Yearly Archives: 2011

Japan’s atmosphere heated before earthquake struck

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For a long time there have been reports of “earthquake lights”, aurora-like lights or colours that appear in the sky during, or just before, earthquakes. Their existence is not widely accepted, but a group of US and Russian researchers claim to have discovered a different earthquake-related atmospheric effect.

After the March 11 quake in Japan the researchers looked at atmospheric data from the days before the quake struck, and their data appears to show a noticeable heating effect in the atmosphere beginning three days before the quake.*

Red lines indicate tectonic fault lines and the star indicates the location of the March 11 earthquake.

The article claims that this heating is due to a complex mechanism called the lithosphere-atmosphere-ionosphere coupling mechanism, in which the lithosphere (the rocky crust of the Earth), the atmosphere (the layers of gas surrounding the Earth) and the ionosphere (a layer of the atmosphere that contains particles that have been ionised by the Sun, where aurora occur) interact with each other.

They posit that small movements of the crust that occurred before the earthquake released radioactive radon gas (created by the radioactive decay of uranium and thorium in the crust) that was trapped there. This radon gas (either Rn-220 or Rn-222) is a highly-ionising alpha emitter. The researchers propose that the radon creates ionised particles in the air that cause water molecules to condense out of their vapour state. This condensation process releases energy, causing the surrounding atmosphere to increase in temperature.

Dimitar Ouzounov et al. 2011. “Atmosphere-Ionosphere Response to the M9 Tohoku Earthquake Revealed by Joined Satellite and Ground Observations. Preliminary results” arXiv:geo-ph/1105.2841v1

* Not everyone agrees with their data.

How to hide a Nobel medal

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In the run up to World War II Niels Bohr’s Institute in Copenhagen had become a refuge for Jewish physicists. The Jewish physicist James Franck and the anti-Nazi physicist Max von Laue, concerned for their gold Nobel Prize medals, both left them there for safekeeping. When Hitler’s army invaded Denmark, Bohr was worried that the invading German soldiers would steal the medals but his friend, the Hungarian physicist George de Hevesy, came up with an ingenious solution.

de Hevesy dissolved both medals in aqua regia, a highly corrosive 1:3 mixture of nitric acid and hydrochloric acid. This formed a transparent yellow liquid, chloroauric acid, that he hid in plain site, simply leaving the jar on a shelf in the Niels Bohr Institute.

When he returned in 1945, after World War II had ended, the jar was still there. The gold was recovered from solution and the medals were restruck by the Nobel Foundation and returned to their rightful owners.

Namaqua chameleon

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The Namaqua chameleon (Chamaeleo namaquensis) is really good at physics, even though he probably doesn’t realise it.

During the morning, when the Namib Desert in which it lives is colder, the chameleon turns its skin black so that it absorbs sunlight more efficiently and heats up quickly.

They can even split their colouring along their spines, white on one side and black on the other; absorbing heat on the black side whilst not radiating it on the white side.

The clip is taken from the BBC David Attenborough series Life.

Boiling point and pressure

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A vapour is created when a substance forms a gas at a temperature below its boiling point; the vapour pressure of a liquid is the pressure of this vapour. A liquid boils when the vapour pressure of the liquid is equal to the atmospheric pressure of the air above it.

You can therefore boil a liquid in two ways: by heating it so that the vapour pressure increases to match the atmospheric pressure; or by decreasing the atmospheric pressure until it matches the vapour pressure at whatever the ambient temperature is.

In this video you can see me boiling water at room temperature: a beaker full of water at room temperature is placed in a vacuum chamber and the pressure lowered until it boils – the pressure gauge is on the right of the picture.

Notice how I can put my finger in the water both before and after boiling without scalding myself. It’s said that it’s impossible to make a good cup of tea at the summit of Mount Everest because water boils at only 70.4°C and this isn’t hot enough to properly brew tea.

Exam Technique in Physics

Download printable version (29.9 kB .PDF)Creative Commons Licence

Basics

  • Don’t panic.
  • Give your best answer; don’t give more than one answer if only one answer is asked for and make sure that you are answering the question being asked, not the one you want to answer.
  • Words like ‘calculate’, ‘define’ and ‘state’ have specific meanings; follow these instructions.
  • If a calculation question asks you to ‘show’ something (e.g. “Show that the speed of the car is 30 m/s.”) and you can’t get the correct answer, use the answer from the question, not your incorrect answer, in any follow-up questions.
  • If you don’t know what to do, do something. Never leave a blank space.
  • Don’t make your answers overly long. If a question has space for four lines, don’t be afraid to write five or six; but eight or ten is too many.
  • Bullet points are your friend when answering longer-answer questions. A three-mark question needs three bullet points, a four-mark question needs four, and so on.
  • Use common sense: if you calculate the speed of a car as a million metres per second or the mass of a person as 800 kg then you have made a mistake.
  • Be specific, e.g. nuclear power plants “create nuclear waste” rather than “create pollution”; “air resistance” wastes energy rather than “friction”.
  • If you have two questions left to answer but only time to answer one, then do the first half of both; questions generally get harder as you work through them.
  • Don’t use terms you don’t understand or that you haven’t actually learnt about, e.g. inertia.
  • Don’t go for complicated risky answers – stick with the most basic, simplest, guaranteed-to-be-correct answer, e.g. the risk from nuclear power is “dangerous waste” not “visual pollution”.
  • Don’t be afraid to use spare space on the exam paper for a sketch diagram, this can be very helpful in thinking a problem through.

    • Calculations

    • Remember the three-step process:
      • Write down the equation you are going to use. If the “standard” form of the equation (e.g. F=ma) needs to be rearranged (e.g. a=F/m) then write down both forms, in case you rearrange the equation incorrectly.
      • Substitute the quantities from the question into your formula and carry out the calculation.
      • Write down the answer and don’t forget to use the correct unit.
    • Always work in standard units: convert centimetres and kilometres to metres, and hours and minutes to seconds before you start answering the question.
    • Watch for multipliers and submultipliers: you must be familiar with centi-, milli-, micro-, kilo-  and mega-.
    • Use standard form for answers with large numbers of zeroes. It is very easy to accidentally add or remove a zero when writing out 0.000000831 but not in 8.31×10−7.
    • Make sure you know how to use your calculator, particularly how to use the calculator’s built in memory and the reciprocal function. Make sure that when doing trigonometry (sin, cos and tan) your calculator is set in degrees mode and not in radians or gradians.
    • Don’t write too many significant figures in your answer: three significant figures is almost certainly enough. But make sure that during calculations you use the full answer – storing it in the memory or using the ‘previous answer’ key – to avoid rounding errors.

    Corrections

    • Make sure that an examiner can tell which answer is the one you want marked. Cross incorrect work through clearly and make it obvious what your answer is; underlining in a different colour or writing “Answer =” is a good idea.
    • Do not use double-headed arrows to swap answers around; examiners may ignore them.

    Graphs

    • When drawing a graph make sure you use the majority of the grid space available.
    • Always think about the features of the graph: what do the axis intercepts, the gradient of the line and the area underneath the line mean – what is their physical significance?
    • When answering graph questions be specific about what you are doing: state “area under graph” or “gradient of line” rather than just showing the calculations themselves.
    • When finding the gradient of a line make sure that you use a large portion (at least two-thirds) of the line to create your rise-run triangle.
    • Read the axes carefully: is the scale in newtons or kilonewtons, seconds or milliseconds?

    Language

    • Use scientific language: “volume is constant” is a much better answer than “container is something that cannot expand”.
    • Don’t write “probably” or “possibly” or “about” or “quite”; don’t write “approximately” unless that’s really what you mean.
    • Match adjectives to quantities, e.g. “long wavelength” rather than “high wavelength” and “high frequency” rather than “large frequency”.
    • Don’t begin an answer with “It” or “Because”, these can create ambiguity in your answer. “The car accelerates …” is much better than “It accelerates …”.
    • To paraphrase Einstein: make your answers as simple as possible, but no simpler. There is no requirement for every answer to be a full sentence, but one word answers will often not get you all the available marks.
    • Don’t waffle. Read your longer-answer question answers to yourself in your head – do they make sense?
    • Don’t use exclamation marks. Nothing in an exam requires an exclamation mark.