Monthly Archives: July 2014

Is the Portuguese Man o’ War an Organism?

man-o-war

Despite its appearance, the Portuguese Man o’ War is not a jellyfish.

It is technically known as a siphonophore, and although it might appear to be one single organism, a man o’ war is actually a colony of four smaller individual organisms known as zooids, which could not survive outside of the colony.

The most noticeable feature of a man o’ war is the “sail” or pneumatophore, a gas-filled balloon filled with carbon monoxide generated by the man o’ war’s gas gland, and nitrogen, oxygen and argon from the atmosphere. The sail allows the man o’ war to trail it’s tentacles through surface water, allowing it to feed, and can be deflated in case of attack from the surface, allowing the man o’ war to escape by submerging itself temporarily.

man-o-war

The sail is the clear “bag” at the top of the image, and the dactylzooids are the bluish tentacles below.

The remainder of a man o’ war is composed of three groups of zooids: the gastrozooids, the gonozooids and the dactylzooids. The dactylzooids make up the familiar tentacles, up to ten metres long, that trail through the water and which are covered in venom-filled nematocysts that are responsible for the tentacles’ highly painful stinging effect. The tentacles capture and guide food to the gastrozooids, which ingest and digest it. The gonozooids are responsible for reproduction.

What is the Point of a Pulley?

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In its simplest implementation, a pulley simply turns a force in one direction into a force in another direction. This might be useful if it’s easier to apply a force in one direction than the other (e.g. to pull downwards rather than push upwards).

A pulley really comes into its own when it is combined with another pulley to create a system known as a block and tackle, allegedly invented by Archimedes in the third century BC.

The simplest block and tackle, the gun tackle, uses two pulleys, which are usually mounted above one another (for clarity, they are shown separated in the diagram below).


 gun-tackle

The load (W) is shared between the tension in the rope and the mount that attaches the block and tackle to the ceiling, and thus you only have to pull with a force equal to half the weight in order to lift it. However, you will have to pull the rope twice as far and thus the conservation of energy is not violated (lifting a one hundred newton weight through one metre is the same amount of work as applying a fifty newton force over two metres).

The Luff tackle uses three pulleys in theory, but this is usually accomplished by running a rope over the top pulley twice. In this case the force required to lift a weight is reduced to one-third of its actual value, but again you have to pull further: three times the height required in this case.luff-tackle

A Luff tackle, shown separated.

The double tackle uses four pulleys, but similar to the Luff tackle, this is usually accomplished by running the rope twice over both pulleys.

double-tackle

In theory, this process can be continued indefinitely (one is reminded of Archimedes’ alleged remark: “Give me a place to stand and I will move the Earth“) but it quickly becomes impractical to do so, as the length of rope required, and the distance pulled through become unmanageable.

block-tackle-eg

Source [PDF]

A block and tackle can also be created using pulleys of different sizes.

Why Can’t We Get to Absolute Zero?

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The temperature of a substance is a measure of the average kinetic energy of the particles in that substance. As the average kinetic energy of the particles increases (i.e. they move faster), the temperature of that substance increases.

Some of the particles in a very hot substance will be moving slower than some of the particles in a very cold substance, but the average speed of the particles in a hot substance will be faster than the average speed of the particles in a cold substance. The number of particles at each speed in a gas is governed by something called a Maxwell-Boltzmann distribution, and is shown for air in the graph below:

air-maxwell-boltzmann

The average speed for particles of air* at 0°C is around 400 metres per second, for air at 100°C it is about 460?m/s and for air at 1000°C it is about 860?m/s. (Note also that at 1000°C there is far more variation in the speeds of particles than for air at 0°C.) At absolute zero, the coldest possible temperature, particles have a minimum of kinetic energy, and therefore the lowest possible speed. (They cannot have a kinetic energy of zero and actually be stationary because of something called degeneracy pressure.)

So why can’t we cool something all the way to absolute zero?

For something to cool down, it has to lose thermal energy. In order to lose thermal energy, this thermal energy has to go somewhere, and thermal energy only ever moves from hot to cold.** For example: a warm can of drink placed into a cold fridge loses thermal energy to its surroundings until it reaches the same temperature as the fridge’s interior.

Therefore, in order to bring something to absolute zero it would have to be surrounded by something that is colder than absolute zero, and this is impossible: hence you cannot achieve a temperature of absolute zero.

The closest we’ve ever got to absolute zero is less than 100 picokelvin, or 100 trillionths of a degree above absolute zero at the Low Temperature Lab at the University of Aalto in Finland. Interestingly though, it would be possible for something to feel colder than absolute zero due to wind chill.

* Obviously air is made up of different gases travelling at different speeds, so this is an average, weighted by the masses and prevalences of the different gases that make up air.

** It would actually be more accurate to say that the net movement of thermal energy is always from hot to cold. Some energy may go from cold to hot, but more will always go in the opposite “direction”.

Desalination

Desalination is the removal of salt(s) from seawater to create fresh water for drinking and irrigation. This is obviously very important on ships and submarines, but is also important on a national level: Israel produces fifty percent of its water via desalination, and the world’s largest desalination plant, the Jebel Ali Desalination Plant in the United Arab Emirates, produces 636 million litres of drinkable water per day.

jebel-ali-desalination

The Jebel Ali MSF desalination plant

There are two primary methods of desalination: distillation and reverse osmosis.

Distillation is the simplest method of desalination: seawater is boiled, and the water boils away as steam and is then collected and condensed back into liquid water, leaving the salt behind. The most common distillation method is Multi-Stage Flash (MSF) Distillation, which operates by feeding seawater through a series of chambers, each at a lower pressure than the first. The low pressure reduces the water’s boiling point (thereby saving energy) and as water reaches each stage it immediately boils (“flashes”) into steam. This steam is collected by condensers and the heat given off in this process (i.e. the latent heat of vaporisation) is used to pre-heat the seawater entering the chambers. MSF distillation uses around 50-90 megajoules of energy total per cubic metre of water produced.

Other distillation methods of desalination include mechanical vapour-compression, in which steam is mechanically compressed into liquid water, thereby generating heat that can be used to generate more steam; and multi-effect distillation (MED) in which steam produced in in one stage (known as an “effect”) is used to boil water into steam for the next effect, reusing energy that would otherwise be wasted. In MED, like MSF, each effect has a lower temperature and pressure than the previous one, so the inevitable decrease in temperature due to lost energy does not affect the boiling of water into steam.

reverse-osmosis-plant

The interior of a large reverse osmosis desalination plant.

Reverse osmosis is the most commonly used method of desalination. In normal osmosis, water moves from an area of low solute (low-salt) concentration to an area of high solute (high-salt) concentration through a semi-permeable membrane along a concentration gradient, thus equalising the concentration on either side of the membrane. (The membrane is constructed so that it allows water to pass through it, but not the solute.) In reverse osmosis high pressures (in the region of five megapascals) are used to overcome the osmotic pressure, forcing water to go from a high-salt concentration to a low-salt one. This is different from filtration, because filtration operates by size-exclusion (a difference in size), whereas reverse osmosis relies on a difference in concentration. Reverse osmosis desalination uses around 11-20 megajoules of energy per cubic metre of water.

Reverse osmosis is only one of a number of membrane desalination processes. Other membrane processes include electrodialysis reversal, which uses an electric current to push salt ions through ion exchange membranes*; and nanofiltration, which uses nanometre-sized (0.1?nm-1.0?nm) filters to remove salts.

* If electrical current is used to move salt ions, then the movement of salt ions can create an electrical current. This is the basis of reversed electrodialysis, a method of electricity generation using the difference in salt concentration between saltwater and fresh water patented in 1977 by Sidney Loeb at Ben Gurion university in Israel. Loeb was also the inventor of a semi-permeable membrane that made reverse osmosis desalination practical.