the early 1900sIn 1900 the mechanical world view seemed capable of explaining just about everything.Did this lead to the materialism and economic rationalism of the twentieth century?</p> <p>13What is the link between our view of the physical world and our philosophical/religious/spiritual views? The mechanical universe was largely responsible for the death of God in the 20th century. In many ways this might have been appropriate, but did we miss something else important? Was the old GOD the only way of considering the great mystery of the universe? Is economic rationalism the social equivalent of the mechanical universe of physics? In other words relativity, particularly when combined with quantum physics can open up big philosophical questions as well as simply a new link between time and space.Relativity could change the way we think!Could relativity really change the way we think?Science is not about collecting facts, but finding new ways of thinking about them. (Bragg)Imagine what could happen if we applied new ways of thinking to, say, energy production and consumption!</p> <p>14Einsteins thinking is characterised by: questioning basic assumptions, no blind acceptance of dogma, creatively thinking of new answers, great use of imagination.Economics seems rather like physics in 1900 rather arrogant and convinced it has all the answers. if the goal of economics became to find new ways to organise the way we do things instead of simply the application of preconceived laws (whether from Adam Smith or Karl Marx or )</p> <p>Its great physics!Relativity is about questioning common assumptions and finding new ways of looking at a situation. Its great thinking!What the world needs now Our students future leaders!So why study relativity?15Physics is about new ideas, about solving problems, about questioning assumptions (not just number plugging).What the world needs .... is a reasoned, thoughtful, and questioning approach to the social, political and environmental challenges we face The biggest problems we face are from fundamentalists of all sorts (religious AND political AND economic) who dont use this approach.Using Newtons laws to solve motion problems is clearly important, but it is not the big thinking that characterises so much of real physics.Relativity for allThis is not (just) for the specialists, it is for future...JournalistsTeachersPoliticiansLawyersHairdressersMothers and FathersCitizens16The specialists will obviously be interested, but they will get it later, the others wont this is their last chance.We can only provide it for physics students all year 12s should be getting it!ok so how do we do it?Developing the story: Overview1. Two principles Einstein did NOT want to give up2. Einstein's crazy idea3. Time is not as it seems: Time Dilation4. If time is strange, what about space?5. Faster than light? Momentum, Energy and E = mcThis sequence includes all of the points in the SD, but orders them in a more historically logical way.18How to introduce relativity?The historical approach enables students to follow the argument their physics can grow just as did physics itself.The whole basis of relativity was the apparent contradiction between the fundamental principles of mechanics and those of electromagnetism.This is the sequence I have developed in Heinemann Ph12.Summary of 1: Two principles Einstein did NOT want to give up1. The principle of relativity (no absolute zero of velocity) seems universal.2. Maxwells (very elegant) equations suggested:light is an electromagnetic waveand has a fixed speed whatever frame of reference!this speed was assumed to be the speed through the aether (to make it consistent with the principle of relativity)But Michelson and Morley could not detect the aether and Maxwells equations didnt want it.The principle of relativity seemed inconsistent with the predictions of Maxwells equations!19We cant develop the whole of the theory in 1 hour! We will go quickly through the content slides just to get a feel for whats there. These black slides give a summary of each main section. Einstein regarded Maxwells equations as very elegant because they seemed to sum up all the best physics that was known to that time in four equations (actually many more in the original version) which seemed eminently sensible! [see them in 4 slides)1. Two principles Einstein did NOT want to give up (1)Galilean/Newtonian principle of relativity:Nothing special about a velocity of zeroVelocity can only be measured relative to some other frame of referenceNo absolute velocityForce changes velocityThe laws of physics are thesame in any inertial frame. </p> <p>Very fundamental physics that seemed to make very good sense!201. Two principles Einstein did NOT want to give up (1) The principle of relativity</p> <p>21Different frames measure different speedsBut the difference in velocity is the same in both cases: 20 m/s forward.1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light In the 1830s Michael Faraday suggested that light may be some sort of electromagnetic wave phenomenon.</p> <p>In the 1860s James Clerk Maxwell developed Faradays idea into his famous electromagnetic equations.</p> <p>PS: Ideal examples of the excellent experimentalist and the excellent theoretician22Faraday actually discovered that the polarisation of light was affected by a strong magnetic field.</p> <p>1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light The equations suggested the possibility of electromagnetic waves travelling through space from an accelerated charge.</p> <p>23Clearly we can not develop the equations to show this here, but a certain amount of hand-waving about electric fields and magnetic fields and the link between them suggested in equations 3 &amp; 4 can help kids see how physics progresses. They will know that electric charges have an electric field (just a region where one charge affects another) and that moving charges (a current) produce a magnetic effect. The question then is, how fast will these effects spread into space? (Answer c). The equations basically tell us: 1) around a charge there will be an electric field (ie. there is a force between electric charges) 2) magnetic fields are continuous (unlike electric fields which originate on charges). 3) A changing magnetic flux (just field spread over area) will generate an electric field perhaps the surprising bit! 4) A magnetic field will be created from an electric current (as we know) or (again perhaps surprisingly) from a changing electric field. This all leads to the idea that a changing electric field can create a changing magnetic field which can create a changing electric field which can create. ie. fields which self propagate through space --- at what speed? Just need to do a bit of maths and we find .1. Two principles Einstein did NOT want to give up (2) Maxwell and the speed of light Maxwells equations predicted that electromagnetic waves would travel at a speed given by a simple expression involving electric and magnetic constants.</p> <p>The speed of light appears to depend only on the two electric and magnetic constants of nature (how m...</p>

Recommended

View more >