Monthly Archives: January 2014

Why does Aurora Borealis occur?

One of the most spectacular phenomena in nature is without doubt the amazing games of light, shades and colors called Aurora Borealis and Australis, depending on which of the two poles it is perceived at.


Those who live at the extreme north and south of Earth might at times experience this colored spectacular lights shimmering across the night sky. But what makes these lights  appear?

Well, it may sound weird but everything begins from the sun.

The temperature above its surface is millions of degrees Celsius. At this temperature, collisions between gas molecules are frequent and explosive. Free electrons and protons are thrown from the sun’s atmosphere by its rotation and escape through holes in the magnetic field. Blown towards the earth by the solar wind, the charged particles get in contact first of all with our planet’s magnetic field which may be thought as been generated by a giant rectangular calamite positioned at the centre of the Earth. The structure of a rectangular calamite’s magnetic field is well known and is based on closed field lines getting out of the south pole and entering the north one. Exactly the same happens on Earth where we have to imagine a giant magnetic shield protecting the whole planet surface, except for the source (south pole) and the pit (north pole) of the field lines which are necessariauroraly more exposed.

The charged particles scattered all around by solar wind are largely deflected by the earth’s magnetic field. In particular these charges are trapped by the force of the magnetic field and they start following the force lines being channeled either towards the south or the north pole.  Therefore some particles enter the earth’s atmosphere and collide with gas atoms or molecules at various heights. These collisions  excite gas particles causing them to light up. Sounds something similar to phosphorescence… 

What does it mean for an atom to be excited? Atoms consist of a central nucleus and a surrounding cloud of electrons encircling the nucleus at increasing distances from the centre. When charged particles from the sun strike atoms in Earth’s atmosphere, electrons move to higher-energy orbits, further away from the nucleus. Then when an electron moves back to a lower-energy orbit, in order to lose the amount of energy it has gained, it releases a particle of light or photon. The color of emitted light depends on the atom and on the size of the inner electron’s jump, but the result is absolutely amazing as it involves billions and billions of particles emitting light at the same time.

aurora3What happens in an aurora is similar to what occurs in the neon lights we see on many business signs. Electricity is used to excite the atoms in the neon gas within the glass tubes of a neon sign. That’s why these signs give off their brilliant colors. The aurora works on the same principle – but at a far more vast scale.

The aurora often appears as curtains of lights, but they can also be arcs or spirals, often following lines of force in Earth’s magnetic field. Most are green in color but sometimes you’ll see a hint of pink, and strong displays might also have red, violet and white colors. The lights typically are seen in the far north – the nations bordering the Arctic Ocean – Canada and Alaska, Scandinavian countries, Iceland, Greenland and Russia. And of course, the lights have a counterpart at Earth’s south polar regions.

The most common auroral color, a pale yellowish-green, is produced by oxygen molecules located about 60 miles above the earth. Rare, all-red auroras are produced by high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or purplish-red aurora.

Several fascinating  myths and legends are connected to the phenomenon of auroras.

aurora2In Finnish, the name for the aurora borealis is “Revontulet”, which literally translated means “Fox Fires.” The name comes from an ancient Finnish myth, a beast fable, in which the lights were caused by a magical fox sweeping his tail across the snow spraying it up into the sky. The Lapps, or the Saami, a people who are a close relative ‘race’ of the Finns, who live in Lapland — that is, north of the Arctic Circle, in what officially are Northern Finland, Sweden, and Norway — traditionally believed that the lights were the energies of the souls of the departed. In Norwegian folklore, the lights were the spirits of old maids dancing in the sky and waving.  Several of the Eskimo tribes also connected the lights with dancing. Eskimos in Eastern Greenland attributed the northern lights to the spirits of children who died at birth.

 

That’s it! Cool, isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti

Why do we slide on ice?

phase

 

Have you ever wondered why we are able to slide on ice? For example why can we sky or skate? Why do we slide on ice and not on other smooth surfaces?

Well, the reason is quite simple and it is completely contained in the above image representing the so called water’s phase diagram.

icewaterPhase diagrams show the preferred physical states of matter at different Temperatures (abscissa – °C) and Pressure (ordinate – bar). Within each phase, the material is uniform with respect to its chemical composition and physical state. At typical temperatures and pressures on Earth water is a liquid, but it becomes solid (ice) if its temperature is lowered below 0°C and gaseous ( water vapor) if its temperature is raised above 100°C, at the same pressure. Each line (phase line) on a phase diagram represents a phase boundary and gives the conditions when two phases may stably coexist in any relative proportions. Here, a slight change in temperature or pressure may cause the phases to abruptly change from one physical state to the other. Where three phase lines join, there is a ‘triple point’, when three phases stably coexist, but may abruptly and totally change into each other given a slight change in temperature or pressure. Under the singular conditions of temperature and pressure where liquid water, gaseous water and hexagonal ice stably coexist, there is a ‘triple point’ where both the boiling point of water and melting point of ice are equal.  A ‘critical point’ occurs at the end of a phase line where the properties of the two phases become indistinguishable from each other, for example when, under singular conditions of temperature and pressure, liquid water is hot enough and gaseous water is under sufficient pressure that their densities are identical. Critical points are usually found at the high temperature end of the liquid-gas phase line.

Analyzing a phase diagram it is generally possible to predict the thermodynamic behavior of the considered substance.

Water is a scientifically fundamental example of this kind of analysis. So, let’s think about what may happen on a skating rink. The temperature is obviously under 0°C. At this temperature and at the pressure of 1 bar the thermodynamically water stable phase is the solid one. There’s no doubt that there would be ice.

skierBut exactly when an hypothetical skater puts the blade of its runner over the surface of ice the situation changes. Or better the pressure conditions change. This pressure variation involves only the ice surface below the blade of the runner. Actually, the skater applies a pressure on the ground with its weight, determining a global pressure increase over the considered ice area.

Looking at the above water’s phase diagram, it is clear that if we increase the pressure the temperature of water solidification (temperature at which water is converted to ice) decreases under 0°C. The natural consequence is that the skater’s weight makes ice melt under the runner’s blade, as in that conditions of pressure and temperature the thermodynamically water stable phase is the liquid one. This means that, actually, the skater is not sliding over ice but over a thin layer of water between the blade and the below ice! That’s exactly what happens with a skier!

Cool isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti

Why does phosphorescence occur?

Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light? What about funny shirts or glasses which are visible at night despite darkness? Why does all this stuff happen?

Well the phenomenon which is behind this cool events is called phosphorescence and we’are about to get a little bit of insight on it!


To better understand what goes on behind the scenes when this phenomenon occurs we have to ask ourselves a simple but fundamental question. What does it happen when a material is exposed to light? Which, in a more basic form, could be reasked in the following way:  what does  happen when a molecule is exposed to light?

Nice one!

fosfo1Without getting into a too detailed analysis of the events we could simply answer the question in this way: the considered molecule absorbs the incident light. Or better, considering that a light beam consists in “little energetic packages” called photons, we should say that when a beam of photons bombs a material’s surface, the inner molecules absorb light in the form of “energetic particles”. The primary consequence of this absorption is that the molecules which were hit increase their internal energy. This energy, however, cannot be kept forever by the molecular system. In general it is quite immediately released by the molecule. This phenomenon is extremely fast and we could never appreciate it to the naked eye!

But let’s see a little bit more in detail what happens inside the molecule right after a photon absorption. There is a huge amount of extremely complex phenomena which are triggered by the absorption of light; all of them can only be explained using quantum mechanics.

fosfo3Nevertheless it is still possible to have an idea of what’s going on in the following way. We first have to accept that each molecule has only well defined accessible energy levels, which means that everything hitting the system won’t automatically be absorbed. We can imagine the reachable energy levels of a molecular system as a building’s several floors. We have also to imagine that these floors are connected one to the other by an internal lift, which lets us reach them from the bottom to the top, and that, in the meantime, we can only use the stairs to go down. That’s it? Absolutely not! There’s another complication. While going down we cannot necessarily access to each floors, as if we had a direct access from the fourth floor to the first one but in order to pass from the third to the second we found a closed keyless door. Forbidden transition there!

Our molecule can be compared to a young man living on the ground floor of this imaginary building and our absorbed photon as a sort of nutritional supplement giving the weak young man some energy to stand up and climb the building to higher floors!

Ok.. So, what does happen after a molecule has absorbed (the right amount of) energy?

Our young man can now stand up and, completely revitalized, takes the lift till the floor allowed by the amount of acquired energy. That’s exactly (more or less!) what happens to a molecular system. It absorbs a photon whose energy excites the molecule to defined level. And what about the energy release?

Our young man has to descend back to the ground floor in order to lose all he has acquired. That’s not easy at all because there is the probability for him to find a forbidden path from a floor to an other. A closed door. What then? Theoretically he should stop and stay there, hopeless. Practically he could, for instance, force the door and access the forbidden transition. Obviously it would not be so fast at all. He would need time to open a passage and finally crash the door. Probably plenty of time. But finally he would succeed and he’d be able to go back to where he began. The ground floor!

After this awesome little story we are able to answer the first real question. Ever wondered why the little stars glued on our rooms’ ceilings go on glowing after we’ve switched off the light?

fosfo2Here’s the answer: when we turn the light on, the stars begin absorbing energy. Or better the molecules inside the material start absorbing photons and get excited to a well defined molecular energy level. Immediately after they try to release this energy but it may happen that the system, attempting to go back to the ground floor, finds itself stuck at a particular energy level. Quantum mechanics reads that, in theory, there are some forbidden transitions. No way to pass! In practice, however, the molecule succeeds in forcing its passage to a lower molecular energetic level. Generally, this operation requires plenty of time, which means that our little star on the ceiling goes on glowing for minutes or hours after its first absorption.

When we switch off the room’s light we will be able to clearly see the phosphorescent star which is slowly releasing the absorbed energy  in the form of light! Phosphorescence! That’s it!

Cool isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti

Why does a microwave oven heat so quickly?

Have you ever wondered why we are able to cook food so fast in a microwave oven?
Why is it so convenient in terms of time to use it instead of a classic oven?

To discover what lies in the backend of a microwave oven let’s start from its main components. This kind of oven contains three most important devices: a vacuum tube called a magnetron, which generates the energy that heats food, a waveguide hidden in the wall, to direct energy to the food and a chamber that holds the food and safely contains the radiation. The real cool stuff which is behind this revolutionary device is exactly this last one: the microwave radiation! After having been generated by the magnetron it is channeled by the waveguide and finally scattered into the main chamber of the oven.

fieldFrom a physical point of view, as all the radiations, microwaves are nothing less than an oscillating electromagnetic field, the same as light, or radiowaves.

In principle a microwave does not heat differently than any other type of heat device; at a molecular level we are dealing about an energy transfer that results in an increased motion of the molecules and eventually in a rise in temperature. Its unique feature comes as follows: in a traditional oven we heat food by placing the it inside a radiated chamber with hot walls which cause the outside of the meal to raise in temperature. The inside of the food cooks by the heat transfer taking place from the hot surface to the inside. In contrast, energy from the magnetron penetrates into the food which means that all its mass can cook simultaneously. But how does he do this?

water

Well our food is generally filled with water which is a funny molecule positively charged at one end and negatively charged at the opposite one. To give this molecule an energy we expose it to the electromagnetic wave generated from the magnetron; this radiation stimulates simultaneously all the water molecules it encounters on its path. The typical microwave electromagnetic field oscillates 2.450.000.000 times per second. Water will try to allign with the oscillating radiation of the electric field, whose very fast variations rock the molecule back and forth rapidly. Well, imagine for a second a water molecule trying desperately to find a stable position in space being punched 2.450.000.000 times per second! That’s a big deal! The natural consequence is that the molecular friction creates heat and the frentic motion destroyes hydrogen bonds which bind molecules to its neighborhood. All this incredibly fast stuff is eventually translated into a progressive, rapid and homogeneous cooking of our food!

Cool, isn’t it?

GO BACK TO MR WHY!

by Francesco Pochetti