SciBlogs

Calculating pi with darts Marcus Wilson Jul 16

I love this one. Really, it’s maths not physics, but there is a bit of experimental physics creeping in at the fringes when the experimenters realize that the first method is biased. The second method is much better designed. 

Regrettably, pi-day (March 14th, 2015, or 3.14.15) only works if you use the US system of recording dates.  But fear not,  e-day (2nd July 2018, or  February 7th, 2018 if you’re American) isn’t so far away…

https://www.youtube.com/watch?v=M34TO71SKGk

A light puzzle Marcus Wilson Jul 13

Here’s a puzzling photograph that Hans Bachor showed me at the end of the NZ Institute of Physics conference last week. It comes from his public lecture on lasers a week ago. And we don’t have the answer to it, so maybe you can enlighten us (pun intended). 

laser-puzzle.jpeg

The photo is of a demonstration of total internal reflection with a laser. Hans is holding a container of water, which has a small hole at the bottom. Consequently there is a jet of water emerging. A laser is held up to the container, and with careful orientation it can be made to shine down the stream of water. The light follows the water, due to total internal reflection at the boundary between the water and the air (rather like a fibre-optic). Actually, it’s not TOTAL internal reflection – if it were we wouldn’t see the light escaping from the stream of water, but a great proportion of it is contained within the water stream. 

Now, in this case, Hans didn’t quite get the hole the right size and shape. Consequently the stream breaks up into discrete droplets, which you can see in the photograph. Now, here’s the puzzle. Look at the droplets and you can see that a couple of them are shining green – i.e. they appear to have laser light in them. 

But how does that work? Light moves so much faster than water one can consider the water to be ‘frozen’ in space as far as the light is concerned. While the laser light will happily travel along the water stream, when the stream breaks up into drops there is no total internal reflection anymore. The drops should not be glowing. Perhaps the light is jumping from drop to drop to drop. Unlikely – each drop will scatter the light considerably so that very little will jump from one drop to the next – let alone across many drops. 

As you think about this, you should bear in mind the conditions the photograph is taken over. It’s a flash photograph, but it’s likely that the shutter is open for longer than the flash illuminates the scence. This might (or might not) be significant, since the flash will capture the position of the water stream, but the shutter will still be letting in light from the laser even after the flash has stopped. So the capturing of the ‘green’ laser light in the photograph is not completely synchronized with the capturing of the rest of the image. 

Our best hypothesis is that the light that is that drops are illuminated directly by light that is emerging from the end of the stream – that is, the light leaves the stream, travels though the air, and hits a drop. In the spirit of Eugenia Etkina’s ISLE approach then, are there other hypotheses and what experiments can we formulate to test them?

NZIP2015 Highlights Marcus Wilson Jul 07

So the NZ Institute of Physics conference is in full swing. I have a bit of a break between the end of the last session and tonight’s conference dinner, so there’s time to give some highlights so far. 

Well, first, the low-light: Like the rest of my family and half of Hamilton I’ve had a horrible cold. On Sunday morning I was wondering whether I’d be able to make any of the conference. But I’ve managed to hold things together and now I’ve stopped sneezing I’m rather less infectious than I was at the weekend. So I’ve been able to get to some of the sessions. 

So what’s been going on? We’ve heard from Hans Bachor that after decades of international scientific research into getting lasers to work, the world’s first funding application for using lasers was for a ‘death ray’. Fortunately, applications have grown well beyond this one (which is still, thankfully, not in place) and far beyond the ideas of the original researchers (i.e. ‘blue sky’ research can have real value). We’ve seen edible fibre-optics (basically jelly), and heard from Jenni Adams about the ICE CUBE detector at the South Pole for detecting high-energy neutrinos. 

The speed talk session last night gave us a rapid-fire mix-and-match bag of physics research from across the country – from Kannan Ridings’ simulations of the melting of metal nanowire’s through to Inga Smith’s (unanswered) question of why do so few women do physics?  

But the real highlight for me has been Eugenia Etkina’s inspiring talk yesterday and workshop this morning, on physics laboratory experiments. The basic idea here is that experimental science is done by experts in a particular way (and she has evidence for this), including a cycle of observation, hypothesis, experimental design, prediction-making, experimental testing, then judgement. Experiments  by experts are done for particular reasons – either to observe, to test, or to apply. Give a group of scientists a practical problem and they will tackle it in a very systematic way, that usually allows them to get to the bottom of what’s happening. Give the same problem to first-year university students, and it’s a mess of hypothesis, tesing, judgement, observation all rolled into one. So it then makes sense for us to give students opportunities to carry out the same scientific processes as real scientists. Too often we give them a series of instructions to follow. This isn’t how real science works. It simply doesn’t help them learn science. 

At the end of her talk, Eugenia asked a very simple but really telling question. “How do you know that Newton’s third law is true?” My initial answer, to be honest, was: “because the text-books say so”. Not the answer of a scientist.  Thinking about it a bit more, I can say “because that’s what I experience…if I hit something hard it hurts…i.e. if I exert a large force on something it exerts a large force on me”. But here’s (roughly) what one of Eugenia’s students said when given the same question:

“I have carried out many independent tests of this law and have not found a single case where it is violated.” Now, that is the response of a real scientist. 

 

 

 

 

 

High-tech, Low-tech, planetary observations. Marcus Wilson Jul 01

First the low-tech:  The conjunction of Venus (the brighter one) and Jupiter as recorded by my very lousy cellphone camera  just after sunset yesterday. 

20150630_174530.jpg

Now the high-tech: A day before that Pluto occulted a star. It moved in front of the star, rather like an eclipse. The significance of the event was that it allowed Pluto’s atmosphere to be studied – by looking at the way the light moved through and around the atmosphere, various properties of the atmosphere can be inferred. The SOFIA project was in action, capturing the event, at the other end of the cost spectrum to my mobile phone. 

https://www.youtube.com/watch?v=_hXIeecp8oU&feature=youtu.be

There’ll be more Pluto excitement coming as the New Horizons probe flies closeby in just a couple of weeks.  

 

P.S. I should add in the conversation I had with my son (just turned 3) yesterday, after showing him the planets outside. 

Benjamin: “I don’t like planets”

Me: “Why not?”

Benjamin: “Because they’re quite noisy”

Me: “How are they noisy?”

Benjamin: “Because Grandad says they’re quite loud, actually.”

Umm…. Work that one out!

Tips on organizing a conference Marcus Wilson Jun 23

With the NZ Institute of Physics conference rapidly approaching, I thought I’d share my thoughts and experiences on how to organize a good conference. Or maybe on how not to organize a good conference. Time will tell.

1. Don’t try organizing a conference when you have 450 exam scripts to mark. 

2. Employ a professional conference organizer. They are worth their weight in free conference alcohol. 

3. Remember that participants at your conference will read ‘deadline’ as ‘guideline’. Don’t expect more than half your abstracts to come before the advertised deadline. While it’s tempting to reject abstracts that are late, the reality is that we can’t afford to do that. So it’s a good idea to work out when the deadline really is, and advertise it for two weeks before then. One could say that exacerbates the problem, because your contributers will learn to take your deadline (whenever you set it for) and add two weeks to it. 

4. Remember, that any ‘extended deadline’ will be read as being ‘extended guideline’. There will still be abstracts coming in well after the two-weeks extra that takes you up to where your deadline really should have been in the first place. And you can’t afford to say no. And they’ll be asking to be placed in premium oral slots, too. 

5. Inter- and intra-organizational politics is just as complex as parliamentary politics. 

6. Don’t be treasurer of the organizing body (in this case NZ Institute of Physics), a key conference organizer, and an employee of the host institution (in this case University of Waikato) all at the same time. Way too many conflicts of interest to manage all at once. 

7. All the above notwithstanding, the conference will probably go very well and the average attendee will have no knowledge of points 1-6 above. So don’t tell them. Whoops. 

And, finally, if you’re a member of the physics community, remember it’s not too late to register. And if you’re not, we have two excellent public lectures lined up for you on the evenings of Sunday 5th July and Monday 6th July. See you there!

 

The equation of time strikes again Marcus Wilson Jun 17

Some of us are rather looking forward to getting to 22 June. That’s when the days get longer again. Yes, the reality is that no-one’s really going to notice much difference for a while, but it’s encouraging to think that the days will be getting lighter again, if only by a little bit. Don’t confuse that with temperatures getting warmer – the coldest day (on average, of course) lags the darkest day quite considerably. Here it’s around the end of July

But there’s an interesting effect going on with sunrise and sunset. We’ve already had the darkest evening (hooray!) yet the darkest morning is still to come. Look at the sunrise and sunset times (for Hamilton) on the MetService website: today we’ve had sunrise at 7.32am and with cloudless skies the sun may stay out all the way to sunset at 5.07pm. But tomorrow sunset is recorded at 5.08pm (later!) and on Saturday sunrise has shifted to 7.33am (also later!). How can that be?

The point here is that the length of a day, meaning now the time between when the sun is at its highest to the next time the sun is at its highest, is only 24 hours on average (for some periods of the year its greater, for some periods is less), and isn’t equal to the time it takes for the earth to spin once on its axis. 

Let’s take this last point first. It’s solar midday, meaning that the sun is at its highest. Now, let the earth rotate exactly once on its axis. Do we get back to solar midday the next day? No. That’s because, in the time taken for the earth to rotate once, it has also moved along its orbit (about 1/365th of the way around). That means it’s got to spin a little bit more before the sun reaches its higherst point. The time to spin once (the siderial day) is about 23 hours 56 minutes – four minutes less than the mean solar day. Note that 4 minutes x 365 = 24 hours – which means one more revolution than you might expect  - the earth actually does 366 and a quarter revolutions each year. 

However, the movement along the earth’s orbit in a day is only on average 1/365th of the orbit. When the earth is closest to the sun (called perihelion – 3 January at present) it moves faster. That’s Kepler’s second law. When it’s further away (at this time of year) it moves slower. That would mean that in January, we should perceive that solar midday gets later every day by our watch (since the earth needs extra time to spin that extra bit more), but that in July, the solar midday should be getting earlier. However, that’s not what is observed. Our prediction for January is true, but for July it’s the other way around – solar midday actually gets later as measured by our watches. 

There’s another effect going on.  This is because the earth is tilted on its axis. However, it’s quite tricky to explain why that makes a difference.  Consider the transition from winter to summer, in the southern hemisphere. If we look at the position of the sun at sunrise and sunset, we see it move southward from one day to the next. What is significant is that at sunset the sun is further southward than for the previous sunrise. That gives us a shift in the measured time between solar midday and the next solar midday. A better explanation is given here.  This effect is ‘zero’ at the solstices and equinoxes, and does two cycles a year. Add this to the effect of Kepler’s second law, and we get the odd-looking curve that is called ‘the equation of time’, and means that, at present, each solar day is slightly longer than 24 hours, giving both ligher evenings and darker mornings. 

You can see a net result displayed on the ground under the sundial in Hamilton Gardens. The elongated figure-of-eight is called an ‘analemma’. It will show you the position of the tip of shadow of the pole at different times at different times of year.

 

 

 

 

Two great talks coming up in Hamilton Marcus Wilson Jun 11

As part of the forthcoming NZ Institute of Physics conference, and to celebrate the International Year of Light, we have arranged two fantastic and very different public talks for the evenings of Sunday 5th and Monday 6th July. 

First up we have Richard Easther, from the University of Auckland. In “Dawn’s Early Light” he’ll be talking about cosmology and the early universe. (Sunday 5th July, 7.30pm, Price Waterhouse Coopers Lecture Theatre, University of Waikato),

Then the following evening Hans Bachor, from the Australian National University will enlighten us (pun intended) on the everyday use of lasers. “Lasers are part of your life.” This will include some exciting laser demonstrations, all done safely. (Monday 6th July, 7:30pm, Concert Chamber, Gallagher Academy of Performing Arts, University of Waikato). Note the different venues for the two talks. 

So if you’re local to Hamilton, do come along and see (another pun) cutting-edge physics from those that do it. Both talks are free.

 

Why you should clean your heat pump filters (the physics) Marcus Wilson May 28

A couple of days ago I cleaned the filters in our heat pumps. What prompted me to do this wasn’t the cold weather, but the visible build up of dust on the casing of the indoor units. It looked horrible. On opening the unit up, it was clear that the filters were well overdue a clean. Eryughhh. But it doesn’t take long to do them, and in just a few minutes they’re back inside the pump and its throwing out warm, toasty air again. 

Aesthetics is just one reason to attend to the filter. The second is that dust clogs up moving parts, which means the fan and the louvre on the front. Getting rid of that dust has to be a good thing in terms of mechanical performance. 

But there’s also a third reason – one driven by physics. Your heat pump will be more efficient. How does that work?

The basic idea of the heat pump is that it takes heat out of the outside air and shifts it inside. It does it with an expansion-compression cycle, rather like a fridge. Although the air outside might be 0 degrees, it still has heat in it, which can be extracted and shifted inside. The result is that, outside, the air leaving the outdoor unit is lower in temperature than the air entering (to the extent that there isn’t a lot that will grow in front of an outdoor unit – event the most stubborn of weeds get frozen out of existence once winter starts), while, inside, the air leaving he indoor unit is of higher temperature than the air entering. Hence the indoor temperature rises. 

But pumping heat from something cold to something warm comes at a cost. It’s not the natural way that heat will flow. The bigger the temperature gap between indoors and outdoors, the harder it is to pump that heat. That means more power usage in the form of electricity. Heat pumps work really well for small temperature differences (e.g. the outdoor air is 15 C and you want to heat the house to 18 C) but not so well for large differences (e.g. -5 C to 18 C). The unit may still work at -15 C, but it’s less efficient – you’ll be getting fewer kWh of heat for every kWh of electricty. 

What has that got to do with the filter? Well, a dust-clogged filter starts restricting the air-flow through the indoor unit. That means there is less volume of air passing the heating element every second, to take away the heat.  If the heating element is still putting out the same amount of heat as before,  it means that it must get hotter. It’s rather like the fan on a car radiator. The fan doesn’t stop the car engine producing heat, but by increasing the air flow it brings the temperature down.  So a clogged filter means that the heating element inside the indoor unit is going to run hotter, if it’s putting out the same amount of heat.  That’s bad, since it means the heat pump now has a larger temperature difference to pump heat over, and therefore is less efficient. 

(I’m sure the reality is complicated by the control systems that heat-pumps use – so rather than running hotter it may simply pump less heat – but you don’t want that either if you want to heat your home.)

So, cleaning those heat pump filters is a good idea, for a good physics reason. 

Lenz’s law – at 3 tesla Marcus Wilson May 20

When I was at school, and introduced to magnetic fields in a quantitative sense (that is, with a strength attached to it), I remember being told that the S.I. unit of magnetic flux density (B-field) is the tesla, and that 1 tesla is an extremely high B-field indeed. Ha! Not any more. Last Friday night  I got to see a MRI machine in action – at Midland MRI at Waikato Hospital – this particular one is a 3 tesla affair. One of my PhD students was making some measurements with it. It needed to be at night – such is the demand for MRI scans we’d never get to play with it during the day. But well worth extending my day’s work for. 

Now, what does 3 tesla do? First you are advised to check pockets very carefully and remove keys and the like. No pacemakers? Good. Now enter the room. Interestingly, I didn’t really ‘feel’ anything until very close to the machine – then there was just a hint of something slightly ‘odd’. Things a little tingly, but nothing really significant. 

Two events, however, confirmed that there was a sizeable field indeed. First, my belt unbuckled by itself. That prompted a quick retreat outside to take that off, before bits started flying through the air. Then our host demonstrated what 3 tesla does to a sheet of aluminium. 

It’s important to remember that alumunium is not ferromagnetic. It is not attracted by a magnet. But it is, most certainly, very conductive. When a conductor moves through a magnetic field, electric currents are induced. These in turn generate magnetic fields, which are such that they oppose the movement. This is Lenz’s law. Consequently there is a force felt by the conductor that opposes its motion. And at 3 tesla, that’s some force. You can stand the sheet of alumnium on its end. Normally, you’d expect it to fall over, pretty quickly. But not at 3 tesla, it doesn’t. Very, very slowly, it topples, taking several seconds to move from vertical to horizontal. I could feel the effect of Lenz’s law by trying to flip the sheet over. It was like trying to turn a rapidly spinning gyroscope. Pretty impressive stuff. 

You can see a movie of this experiment (not ours, I should add), here. https://www.youtube.com/watch?v=JNUVfmy-iqM 

Special relativity – not so easy to grasp Marcus Wilson May 18

I’ve just given a couple of lectures on special relativity to a class of first years. It’s clear that grasping the key ideas is going to take some time. The results are so far removed from everyday experience that there is a certain air of bewilderment in the classroom. Here’s an example of what I mean. 

Suppose two students are travelling on skateboards, both at 10 km/h, but heading towards each other. In the frame of reference of one, what is the velocity of the other? 

The answer is simple: Add up the speeds – one sees the other coming at a speed of 20 km/h. 

Now make the skateboards a little quicker. To be precise, make them both travel at 0.8 times the speed of light. Now what does one of the skateboarders experience?

Our immediate reaction might be to say 0.8 + 0.8 = 1.6  -they see the other approaching at 1.6 times the speed of light. But that would be wrong. At high velocities, it just doesn’t work that way. The correct answer would be (from the Lorentz addition formulae) 0.98 times the speed of light. That is hard to grasp. There are at least two reasons. First, we never experience skateboards going at 0.8 times the speed of light, so the question is not physically meaningful. Secondly, it is so far removed from our physical experience it just doesn’t make any intuitive sense. 

One can do the correct relativistic calculation on our first example – two skateboarders each heading at 10 km/h (or 9.26 billionths of the speed of light). This time we end up with 19.999 999 999 999 998 3 km/h. It is little wonder that calling it 20 km/h is an approximation that works for us! Putting it in context – if we travelled at this speed for an hour (thereby covering 19.999 999 999 999 998 3 km), we would be about 2 picometres short of 20 km. That’s something much less that the size of an atom (but rather larger than the size of a nucleus). Little wonder we get away with calling it 20 km without any trouble. 

A consequence of special relativity is that time and space are ‘relative’ – meaning that different observers will disagree on the time between two events, and the distance between two events. This is measurable – put an atomic clock on an aircraft and one on the ground, and fly the plane around for a few hours. On landing, the two clocks will be different, showing that time has been experienced (very slightly) differently. 

There is, however, one very readily measurable consequence of relativity – one for which we are all familiar. That is magnetism. Magnetic fields and electric fields are part of the same entitiy. Just as observers will disagree on the time and distance between two events, so two different observers will disagree on the strengths of magnetic and electric fields in a system. A magnetic field becomes an electric field to a different observer. The reason we experience magnetic fields at all is down to the extreme neutrality of matter – the number of electrons and protons in a sample of material being incredibly well balanced. I didn’t try to explain that one – but it makes a nice bit of analysis for third-years. 

 

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