In the past, we’ve had students who were still dissatisfied with their physics grades joined us around this period. For some, it could be due to CCAs and other commitments, for others, it could be due to having to constantly new concepts before having reinforced older ones. It doesn’t help that in schools, lectures and tutorials are often not in sync so students are unable to clarify their doubts quickly.
However, now that most CCAs have stepped down and schools have more or less finished teaching the syllabus, students are able to concentrate on preparing thoroughly for their upcoming papers. Working hand in hand, we are delighted to have helped numerous students improve by leaps and bounds, many from Us before joining us to As in their final exams.
Through the coming months of physics revision, we will go through the most important concepts in each topics and ensure that students have a crystal clear understanding of how these can be applied. From experience, we will also clarify the common misconceptions students have and highlight the common pitfalls to look out for.
J2 Physics revision classes have started at Learners’ Lodge!.
In the intensive 8-lessons revision in June, we will go through the foundational topics of
- Dynamics, and
In addition, we will also cover the critical J1 topics of which often come out for structured questions:
- Thermal and
For the J1 June revision classes, we will be covering the following foundational topics:
- Work, Energy and Power.
Besides preparing students who have their mid-year exams in early July, these lessons also seek to ensure that students have a solid foundation before going on to the more difficult topics in semester 2.
Be prepared to be worked hard! After all, it is only after we have truly toiled will we savour the fruits of our labour. For more information about our intensive physics revision programme, feel free to contact us!
Whoever thought engineering is boring? 🙂
What happens when stars die?
In Chapter 20 Nuclear Physics, we learnt that stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.
When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.
If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star’s internal nuclear fires become increasingly unstable – sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.
Average Stars Become White Dwarfs For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf. White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers – why didn’t they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation. A phenomenon called electron degeneracy prevents further collapse. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down.
This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron degeneracy cannot support the core against further collapse. Such stars suffer a different fate as described below.
Supernovae Leave Behind Neutron Stars or Black Holes Stars over eight solar masses are destined to die in a titanic explosion called a supernova. In a supernova, the star’s core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion – fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses. In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions as stardust. On average, a supernova explosion occurs about once every hundred years in the typical galaxy. About 25 to 50 supernovae are discovered each year in other galaxies, but most are too far away to be seen without a telescope.
Neutron Stars. If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star. Neutron stars are incredibly dense – similar to the density of an atomic nucleus.
Black Holes If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly. Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material – often the outer layers of a companion star – is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion.
If you are interested in black holes, you can watch the video below (The Largest Black Holes in the Universe) for more information! However, you might encounter certain terms being used, so attached are brief explanations of some of the technical terms used.
|Quasar||A Quasar, or quasi-stellar radio source, is a very energetic and distant active galactic nucleus. They exhibit high redshift amount of electromagnetic energy, including radio waves and visible light. There is now scientific consensus that a quasar is a compact region in the center of a massive galaxy surrounding its central supermassive black hole.|
|Supernova||A supernova is a stella explosion that results when the production of energy at the core of a star via nuclear fusion is suddenly turned on or off.|
|Chandra X-ray observatory||A space based telescope, the third of NASA’s four great satellite observatories – the first was Hubble Space Telescope; second the Compton Gamma Ray Observatory, and last the Spitzer Splitzer Space Telescope. X-ray telescopes do not work on Earth as the Earth’s atmosphere absorbs the vast majority of X-rays.|
|Solar Mass||The solar mass, kg is the standard unit of mass in astronomy, used to indicate the masses of stars and other galaxies in relation to the mass of the Sun.|
|Accretion Disc||An accretion disc is a structure formed by diffuse material in orbital motion around a central body. The central body is typically a star. Gravity causes material in the disc to spiral inward towards the central body. Gravitational forces compress the material causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object. Accretion discs of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the x-ray part of the spectrum.|
|Event Horizon||In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman’s terms it is defined as “the point of no return” i.e. the point at which the gravitational pull becomes so great as to make escape impossible.|
Imagine walking into a room and all your electronic devices start charging instantaneously, without you having to fumble through numerous wires to find the correct one to plug into your devices. Sounds like something from a futuristic sci-fi movie? Well, this future is not too far off, thanks to a newly invented technology – Witricity.
Witricity, short for “wireless electricity” is a technology invented by physicists at MIT to allow wireless energy transfer over large distances. Current wireless charging methods employ induction plates on which you can place your devices such as handphones. As we learn in Electromagnetic Induction, a changing magnetic field induces a current in a nearby conducting wire. As such, the induction charging plate consists of a primary coil that generates a changing magnetic field. This oscillating field is then picked up a secondary coil in the device to be charged, which in turn induces a current to charge the device.
However, one major drawback of this method is that the device must be in close proximity to the induction plate, practically rendering the device immobile while it is being charged. One might wonder, what if we increase the strength of the magnetic field? Indeed, increasing the magnetic field might be a plausible solution, except that too strong a magnetic field can disable magnetic devices such as credit cards and hard disks!
How then can we send large magnetic fields through distances such that only the device(s) of interest pick up these magnetic fields? Fortunately, oscillations provide us with the answer. One might have seen a video where an opera singer is able to shatter a distant wine glass simply through her voice.
To understand this effect, we first have to appreciate that all objects have a natural frequency – the frequency that the object will vibrate at when given a disturbance. To find the natural frequency of the wine glass, we can simply tap it with a spoon and let it vibrate. When an external oscillator (in this case the opera singer) emits a wave that is of the same frequency as that of the natural frequency of the object (wine glass), resonance occurs. Maximum energy is then transferred from the external oscillator to the object.
Similarly, Witricity makes use of this phenomena. The devices to be charged have a high natural frequency that is matched by the frequency at which the magnetic field oscillates. This way, the magnetic field would be picked up only by the devices to be charged.
Image courtesy WiTricity via the IDTechEx Energy Harvesting Conference
By using resonant transfer, efficiency is increased. Furthermore, since there is no radiation produced, this wireless method of charging is safe to humans as well! One particular useful application of this wireless charging method is in pacemakers. Modern pacemakers used to regulate heartbeats require their batteries to be replaced every 7 to 8 years, meaning the patient has to undergo surgery. With Witricity, pacemakers can be charged while they are still in the patients’ bodies!
Visiting Norway to catch the aurora borealis has recently grown in popularity, and is making its way into the bucket list of many. From the encounters of those who have seen it, it is truly a magical experience! To these lucky few, no videos or pictures could do justice to the actual experience of being under the vast open night skies, lit ablaze by the tapestry of colours.
However, one might wonder, how did the aurora come about? As it turns out, the beautiful display of the aurora borealis is actually formed thanks to the Sun! Interestingly, despite the Sun not being present in the night sky, the effects of its invisible plasma still manifest in the aurora.
To understand how the aurora works, we must first start with the solar wind. This is a cloud of plasma, comprising of mainly electrons, which is being spewed out from the Sun constantly. Eventually, some of this solar wind finds itself in the Earth’s magnetic field, also called the magnetosphere. As the two interact, the magnetosphere is compressed on the day side, and is stretched into a long magnetotail much like the wake of a speedboat on the night side.
This compressing and stretching process stores electrons in the magnetotail, thus building up a potential difference between the tail and the poles. This is akin to the potential difference stored between a battery. Recalling chapter 12 on Electric Field, this will then cause the electrons to accelerate towards the Earth’s North and South poles as electric potential energy is converted to kinetic energy, much like how the potential difference in a battery causes the electrons to flow in the circuit, firing up electrical components along the way.
As we have learnt in Electromagnetism, the negatively-charged electrons will be deflected by the Earth’s magnetic field into a circular path. This combined acceleration towards the poles and circular path thus result in the electrons moving in a helical path (although it is not a perfect circle due to winds in the atmosphere). Finally, as the fast-moving electrons collide violently with the air particles, the kinetic energy of the electrons is lost and emitted as photons, or light energy*. As such, a helical ring of light is formed up in the sky. From the ground, what we see as resembling curtains of light rising up in the sky is actually part of a helix cascading towards the Earth. From an extraterrestrial vantage point, we can see this circular path clearly.
So, in future if you do get to see the aurora, remember that you are not alone. In fact, everyone who is under that ring of light can see the same magical display as well! 🙂
* Finally, we will learn how electrons colliding with gas atoms can convert kinetic energy to photons in a later chapter on Quantum Physics!