Monday, December 26, 2011

Energy Transformation

This is an important concept you must know, it is studied in the firsts courses or High school and University, but still a very hard topic to understand. the hardest course about energy i took was Thermodynamics.

Matter and Energy

The entire Universe is formed by matter and energy. The matter is relatively easy to distinguish: wood, plastic, water or steal are different examples of matter; but the energy is a harder thing to describe and define. In our everyday lives we rarely stumble upon pure energy, but with the effects of it over matter. The water of a mountain river has energy because it is capable of moving rocks, but when it gets to the plain and calms down it can no longer do anything: it has lost its energy. A new battery can light a bulb, but after hours of work it stops: it has consumed all the battery energy.

Even though we can only explain energy through its effects, we can measure it. We can know, for example, the amount of energy that there is in the water hold in a dam, in a gallon of gasoline or in a battery.
The light, from the Sun or from devices made by man, is one of the purest forms of energy. Its effects are present constantly in our lives. Today we can see how this type of energy is being used, with solar panels, the energy efficiency of these devices is still low but in the next years we can expect improvements on it.

One of the ways to recognize energy is by its capability to perform a work. A brick in the ground doesn’t have energy, but if we lift it, it has stored energy that will be released upon drop. To break a nut, you have to apply a force on it. If you put a nut in the ground and drop the brick, the nut will break. The work of breaking the nut has been performed by the brick with the energy you transmitted to it when you lifted it.

The water in a dam looks placid and calm, but it has an enormous amount of energy stored. When the gates open, the water goes out with extremely force. If we channel towards a turbine, we can take advantage to make clean energy, but if the dam breaks it can destroy everything on its path.

The solar energy hits the Earth in the form of light and heat; the plants use it for their growth. On this wheat field the solar energy is transformed into substances on the grains that are used as food.

Source: Ciencia Recreativa (1992) - Planeta Agostini 

Thursday, December 22, 2011

Density Facts

So in the end, in relation to the floatability of a body the most important thing is not the absolute density, but the relative density of the object and the fluid in which the object is immersed. For example, the wood is a dense object and heavy in the air, but it is not if its in the water, because the density of the water is bigger than the wood’s.

This concept is used in many aspects of our life. The most common gases used are fuels, butane, and natural gas, and the most common residue from combustion, the carbon dioxide, all are denser than air, therefore they do not float in it, but occupy the lowest part of their location. The vents on kitchens and other places where these gases are used or produced are located in the bottom part of doors and walls that lead outside to allow this dense gases to flow out. The vents located in the upper part allow the entrance of new fresh air.

Salty water

The dissolution of Salt in the water increases its density. This is the reason for the sea water has more density than river water. The difference is not that big, but enough to allow some objects with density almost the same as river water to float in sea water.

Couple of tourists in the Dead Sea. They are no sitting down on the bottom: they float because of the high salinity
The composition of sea water is not the same in every part of the world. In closed seas, or almost closed, and medium depth, as the Mediterranean, the Black Sea or the Dead Sea, the higher evaporation of the water produces bigger concentrations of salt and therefore increased density. The open and deep oceans like the Atlantic or Pacific have waters with less density.

Swimming in the Dead Sea

 An extreme case of high density caused by high salt concentrations is the Dead Sea. The dead Sea is, in reality, a salty lake of about 80 Km long and 17 Km wide, located in Israel and Jordan. Its depth is low, about 40 m average, and it changes a lot depending on the seasons. The low depth and the fact that its surface is located about 390 m under any other ocean and sea has favored the evaporation of the water for thousand of years, producing a high concentration of salt like any other sea. The density of these waters is high, so floating in them is quite easy. In fact the hard part in this sea is to drown, but at the same time these conditions have banished any kind of vegetal or animal life form in it.

Source: Ciencia Recreativa (1992) - Planeta Agostini 

Monday, December 19, 2011

Taking Advantage of Density

As we already know, the floatability of an object depends of the hydrostatic push of the fluid in which it is immersed. Objects which density is lower or equal to the density of the fluid will float.

Float in the water

Sometimes you would wonder, why all those big ships made of steel can float in the water? Even though the iron and steel are like 8 times denser than water, a boat can float because in its design, the major part of its volume it’s occupied by air, which is like nine hundred times less dense than water. Therefore the ship and the air contained inside of it, is less dense than water.


The submarines obey the same principle we have applied to ships, but they are able to increase their density to submerge and navigate under water, and decrease it when they want to float and sail in the surface.

The increase or decrease of the density of a submarine is accomplished by pumping water from and into the ship. The submarines have, distributed along its structure, tanks that are filled with water to increase the density of the ship to submerge, or air to make it float.

 Up: the submarine in the surface; Middle: as the tanks fill the submarine submerge; Down: emptying the tanks makes the submarine go up again.

Fish Swim bladder

Most of Fish balance their density with the water surrounding them with a special organ, called the Natatory Bladder or Swim Bladder, located in its abdomen, and in some cases connected to the digestive system. This Bladder is a bag of muscle tissue filled with a mix of gases.

This Bladder may assist the fish increasing or decreasing its density. By compressing the air which is contained in it, the fish descends in the water; and it rises again by relaxing these muscles which had served to compress the bladder. The gas density is less than the density of water.


Friday, December 16, 2011

Globes and Zeppelins

Everything I said about density and hydrostatic push is valid for all fluids, gas and liquid. Therefore it can also be applied to the air. The bodies immersed in air also experiment a push towards up that equals the weight of the displaced volume of air, but this push has a low value for objects with high density, as the human body for example, and its effects are not noticeable. This is not the case for low-density objects.

Some gases, like the hydrogen or helium, are less dense than the air. If we fill a bag of an impermeable material with any of these gases, the bag will experiment a push towards up superior to its weight and, if we set it free, it will ascend. This is the principle in which globes and zeppelins go by.

When we talk about the atmosphere, the lower layers of air close to the surface of the earth are denser, partially for the pressure of the superior layers.
We also need to know that the gas density decreases when the temperature of that gas increases, that is if it doesn’t find obstacles to increase volume.

Open and Closed Globes

The Globes that use the mentioned gases (Hydrogen, Helium) have to be necessarily closed, other way the gas would just escape and the globe would crash. But there are ways you can make an Open Globe fly. In this case the gas used to fill the Globe it is the same air, but increasing its temperature above the temperature of the surrounding air.

As long as the temperature of the air contained inside the globe it’s higher than the one outside of it, the globe will ascend, but there is a limitation. We said already that the air density in higher atmosphere layers is low, As the globe ascends, it will eventually find these high-density air layers and to keep ascending it will need to increase its temperature even further. Finally, in higher atmosphere layers, the density of the air is so low he globe won’t be able to ascend anymore no matter how much it increases the temperature of the air inside of it.

The Zeppelin

The Globes, open or closed, depend of the airstreams and wind to come and go. The zeppelin is a kind of closed Globe that has an engine attached to it that moves some propellers. This way it no longer depends on the wind. At the beginning the zeppelins were full of Hydrogen, but this gas is very flammable so it was replaced by Helium, although more expensive it was safer.

The last zeppelin dedicated to passenger transportation was the Hindenburg, of 247 m. of length and capable of lifting 22200 Kg. It was full with Hydrogen, and in May 6th 1937 it went on fire when it was landing on New York.
Source: Source: Ciencia Recreativa (1992) - Planeta Agostini

Wednesday, December 14, 2011

Everything Floats

Today I'm gonna introduce a physical concept that is used a lot in Chemistry. Like some physical properties, this one is hard to explain; I never had a clear concept on college and only understood it correctly in university.

Let’s use this example:

What weights more, a pound of rocks or a pound of feathers?


Both are the same, a pound is a pound, the difference is that to have a pound of rocks we would only need a few smaller rocks, but to get a pound of feathers we will need a lot of them.

The difference of the volume occupied by these 2 objects is because of the difference of DENSITY between them.

The density of a material is its weight divided by the volume occupied by that mass. The density of liquid water is 1, this means I liter of water weights 1 Kilogram, but the density of ice is 0.9 because 1 liter weights 900 grams. This is the explanation of why the ice floats in your drink.

Hydrostatic principle

If we tie a rock to a string and we immerse it into water we observe that the necessary effort to hold it is less than when it is in the air; it’s like if its weight was decreased. In the same way, if we try to sink a glass pushing its bottom into the water we will notice a big resistance.
These 2 phenomenons obey the same reason: Every liquid exert over the objects immersed in them a force in the contrary direction of the weight, called Push or Bouyancy.

Archimedes discovered that the push experimented by any object immerse in a liquid or a gas is equal to the weight of the fluid being displaced but in opposite direction.


Therefore, on any object immerse in a liquid act 2 forces: its own weight, directed to the center of the Earth (gravity) and the push force of the fluid, directed in the opposite direction. When the weight is bigger than the Push, the object sinks. When the push is equal to the weight, the object remains suspended in equilibrium without movement. And when the push is bigger than the weight, the object goes up to the surface until the weight of the immersed section equals the weight of the displaced fluid.

 Any object can float, as long as it is immersed on a substance more dense than itself, like this bronze object that is floating in Mercury. 

 Source: Ciencia Recreativa (1992) - Planeta Agostini

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