Answers to Last Issue's Do You Know?

1. What is the difference between a jet and a plane?

Answer: First thing, every jet is an aeroplane, but there are planes which are not jets. So it is like the difference between a square and a rectangle. 
The word jet refers to the ``jet engines'' inside these particular type of planes. Non-jet planes may be driven by propellers at the front of the plane, and some are not powered at all, they just glide!
Mostly, jet engines are found at the bottom of the jet planes, and they use propulsion: they suck in air, then mix the air with a special type of fuel that makes the air explode in such a way that the exploding air can only exit from the back of the jet engine. This exerts a really strong force pushes the plane forward. The turbochargers used in jet engines can use even the very thin air present in the upper atmosphere for combustion.
Why should one want jet engines? One reason is that jets can travel much faster than propeller planes, up to and beyond the speed of sound. Another reason is that they can travel at higher altitudes. Propellers require dense air to engage their spinning blades, whereas jets can work with thin air. Flying higher allows planes to avoid turbulence and also increases the number of aircraft in the skies since they can operate at different altitudes.

2. What kind of chemical reaction is photosynthesis ?
Answer: Photosynthesis is a chemical change process, that can be summarized in a single chemical equation, but that equation is actually the sum total of a collection of chemical reactions. In words: 
carbon dioxide + water + light -> glucose + oxygen
More quantitatively:
6CO2 + 6H2O (+ Light) --> C6H12O6 + 6O2
Although the chemical equation appears straight forward the process actually involves several "steps" occurring in two major groups of reactions: the light reactions and the dark reactions.
As light energy from the sun (in the form of photons) reaches a plant, chlorophyll molecules forming a light harvesting complex absorb that energy, exciting electrons. These electrons move along an electron transport chain, eventually transferring their energy into the bonds of special molecules called ATP and NADPH. These act as highly charged energy carriers ready to provide energy to continue photosynthesis in the dark reactions.
Using the energy of these molecules and others (including water and carbon dioxide), a carbohydrate called glucose is formed. Each chlorophyll molecule replaces its lost electron with an electron from water; this process essentially splits water molecules to produce oxygen.
Thus photosynthesis not only drives the carbon cycle, it also creates the oxygen necessary for respiring organisms. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth. 

3. Angles are so easy to understand in degrees. What are radians and why do we need to understand angles in radians ?
Answer: This is such a reasonable question that it is rather surprising that it is never clearly answered in school. The message always is: ``Learn radians because they make math easier.'' But why?
We all seem to find `degrees' natural: at least 30 degrees, 45 degrees and 90 degrees. But where do degrees come from?
Like most things in mathematics and science, the answer lies in astronomy. Ancient civilizations used astronomy to mark the seasons, predict the future, and appease the gods. They looked up at the sky, and realised that constellations make a circle every day. If you look at the same time every day (midnight), you realise that the constellations also make a circle throughout the year. Thus, every day, the constellations they were off by a tiny bit ("a degree"). Since a year has about 360 days, they decided that a circle had 360 degrees.
Please do not complain that it should really have been 365.242199 degrees! 360 is a nice number, with many factors: 2, 3, 4, 6, 10, 12, 15, 30, 45, 60, 90, 120, 180. It fits nicely into the Babylonian base-60 number system. Basing mathematics on the (apparent) movement of the sun is quite reasonable.
Thus, a degree is the amount you, as an observer, need to tilt your head to observe movement in a circle. Instead we could be looking at the distance travelled by the mover within that time; this would give us radians.
Degrees measure angles by how far we tilted our heads. Radians measure angles by distance traveled.
But then absolute distance is not very useful, since it depends on how large the circle is. So we divide by the radius to get a "normalized angle".
Thus angle in radians (theta) is arc length (s) divided by radius (r). If we go all around the circle, that is 360 degrees, we are travelling a distance of 2 * pi * r (the circumference!) and thus we get: 360 degrees in radians = (2 * pi * r) / r, which is 2 * pi radians. Therefore 180 degrees = pi radians.
Why are radians better for doing math ? Consider a giant truck with each wheel having a radius of 2 metres and you want to see how fast the wheel is turning and learn how fast the truck is moving. If it is turning 2000 degrees per second, it means 2000/360 rotations per second, which means a distance of 2 * pi * (5/9). It is a little messy. On the other hand, if we say it is turning 6 radians a second, we simply scale by the `real radius' to get 5 * 2 = 12 metres per second. This is so much better!
When you get to calculus, radians make life enormously easier, and expressions like (sin x / x) make sense. I suppose by now you have figured out that (sin x/ x = 1 for small x when x is measured in radians!

4. What causes a volcano to erupt ? Can we predict this accurately ?
Answer: We think of the earth as solid rock, but you know that there is an upper mantle and a lower crust. Interestingly a part of these can melt, and when that happens, something called magma forms. A volcano is essentially an opening or a vent through which this magma erupts. Along with it the dissolved gases it contains also escape. 
What triggers a volcanic eruption? There are many factors, but three are important: the buoyancy of the magma, the pressure from the dissolved gases in the magma and the injection of a new batch of magma into an already filled magma chamber. 
Buoyancy: As rock inside the earth melts, its mass remains the same while its volume increases. This produces a "melt" that is less dense than the surrounding rock. This lighter magma then rises towards the surface by virtue of its buoyancy. If the density of the magma between the place where it is generated and the surface is less than that of the surrounding and overlying rocks, the magma reaches the surface and erupts.
Gases: Magmas contains many dissolved volatile elements such as water, sulfur dioxide and carbon dioxide. Experiments have shown that the amount of a dissolved gas in magma (its solubility) at atmospheric pressure is zero, but rises with increasing pressure. For example, in magma saturated with water and six kilometers below the surface, about 5 percent of its weight is dissolved water. As this magma moves toward the surface, the solubility of the water in the magma decreases, and so the excess water separates from the magma in the form of bubbles. As the magma moves closer to the surface, more and more water escapes from the magma, thereby increasing the gas/magma ratio in the path. When the volume of bubbles reaches about 75 percent, the magma disintegrates into partially molten and solid fragments and erupts explosively.
Injection: When a `chamber' already filled with magma and some more magma comes in, it pushes some of the magma in the chamber to move up through the pathway and erupt at the surface.
Determining the timing of an eruption in a monitored volcano is very difficult, and scientists cannot also tell how large the eruption can be. They keep watching for earthquakes, changes in ground formation nearby and gas emissions. Moving magma shakes the ground and hence measuring seismic activity helps. Gas and magma can push the slope of the volcano upwards, and this can be measured by "tilt-meters". Measuring gas emissions can be done at site, but these days, remote sensing satellites are used.

Sources: Scientific American, Jefferson Lab