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Transcript
PROTONS FOR BREAKFAST AUTUMN 2013 WEEK 2 LIGHT 1 This is what you said on the feedback forms in Week 1 You said… Michael said… You have obviously got I do indeed have a vast library of songs and recently a vast and fantastic they have become available on iTunes. Details here: library of science http://protonsforbreakfast.wordpress.com/songs/ songs. Please could but don’t look if you want the songs to be a surprise you provide links to each week . them. No, fields are not made up out of particles of ‘stuff’. Are the fields made We will look at this again this week. And up of electrons? demonstrate the nature of fields with Jelly babies. Qu: Can you Mmmm. I am not quite sure I have got the gist of this. demonstrate electrical I think you are saying that if after rubbing the balloon repulsion using your on my pullover, I had taken off my pullover, would the jumper instead of the paper have been repelled rather than attracted? balloon, or would the If that is your question then the result would be the paper rotate and still same. Why? Because overall the paper remains be attracted but on electrically neutral. And the balloon (or pullover) acts the opposite side. If to polarise the paper – i.e. to induce a tiny separation so can you colour each of charge., rather than to add or subtract charges side to visually If I had used my pullover rather than the balloon, my represent the pullover would have been oppositely charged compared rotation. to the balloon and would have induced the opposite polarisation than that induced by the balloon. And the net result would still be an attraction. 2 You said… Michael said… I like the I think this distinction is very important for appreciating distribution between ‘how the world works’. One can’t make sense of things matter and fields as without appreciating this fundamental ‘duality’. two different kinds ‘Action at a Distance’ was the phrase used to summarise – of ”entity” that indeed mock – the concept of a ‘field’. At the start of the exist in “space|”. 19th Century people could only envision a force being Q: Can we communicated by direct contact. It was really James understand action at Clerk Maxwell who understood that the electric field was a distance, or just real. This gave rise to the ‘three-step process I describe it in described. mathematical ways? o A particle with a property we call electric charge (I’m not sure this disturbs the field in its immediate vicinity. makes sense!). What o The disturbance – or wave – travels away is a “field”? o And a short time later, other particles are affected by their local field. I think we do understand it, but not completely. And in some sense we can only describe the world and say what fields are ‘like’. In a fundamental sense, we can never know what either fields or particles ‘are’. All really interesting! I am glad you found it so. How quickly It depends on the atmosphere. If the atmosphere is dusty or does a charge moist, then electrical charges on the balloon can polarize piece of paper microscopic dust particles – attracting them, and then when they lose its charge? touch the balloon, they become charged and then repelled, carrying away the charge. In normal air it typically takes a few hours, but occasionally things can stay charged for days – we still had ‘floating foil’ pieces stuck to the ceiling for a whole day after last week’s session. How is the strong When I said ’it’s all electricity’ I was speaking of the force different forces which we experience around us: i.e. chemistry, from electricity? biological processes, mechanical forces etc. are all If it’s all fundamentally electrical in nature. electricity, what is We do not know of any connection between the strong the strong force? nuclear force that acts between: Looking forward to o protons and protons, the answer next o protons and neutrons, and week – thank you o neutrons and neutrons. for this evening. And the electric force. They are – as far as we know fundamentally different. The strong force is very short range. So that it acts in effect like a ’contact’ force – as if the particles had a sticky surface. 3 You said… I like the toys. What is the strong nuclear force? How much static electricity does it take to move a human? Michael said… Glad you liked the Fun-Fly Sticks. I have written an explanations of the strong nuclear force both above and below this answer! Mmmm. Not sure what you mean here. If I applied a few nano-coulombs of charge from a Van der Graaff generator, then the flow of electricity would cause you to feel pain, and the resultant muscle contractions would make you move. Perhaps that is not what you meant. If I hung you on a balance like I hung the sausage, then I could move you with just a single balloon rubbed on my pullover. The force would be very similar to that on the sausage, but because you are so much heavier, you would accelerate slower. However you would eventually swing around! The largest thing I have accelerated in this way was a guitar weighing around 10 kg. By week 6 will I be able to converse with Brian Cox? Yes When an We will talk more about isotopes in week 6. But briefly, no, more atom has a neutrons do not always make an isotope more stable. For atoms with higher small numbers of protons, the optimum number of neutrons is the isotope same as the number of protons. So for example, the main stable (more isotope of carbon has 6 protons and 6 neutrons and is called 12C neutrons) where the ‘C’ tells you it is carbon and so has 6 protons and 6 does it mean electrons and the ‘12’ tells you that the total number of protons and that it is neutrons is 12 i.e. that it has 12 – 6 = 6 neutrons. more stable? If you take away a neutron to make 11C then the nucleus falls apart after about 20 minutes. If you add a single neutron to make 13C then the nucleus is stable and this isotope makes up 1.1% of all the carbon around us. If you add two neutrons to make 14C then the nucleus is unstable and the nucleus falls apart after about 6000 years. So too few or too many neutrons makes the nucleus unstable. For heavier elements, such as gold (Au) which has 79 protons, the nucleus needs more neutrons to act a nuclear glue to hold all the protons together. So the only stable isotope of gold is 197Au which has 197 – 79 = 118 neutrons. However adding (198Au) or subtracting (196Au) a single neutron makes the nucleus unstable. Atoms with unstable nuclei are said to be ‘radioactive’. 4 You said… Comment - loved doing the demos at coffee break! Question - Why is the middle of the periodic table shown separately? Thought - I feel much better knowing there are only 2 kinds of thing in the Universe that I have to understand (i.e. stuff/nonstuff). Learning about science seems much less daunting now! (That’s not sarcasm either!). Thanks for a great evening. Michael said… Glad you enjoyed it. Great Question: The Periodic Table shows elements in order of the number of protons in the nucleus of the element (called the atomic number). It starts at the top left (H) and reads left to right, and then we go down a row. The shape of the conventional diagram reflects the way electrons arrange themselves around the nuclei of atoms in so-called ‘shells’. Let’s imagine creating new atoms by adding protons to the nucleus, and corresponding orbiting electrons. We start with hydrogen (H) which has 1 proton and 1 electron, then make helium (He) which has 2 protons and 2 electrons. 2 electrons form a closed-shell i.e. the orbits make a perfect sphere. As we add more protons and neutralising electrons, the electrons arrange themselves around the first closed electron shell – but it now takes 8 electrons to complete this 2nd shell (Ne). These closed shell elements are chemically similar (and very un-reactive) and they are plotted on the right-hand side of the table. The next shell also takes 8 electrons to complete (Ar). The next shell permits more complex orbital trajectories and can accommodate 14 electrons. The elements with the complex orbitals – called d-orbitals – are plotted in the middle of table. After that – it gets ‘even more complicated. 5 You said… Michael said… Very interesting – I Thank you. That comment makes me at the same time learnt more tonight on happy, but also sad! the subject that I did ‘Fun-Fly-Sticks’ cost £20 and we buy them from http://www.grand-illusions.com/acatalog/FunFlyStick.html at school! which is a British company which pays tax. They are How much does the available cheaper from Amazon. portable VDG generator cost? Well explained. Thank you. God said he created I am not aware that we have God’s authentic voice on everything at the any issue, but there are many competing and conflicting beginning, would human claims to represent ‘him’. dinosaurs have been Dinosaurs existed more than 65 million years ago. alive with humans such Humans have only existed for the last 1 million years or as in times like end of so, and the regular advance and retreat of the polar ice Triassic period and that we call ‘ice ages’ have only been taking place for early ice age. probably a similar period. You said about Yes, exactly correct. As we pack extra electrons around an valancies in the atom they ‘stack’ in so-called ‘shells’ at different distances talk. Are they the from the nucleus different amounts o the first shell can accommodate 2 electrons and the of electrons in the element with the filled shell is non-reactive helium. outer shell of an o the next shell can accommodate 8 electrons and the atom, with the element with the filled shell is non-reactive neon. shells having 2, 8, o the next shell can accommodate 8 electrons and the 8 etc per thing? element with the filled shell is non-reactive argon. And the ones with o the next shell can accommodate 18 electrons and the complete outer element with the filled shell is non-reactive argon. shells are I feel obliged to say that the patterns that electrons unreactive as they make as they orbit nuclei are complicated, but this simple can’t form ions representation is a fair summary. right? The thing which makes them unreactive is that the filled shell is completely spherical symmetric and so nearly perfectly cancels out the effect of the electric charge on the nucleus in every direction. Unfilled shells are not symmetric and there are ’gaps’ though which the nuclear charge can be detected by other atoms – pulling them in for an electrical interaction that we call a ‘chemical reaction’. 6 You said… How many protons/electrons are there in a banana? How many electrons are there in a sausage? Very good speech – detailed, understandable and informative. Loved the experiments. Michael said… I have just weighed a banana – a large one – and it weighed 0.243 kilograms. Scientists have worked out the protons and neutrons weigh roughly the same – approximately: 1.66 x 10-27 kg Electrons weigh approximately 1/2000th of the mass of a proton or neutron so we can neglect their mass in this calculation. So the total number of protons and neutrons in anything weighing 0.243 kg is roughly; 0.243 kg ÷ 1.66 x 10-27 kg = 146 x 1024 If we assume this number is roughly half neutrons and half protons – which is pretty accurate for a banana made mainly of oxygen and carbon – then a banana contains approximately 73 x 1024 protons. I am pretty sure that answer is within 10% of the correct answer. You can do the same calculation for a sausage – but I will leave that up to you – it depends on the mass of the sausage. Glad you enjoyed it 7 You said… Really enjoyable, entertaining and very informative. My question: Do fields – electrical, gravitational always behave in the same way irrespective of their surroundings. Are they affected by, for example, pressure. Would an electrical field be the same at the bottom of the ocean or deep in the earth as it is with no pressure – or even in space? Michael said… I am glad you enjoyed it. Mmmm. I am not sure how to answer this, but here we go. Imagine two charged particles, both positively charged – so they repel each other. Imagine they are in a box which is evacuated so it has no air inside. Then it wouldn’t matter where that box was: in the deep ocean or deep space:– the force between the particles would be the same. However if you let some atoms into the box, then they would be affected by the electric field, and in return they will then affect the electric field, and so the force between the two charged particles would be slightly modified. OK. Now I get it! Great ;-) Why do some Both the normal arrangements (6 protons and 6 neutrons) and carbon atoms the rare arrangement (6 protons and 7 neutrons) are stable. have seven So when the elements were formed, they stayed the way neutrons rather they were. than the usual When were the elements formed? Well: six. o The nuclei of nearly all the hydrogen and helium in the universe was formed in the first few seconds after the Big Bang, This includes all the hydrogen atoms in the H2O in your body. Does that make you feel old? Because the protons in all the hydrogen atoms are around 13 billion years old. o The elements lighter than iron were subsequently created inside stars (this is where the carbon came from), and when the stars exploded they were scattered into clouds of dust o The elements heavier than iron were created during the stellar explosion that scattered elements into clouds of dust o We are literally (as Joni Mitchell sings) stardust. 8 You said… I want a fun fly stick! Feedback: I like the extensive talk about the subjects. Especially about the atoms because it will definitely help me in school. Questions: What is the reason why atoms can only hold 82 protons before it becomes radioactive? Overall: I loved the experience, and can’t wait to return next week. Michael said… Everyone does, me included. But to be brutally honest – there is only about an hour or so of real play value in these things – so you have probably had the best of it already Good. Happy to help. The limit comes from the balance of the electrical repulsion and the strong nuclear attraction. Consider the case of a proton on one side of the nucleus. Because electric repulsion is a long range force, the ‘edge’ proton feels the repulsion of all the other protons. This repulsion is shown by double ended arrows in eth picture above. It is bound into the nucleus by the strong nuclear force which is a short range force – essentially a contact force. The ‘edge’ proton is held in place by at most 9 neighbouring neutrons. As we add more and more protons the repulsion gets bigger and bigger, but the binding force can’t keep growing because the ‘edge’ protons can’t make contact withany more neutrons. Above 82 protons (Lead), it isn’t possible to find an isotope with a perfectly stable arrangement. Element 83 (Bismuth) is long lived, but not quite perfectly stable. Thank you. I am looking forward to it too. The dogma is that:- I am afraid I can’t understand this. DNA makes RNA My guess is that you are commenting about some aspect of make PROTIENS! our understanding of how biological cells function. In So, how come:- DNA particular on the mystery of how archaic cells are thought to makes RNA make have evolved using RNA only but now cells use bothe DNA PROTIENS? and RNA. I am not the right person to comment on this – personally I think it will take a while before we get all this sorted out. 9 You said… Michael said… Questions: Do you see a parallel between Mmmm. Tricky. the fact that matter and anti-matter The universe as we find it has must exist together, but being kept only the tiniest amounts of antiapart, otherwise cancel each other and matter. And I am not expert on the Bible’s description of Heaven and Biblical interpretation, but I don’t Earth? entirely see a parallel. Maybe I Maybe a parallel between how the tiniest have missed your point. Sorry. particles behave and how our universe is set apart into two worlds is possible? Here is the Powers of Ten Video http://www.youtube.com/watch?v=0fKBhv Can I show the Power of Ten’s video to Djuy0 my son? Is it available on the internet? We will indeed go deeper: glad Feedback: Great overview, hope we go you enjoyed it. deeper. Memorable lessons. Very interesting presentation, Glad you enjoyed it. The periodic table is a learned lots. Great to see a periodic great cultural icon – a tremendous table after 25 years! achievement our culture. How many atoms thick 250 sheets of printer paper is approximately is the thinnest sheet 50 mm thick so I estimate that 1 sheet of printer of paper? paper is 50/250 = 0.2 mm. Also great The spacing between atoms in most substances is presentation. Michael about 0.3 nanometres, so the sheet paper is about is so funny! It would be 1 million atoms thick. great to have him as For a few weeks at least I think I am your my science teacher! science teacher. In a field, Great question and we will discuss this more this week, when we use what is the giant Jelly-Baby wave machine. actually The behaviour of a ‘field’ is like the surface of a pond. moving? When you poke a stick up and down in a pond – a wave travels away from the stick across the surface. Water isn’t travelling away from the stick. The water level at each point on the surface just moves up and down, but the disturbance is passed from each region of the wave to its neighbouring region and so travels away from the origin. In this way a disturbances spreads even though each part of the surface just moves up and down. 10 You said… Why is an Angstrom 1010? Not a multiple of 3! 10-9, 10-6, 10-3, 100? If everything is electrical what about neutrons? Michael said… An Angstrom (symbol Å i.e. an ‘A’ with a ‘diacritical’ mark) is a unit of length named for Swedish physicist Anders Ångström (1814–1874). Though still widely used, particularly by chemists, it is not part of the International System of Units (the ‘SI’) and so is an obsolete unit. As you noticed, the SI uses special prefixes to give names to units which differ in magnitude by a factor 1000. I think I mentioned this somewhere above. You are quite right that within the nucleus of atoms both electricity and the strong force are important. Why have you got Yes, they are cheaper to run and should last for 25 new LED lights? years. They can be much brighter than they are. When - are they cheaper they were commissioned, their brightness was to run? measured and turned down to give just the correct - are they brighter level of illumination. than the old ones? What do you mean by ‘electricity and water don’t mix’’? If electricity holds In our homes we keep electricity quite separate from water together water. That’s because water is an electrical conductor why don’t and so if water spilt, electricity could flow into electricity and wherever the water went, which could be quite water mix? dangerous. But the fact that water is an electrical conductor arises from its intrinsic electrical properties. Will look at the world in a Great…I think. As long as you are completely different way. happy with the new view. Have scientists ever thought of I don’t think so. Because the word giving the word ‘electricity’ a comes from the Greek, it has been different name. It is such a inherited more or less directly into all universally important concept, and modern languages. in my mind not completely It is definitely not appreciated how appreciated. amazing and universally important the concept is. 11 You said… What size is the Milky Way to the Sun to the Earth to the moon? Cause in the first video the Milky Way was further than the Sun and yet almost the same size as the sun and moon when I’m standing on Earth. What is the strong force? Why are protons and electrons charged? Why don’t neutrons have a charge? How much does one of those ???? things cost? Where can I get a magic wand…? Michael said… Not sure exactly what you are asking. I think what you are commenting on is the angular extent of an object. For example, when you hold out your thumb at arm’s length, you can block from view: A 0.5 metre-wide person 50 metres away The 3400 kilometre-wide Moon 380,000 kilometres away The 1.4 million kilometre-wide Sun 150 million kilometres away In each case the blocked object has a diameter of around 1/100th of the distance to the object Our estimate for the size of Milky Way is uncertain because we are right in the middle of it, but we think it is about 100,000 light years in diameter but only (!) around 1000 light years thick. I have answered this a couple of times above and below. Briefly the strong force acts between: o protons and protons, o protons and neutrons, and o neutrons and neutrons. Great question. Fundamentally, we don’t know what electric charge ‘is’ – we can’t explain it in terms of anything else – it is a fundamental property of the ‘stuff’ of the universe. As I mentioned somewhere else – electrons are – to the best of our knowledge – truly fundamental particles: no matter how hard we bash them together, we don’t see evidence of any internal structure. Protons and neutrons are each composed of three even tinier particles called quarks. Quarks too possess electric charge, but in a neutron, the quark charges add up to zero, while in a proton they add up to make a charge we observe. Glad you enjoyed the experiments. The Fun Fly Sticks cost about £20 – a web link is given somewhere else in these answers. I think you mean a Fun Fly Stick – I put the links somewhere earlier in this document. Magic? It’s not magic, its science, which is IMHO just as cool – if not cooler – than magic. 12 You said… Why is there a charge? Where does it come from? How are electrons and neutrons and protons made? Michael said… As I mentioned above, fundamentally, we don’t know what electric charge ‘is’, and we can’t explain it in terms of anything else – it is a fundamental property of the ‘stuff’ of the universe. We don’t know where it comes from but we do know that it is ‘conserved’ - we can’t get rid of it! And there seems to be an equal amount of the positive charge and negative charge in the universe – coincidence? I think not! In our current understanding, all the protons and neutrons in existence now were formed about 2 seconds after the big bang. The electrons were formed a few seconds later. Obviously you can ask – How can we possibly know stuff like that ! And of course we don’t. But we can say: If the big bang happened as we think it did, how much hydrogen and helium would we expect to find in the universe? And when we do the calculation we find a very reasonable match with what we observe – which makes us think that we must at least be close to the right idea. Yep! You were right. I’m boggled. Wow! Boggled is OK as long as you are not Thank you. panicking. Great presentation – Thank you and well done for re-visiting the exam. It wish I’d had that 50 is interesting to me how long your memory of having yrs ago when I failed ‘failed’ persists. Based on your comment, you must physics O level now be 65-ish and yet this ‘failure’ still sounds like it (passed 2nd is somehow alive. As you may know, in the last 50 attempt!) (But still years, schools have become obsessed with exams and pretty boggled.) I worry about the large numbers of citizens that can Q: Why does light feel that sense of failure and thus perhaps exclusion sometimes seem to from ownership of science. Which is wrong, because “bend” round if you are a citizen of the UK, your taxes over the objects? Is it due to last 50 years have paid for pretty much all the the electric charge science in the UK: in other words: it’s yours! of the object? Excellent observation. The phenomenon is called diffraction and arises because light is a wave. For light, yes, the electric charge within an object is involved. We will look at this week. You can observe other waves diffracting: sound diffracts easily which is why we can ‘hear around corners’; and water waves in the sea diffract around islands and headlands. 13 You said… If you flipped electrons and protons would things be different? It was electrifying. Michael said… Assuming by ‘flipped’ you mean we somehow instantaneous replaced every proton with an electron and vice versa: yesthings would be very different. o The nucleus of every atom would be composed of neutrons and electrons. The strong force does not affect electrons – they only feel the electro-weak force, and so the nuclei of every atom would instantly fall apart. o Some of the neutrons would meet the protons and form new nuclei. But the repulsion between the protons means that heavy complex nuclei would not form – they are only formed at very high temperatures and pressures inside stars. o Finally electrons would begin orbiting the new nuclei. o The net result is that basically the whole world would end up being made of hydrogen. So yes, things would be ‘different’. Glad you enjoyed it. 14 You said… Can you explain the means by which electricity is measured e.g watt, volt, amp, ohm, etc please. Thanks. Michael said… It’s complicated, but you did ask! Three of these units (volt, ampere and ohm) describe the ‘flow’ of electricity which is described by analogy with the flow of water along a pipe under gravity. For water flows: The ‘current’ is the amount of water passing one point in the pipe per second. This is analogous to the amount of electrical charge passing a point in a wire per second Tilting the pipe gives the water more energy for each extra distance the water flows. The extra energy per unit mass is analogous to electrical voltage, which is related to the energy per unit charge. The resistance of a pipe makes it hard for water to flow. Similarly the electrical resistance of a wire describes how hard it is to pass a given electric current Coulomb: The unit of electrical charge is called a Coulomb. One Coulomb is a massive amount of electricity – the experiments with the balloons typically involved a few nano-coulombs and an individual electron has a tiny charge 1.6 x 10-19 coulomb. Ampere (or ‘amp’ for short): when one coulomb of electrical charge per second flows along a wire then we say a current of 1 ampere is flowing. Because the charge on each electron is so tiny, this involves the motion of massive numbers of electrons. Volt: when some electrical charge has flowed far enough so that its energy has changed by 1 joule for every coulomb of flowing charge, we say it has flowed through a ‘potential difference’ of 1 volt. So in a 9 V battery, the electrons leaving the – terminal flow ‘downhill’ to the positive terminal and for every coulomb of charge that moves along the wire it gains 9 joules of energy Ohm: The electrical resistance of wire is measured in ohms. If it takes a voltage of 10 volts to drive 2 amperes of current we say the electrical resistance is 10÷2 = 5 ‘volts per ampere’ or 5 ohms. Watt: A watt is a unit of power (not necessarily electrical) which specifies how much energy (joules) per second a process uses. If a 240 volt mains source passes 4 amperes through a resistance then it will dissipate V x I = 240 x 4 = 960 watts or just under 1 kilowatt. 15 You said… Good presentation. Can you give a summary of the half-life of elements, e.g. uranium? Michael said… In elements with unstable nuclei, not all the nuclei decay at same time. There is a uniform chance per unit time that a nucleus will undergo a change to a new configuration of protons and neutrons. Some nuclei are very unstable, and the chance per unit time of a nuclear change is high, so that only a few atoms of the substance can exist on Earth at any one time. Some nuclei are nearly stable, and the chance per unit time of a nuclear change is low, so that we can still find lots of the substance on Earth even billions of years after the elements were created. Scientists summarise this chance per unit time of a nuclear change by specifying the time during which on average half the atoms of particular substance will decay: the half-life: For Uranium (U): 232U has a half-life 68.9 years 233U has a half-life 159 thousand years 234U has a half-life 245 thousand years 235U has a half-life 700 million years 236U has a half-life 23 million years 238U has a half-life 4468 million years Because of this, most natural uranium we find on Earth now is mainly 238U. Other isotopes may exist but their half-lives will be very short – days, minuets or even small fractions of a second! More in Week 6 Great, really enjoyed it. Good If the number of We don’t create water, but we can move it around electrons are all already and make it appear at convenient places – such as in existence how do we coming out of taps! So it is with electricity. Using generate Eeeeee-lecbatteries, generators, conducting wires and tric-ity. insulators we channel electricity to appear just where and when we want it. Thanks for making ‘electricity’ so entertaining!! You are welcome Wish science was/had been taught like Me too. Perhaps it will be one day this at school! How many electrons do There are various estimates, all highly uncertain. The we think are in the basic approach is to neglect everything other than universe? hydrogen, and then estimate the number of atoms in an average star, the number of stars in the average galaxy, and the number of galaxies in the observable universe. Of course we have no idea about the un-observable universe. But in the bit that we can look at we think the number is 1080-ish but a factor a million either way is quite plausible. http://bit.ly/16SAUay 16 You said… What is the strong force? Michael said… This is the force between: o protons and protons, o protons and neutrons, and o neutrons and neutrons. It is not a long range force like electricity, but instead just acts between neighbouring particles – like a super-strong contact ‘glue’ making the particles stick together What keeps Do you mean “What stops electrons flying off into space?” or electrons in “What stops electrons spiralling into the nucleus?” orbit around the “What stops electrons flying off into space?” Basically the nucleus? Is it electrical attraction of the nucleus – the electrons just can’t centrifugal get away. force – Earth “What stops electrons spiralling into the nucleus?” This is a around the Sun? much harder question which stumped people for decades. Obviously electrons don’t do this – otherwise atoms would not exist. Now we understand that although the force between electrons and protons within an atom is exactly the same as the one between the balloon and the paper, the laws of motion of small particles are not the same as Newton’s laws of motion which govern large objects. Tiny particles such as electrons obey the laws of ‘Quantum Mechanics’. In quantum mechanics the electrons find stable ‘orbits’ and ‘trajectories’ which balance the kinetic energy and potential energy. Loads of ‘Wow’ I am glad you had some ‘Wow’ moments. To be honest, I moments. I am was exhausted and fed up before I began last week’s actually starting to talk, but I too find this stuff fascinating and as I spoke, understand Y9 I too was reminded that “Wow! This stuff is amazing!” chemistry – 34 years At the age of 53 I believe – or is it hope? – that it is late! never too late Still with it so far…. Great! Week 2 is the most difficult week, but people Brilliant! usually catch up in Week 3 17 You said… What about anti protons and positrons? Q: What are quarks? I really enjoyed the experiments. Michael said… Good question. The list of ‘stuff’ particles that I gave: protons, neutrons and electrons, is sufficient to describe 99.99…etc% of the universe. But there are other particles, including a class of particles that I might call anti-stuff: anti-protons, antineutrons, and anti-electrons – also called positrons. To the best that we have been able to measure them, anti-particles have identical properties to their matching particle, but they have the opposite electric charge. When a particle meets its anti-particle, they annihilate each other and both particles are destroyed absolutely – and all that is left is typically – a flash of light. Just to be clear about this – this is astonishing! And investigations of why particles and anti-particles behave this way have given great insights into the fundamental nature of all particles and fields Quarks are particles even tinier than protons and neutrons and there are a whole family of 6 of them plus their anti-particles (see above) Electrons are – to the best of our knowledge – truly fundamental particles: no matter how hard we bash them together, we don’t see evidence of any internal structure. Protons are quite different. When we bash electrons against either protons or neutrons, we see evidence of internal structure – even tinier point like scattering centres within the protons and neutrons: these scattering centres we call quarks. (This is a bit like a souped-up version of Rutherford’s Gold Foil Experiment that I mentioned somewhere later in these notes.). Glad you enjoyed the experiments. 18 You said… Amazing, really funny. Has anyone ever travelled a light year at one time. See you next week. Michael said… I don’t think anyone one has ever travelled anywhere near one light year. The speed of light in a vacuum is approximately 186,000 miles per second. A light second is the distance light travels in one second – which is 186,000 miles. A light year is the distance light travels in one year – which is 186,000 miles x 31.5 million seconds which is 5.8 million million miles. To travel that distance in your lifetime (say 100 years) you would need to be constantly moving at 1/100th of the speed of light or about 1800 miles per second. Obviously – no one has done that. But… The Earth itself travels around the Sun at approximately 18.5 miles per second. And the Sun orbits the centre of the Milky – but I am not sure of the speed of that. However even taking that into consideration I doubt any of us ever manage to travel one light year in our lifetimes. Excellent presentation and brilliant explanation Glad you enjoyed it: Thank you. especially on measurement of length. Good reminder of Glad you enjoyed it. things learnt at Sorry we couldn’t read that word! But I am glad you school many years found it fascinating. ago! I don’t think it is possible theoretically or practically A fascinating look If we drained the electrons from your body (which at a subject once would be fatal ) this would make about a million thought of in a coulombs of electric charge. great ??????. If we moved this negative charge 1 metre away from the Very well left over positive charge, the force of attraction would presented. be approximately 1022 newtons – an unimaginably large Theoretically force. So I can’t really see how one would drain all of would it be existence of its electric charge. possible to drain If we had antiparticles for the whole universe we could or extract completely destroy it – and the charge in it. But as far electricity from as we know, it is not possible to create an antiparticle the whole universe without simultaneously creating a partner particle. to an alternative one (if it exists). 19 You said… Michael said… You mentioned the The Greeks did not ‘discover’ atoms experimentally, Greeks knew about the but by discussion they deduced that something like atom. How was it atoms (i.e. in-divisible particles from which discovered? everything is composed) must exist. What is the force that As I think I have mentioned above, it is the strong attracts the neutron to force which attracts the neutrons to protons. It is the proton? What is a short range force – essentially a contact force – the strong force? Does and its origin is in the interactions between the it exist outside quarks that make up each proton and each neutron. proton/neutron The force acts between all particles that contain relationship? quarks, i.e. between: You say everything is o protons and protons, electric, but you o protons and neutrons, and wouldn’t have atoms o neutrons and neutrons. without the strong Yes, the strong force is very important, and indeed force holding neutrons surprising details of the arrangements of neutrons and protons together – within nuclei are important in our lives – this is the so isn’t strong force subject of current negotiations between Iran and equally important? ‘the West’. However, we do not in general need to A very enjoyable know the details of the strong force to understand lecture! Thank you! the world we experience daily. Sorry for any confusion. Glad you enjoyed it. If light gets bent around stars and When light is bent around galaxies in galaxies, then how do we measure the process known as ‘gravitational the distance of the galaxies on the lensing’, it is usually by a very small edge of the universe. As we are not angle – tiny fractions of degree. So measuring a straight line. although its path is slightly I understand it’s the red shift lengthened, this is insignificant when which is measured, but is the red compared with the uncertainty shift we measure the true distance associated with measuring the of the galaxy and not just the line distances to galaxies. of light which has bent around the Does that make sense? galaxies. Balloon experiment. Yes, if we used the Van der Graaff - With a much larger charge, cannot generator for example we could easily move metallic objects be moved that are much larger objects much larger than paper? 20 You said… Was the universe designed by an unknown force (a God) or do you believe it is just evolution that created the entire world? How do electromagnetic waves travel through a vacuum? Why is science so mind boggling? Michael said… Mmmm. Interesting. I don’t know. Over the last few hundred years many processes which were believed to be ‘supernatural’ – for example rainbows, or many diseases - have been understood as being perfectly ‘natural’ processes which we can study and analyse and modify. So if you believed these to be works of God you find yourself in a tricky position, because acknowledging them as being ‘natural’ means changing your beliefs about God – something which is traditionally fraught with difficulty. Following this line it is possible to end up believing in a ‘God of the Gaps’ – i.e. a God which explains all the things science hasn’t explained yet. Personally I don’t feel the need to invoke the idea of God to explain things I don’t understand. I just say ”I don’t understand”! We will look at electromagnetic waves (literally!) this week. The key part to understand is that although a vacuum by definition has no matter – no ‘stuff’ - in it, it is not empty of fields – and these are real entities which are always present in space. Errr. Because the world in which we find ourselves is itself mindboggling in its beauty and complexity and size. From an evolutionary point of view, you could say we are coping pretty well for a group of evolved apes. 21 You said… Fantastic! I feel fully charged! Is this just a graphic representation Michael said… Terrible pun: well done It is a graphic representation. We know electrons do orbit nuclei – but atoms are so small that although we see atoms every day – that is what you are looking at now! - we never see this structure directly. There is a massive space between the electrons and the nucleus. This was deduced from one of the most famous experiments in Physics known as Rutherford’s gold foil experiment carried out in 1909 http://en.wikipedia.org/wiki/Geiger%E2%80%93Marsden_experiment Or is it what an atom really looks like? It feels like there is too much space between the nucleus and the electrons. If you pack all the atoms together into a solid, is there still enough room for the electrons to move around? Please explain. Ernest Rutherford and colleagues fired the nuclei of helium atoms (2 protons and 2 neutrons bound together and called an ‘alpha particle) at a thin gold foil. He observed the alpha particles by the flash of light they gave off when they hit a fluorescent screen. He observed that most alpha particles went straight through the foil completely undeflected. But that occassionaly, one or two would be deflected almost totally through 180° and end up going back in the direction they came from! It was from this experiment that we deduced the basic structure of atoms. Rutherford said It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge If we cannot see electrons and protons how do we know the electrons orbit the proton? Great question. I will explain that this week. The answer above explains how we deduced that protons and neutrons clumped together in the middle of an atom. We can also deduce details of the electron orbits by looking at the light that atoms emit. 22 You said… Michael said… How do super This is too complicated to explain here. But briefly, conductors work? superconductors are materials in which – when cooled Do electrons below a certain critical temperature – the electrical actually move down resistance of the material falls to a value a wire? indistinguishable from zero. Fire isn’t an Normally electrons in an electric current move through element? Right? If the material independently of each other 0 like millions not, what is it made of individuals jostling in a crowd. In the of (the flames)? superconducting state, the electrons move coherently and in synchronised motion. Even though the change is incredibly dramatic, the origin of this new mode of motion took scientists around 50 years to work out. Yes, electrons do move down a wire – but not very quickly. On average, for a typical flow of electric current, the flow speed is just a few millimetres per second. Fire – the flame that we see – is just hot gas and unburned particles – soot. o In ‘clean’ flames – such as from a gas burner – there is not much soot and even though the gas is at temperatures approaching 2000 °C, the atoms do not give off much light. o In sooty flames – such as a candle flame – unburned fuel particles (carbon) glow brightly and give off light and keep the flame temperature much lower – but still approaching 1000 °C. o Flames also contain a small amount of plasma – a fourth state of matter in which atoms and molecules are ionised. We discuss why hot stuff gives off light in week 3. 23 You said… Michael said… Is it OK to bring I don’t know. If we can’t tell the difference then another person if the I guess it doesn’t matter. named (child) cannot Yes the video was made by the Charles and Ray make it? Eames, who are quite interesting people for all Looking forward to kinds of reasons, but who were at heart designers showing my family the of chairs! video from 60s Good luck with the Fun Fly Sticks – I think I gave furniture couple and some web links somewhere above. You should be periodic table video. aware that the delicate floating foils are quite I am buying the fun fly expensive. Perhaps you could get children to make stick to share with them out of the metallised-plastic foil found pupils at my son’s around tea bags – or similar. schools. Glad you enjoyed it. Excellent and hands on. Why did you If I did, that was unintentional. In some materials some only rub the directions might be a little better, but the key things are balloon one that the material should be dry. way? (In one What an excellent observation. Imagine two nuclei – lets say direction on they are hydrogen nuclei i.e. just single protons. When they your shirt?) form a molecule (H2) the two electrons make what is called a Does create ‘co-valent’ bond i.e. they orbit both protons instead of just a better one. So in two-dimensions you can imagine their orbits as charge? being like a figure ‘8’. The key part of the orbit is the region What makes in between the protons. If the electrons spend time there 2 atoms join, then they will pull the two protons together. if the If the orbits are such that the electrons are in that region electrons at the same time, then the electrons repel each other and seek to avoid the molecule falls apart – these are called anti-bonding each other? orbitals. But if the electrons orbit so that they avoid each other, then there is very little electron-electron repulsion and the orbits successfully bond the two atoms together It’s all a bit more complicated in 3-dimensions. Terrific! Thank you. You are welcome 24