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2006-10-17T17:58:00.001+01:00

Ecology: challenges of globalisation

2006-05-16T22:12:00.000+01:00

Modern transport, particularly the volume and speed of air transport has facilitated the rapid migration of bacteria and viruses which cause diseases. One of the earliest examples is the infamous plague epidemics or "Black Death" which arrived in Europe along trade routes via the Middle East from the Orient. More recently, virulent strains of influenza and AIDS.

this also brings about A biological invasion of non-native plants spreading into our nations' fields, pastures, forests, wetlands and waterways, natural areas, and right-of-ways.

Economics: Debit card cashback

2006-05-16T00:33:00.000+01:00

Debit card cashback is a service offered to retail customers whereby an extra amount of money is added to the total purchase price of a transaction (paid by debit card) and the customer recieves the extra amount in cash along with their goods. For example, a customer purchasing £18.99 worth of goods might ask for twenty pounds cashback. They would pay a total of £38.99 (£18.99 + £20.00) with their debit card and receive £20 in cash along with their goods. Many customers find this a useful way to obtain cash, instead of making a separate trip to a cash machine.

The service is offered by both banks and merchant service providers in countries such as the United Kingdom, Belgium, Canada and The Netherlands because of the fee structures in use in these locales:

When accepting payment by debit card, merchants pay a fixed commission fee (as opposed to a percentage) to their bank or merchant services provider. (This is because the commission paid by the merchant for accepting debit cards, unlike credit cards, does not need to fund interest free credit or other incentives).

Accepting payments in cash can be costly for merchants, given that many British banks charge around 0.5% for depositing cash into a business bank account, along with the costs of transporting and insuring the cash.

The combination of these two points means that the retailer can save money by offering the cashback service. It does not cost the retailer more in commission to add cashback to a debit card purchase, but in the process of giving cashback, the retailer can "offload" cash which they would otherwise have to pay to deposit at the bank.

Merchants do not offer cashback on payments by credit card because they would pay a percentage commission of the additional cash amount to their bank or merchant services provider.

Some vendors enforce a minimum purchase amount or add a fixed fee when providing cashback to a customer. In many cases, retailers require customers to initial the cashback entry on the till receipt to confirm that they have received the cash. This system is used to prevent cashiers surreptitiously adding cashback amounts to a transaction and keeping the money for themselves (or accusations of same).

Cashback can be useful in many scenarios. In locations where there are no ATMs nearby, or nearby ATMs are out-of-order, a local retailer may be able to supply the required cash instead. Sometimes it is simply more convenient to combine the transactions at the retailer and ATM into a single cashback transaction with the retailer.

Cashback is particularly useful in pubs, where it is usually considered somewhat impolite to pay for each drink (or round, unless very large) with a card. Customers finding themselves without cash can make one payment by debit card, asking also for cash with which to pay for the remainder of the evening.

from: http://en.wikipedia.org/wiki/Debit_card_cashback

Physics: Loudness

2006-05-15T00:35:00.000+01:00

Loudness is not simply sound intensity!
Sound loudness is a subjective term describing the strength of the ear's perception of a sound. It is intimately related to sound intensity but can by no means be considered identical to intensity. The sound intensity must be factored by the ear's sensitivity to the particular frequencies contained in the sound. This is the kind of information contained in equal loudness curves for the human ear. It must also be considered that the ear's response to increasing sound intensity is a "power of ten" or logarithmic relationship. This is one of the motivations for using the decibel scale to measure sound intensity.

A general "rule of thumb" for loudness is that the power must be increased by about a factor of ten to sound twice as loud. To more realistically assess sound loudness, the ear's sensitivity curves are factored in to produce a phon scale for loudness. The factor of ten rule of thumb can then be used to produce the sone scale of loudness. In practical sound level measurement, filter contours such as the A, B, and C contours are used to make the measuring instrument more nearly approximate the ear.

"Rule of Thumb" for Loudness
A widely used "rule of thumb" for the loudness of a particular sound is that the sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud. A common way of stating it is that it takes 10 violins to sound twice as loud as one violin. Another way to state the rule is to say that the loudness doubles for every 10 phon increase in the sound loudness level. Although this rule is widely used, it must be emphasized that it is an approximate general statement based upon a great deal of investigation of average human hearing but it is not to be taken as a hard and fast rule.

Why is it that doubling the sound intensity to the ear does not produce a dramatic increase in loudness? We cannot give answers with complete confidence, but it appears that there are saturation effects. Nerve cells have maximum rates at which they can fire, and it appears that doubling the sound energy to the sensitive inner ear does not double the strength of the nerve signal to the brain. This is just a model, but it seems to correlate with the general observations which suggest that something like ten times the intensity is required to double the signal from the innner ear.

One difficulty with this "rule of thumb" for loudness is that it is applicable only to adding loudness for identical sounds. If a second sound is widely enough separated in frequency to be outside the critical band of the first, then this rule does not apply at all.

While not a precise rule even for the increase of the same sound, the rule has considerable utility along with the just noticeable difference in sound intensity when judging the significance of changes in sound level.

Adding Loudness
When one sound is produced and another sound is added, the increase in loudness perceived depends upon its frequency relation to the first sound. Insight into this process can be obtained from the place theory of pitch perception. If the second sound is widely separated in pitch from the first, then they do not compete for the same nerve endings on the basilar membrane of the inner ear. Adding a second sound of equal loudness yields a total sound about twice as loud. But if the two sounds are close together in frequency, within a critical band, then the saturation effects in the organ of Corti are such that the perceived combined loudness is only slightly greater than either sound alone. This is the condition which leads to the commonly used rule of thumb for loudness addition.

Critical Band
When two sounds of equal loudness when sounded separately are close together in pitch, their combined loudness when sounded together will be only slightly louder than one of them alone. They may be said to be in the same critical band where they are competing for the same nerve endings on the basilar membrane of the inner ear. According the the place theory of pitch perception, sounds of a given frequency will excite the nerve cells of the organ of Corti only at a specific place. The available receptors show saturation effects which lead to the general rule of thumb for loudness by limiting the increase in neural response.

If the two sounds are widely separated in pitch, the perceived loudness of the combined tones will be considerably greater because they do not overlap on the basilar membrane and compete for the same hair cells. The phenomenon of the critical band has been widely investigated.
Backus reports that this critical band is about 90 Hz wide for sounds below 200 Hz and increases to about 900 Hz for frequencies around 5000 Hertz. It is suggested that this corresponds to a roughly constant length on the basilar membrane of length about 1.2 mm and involving some 1300 hair cells. If the tones are far apart in frequency (not within a critical band), the combined sound may be perceived as twice as loud as one alone.

Critical Band Measurement
For low frequencies the critical band is about 90 Hz wide. For higher frequencies, it is between a whole tone and 1/3 octave wide.

from: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/loud.html

Chemistry: Fire

2006-05-13T18:46:00.000+01:00

Fuel and Oxygen

Most combustible fuels begin as solids, such as wood, wax, and plastic. Many fuels that people burn for energy, including gasoline and methane (natural gas), begin as either a liquid or a gas. Any fuel must be in a gaseous state (so that it can react with oxygen) before a fire can occur. Heat from the fire’s ignition source, and later from the fire itself, decomposes solid and liquid fuels, releasing flammable gases called volatiles. Some solids, such as the wax in a candle, melt into a liquid first. The liquid then evaporates, giving off volatiles that may then burn. Other solids, such as wood and cotton, decompose and evaporate directly. In a wood fire, gases given off by the decomposing wood enter the flame, combine with oxygen from the surrounding air, and ignite. The heat from the flame decomposes more wood, thus adding more flammable gases to the flame and creating a self-supporting process.

Most common fuels consist of compounds containing the elements carbon and hydrogen. Fuels often also contain oxygen, nitrogen, chlorine, and sulfur. Cellulose is the principle combustible compound in wood, paper, and cotton. It contains carbon, hydrogen, and oxygen. Plastics that burn, such as polyvinylchloride (PVC), polystyrene, polymethyl methacrylate (PMMA), nylon, and polyurethane, are composed mostly of carbon and hydrogen. Liquid fuels include oil and gasoline, while gaseous fuels include methane, propane, and hydrogen. All of these fuels (except pure hydrogen) contain both carbon and hydrogen.

The final requirement for a fire is a chemical chain reaction. The heat of the ignition source starts the reaction, and heat from the fire’s flame continues the reaction. The flame needs to heat the fuel and make it release enough flammable gases to continuously support the chemical reaction. A common example of combustion is the burning of wood. When an ignition source heats wood to a sufficient temperature, about 260°C (500°F), the cellulose in the wood decomposes, producing volatile gases and char. The average composition of the gases can be represented by the compound CH2O, where C stands for carbon, H stands for hydrogen, and O stands for oxygen. Under ideal conditions, CH2O reacts with oxygen in the air and produces carbon dioxide (CO2) and water vapor (H2O). In the real world conditions are not ideal, so fires often produce other products as well, such as carbon monoxide (CO) and soot.

Products of fire

1. Light and Heat
Once a material ignites, a flame forms. The flame consists of volatile gases moving upward, and it is the region in which the combustion reaction occurs. The gases in the flame move upward because they are hotter—and therefore lighter—than the surrounding air. The colors in the flame come from unburned carbon particles that glow and travel upward as the flame heats them.

The flame continues to burn as the volatile gases streaming from the fuel combine with oxygen from the surrounding air. Different parts of the flame have different temperatures. Most common fuels are compounds called hydrocarbons, and they produce about the same flame temperature, roughly 1200°C (2200°F). The maximum theoretical flame temperature for most hydrocarbons is about 1300°C (2400°F).

Different fuels produce varying amounts of heat. The rate at which a fire generates heat is equal to the burning rate of the fuel (measured in grams per second, or g/s) multiplied by the amount of heat produced by the combustion reaction. This second factor is called the effective heat of combustion, and scientists measure it in units of kilojoules per gram (kJ/g). When a gram of wood burns, for example, it produces 8 kJ of heat energy. Wood’s effective heat of combustion is therefore 8 kJ/g. Polyurethane’s effective heat of combustion is about 18 kJ/g. Polyurethane’s burning rate is also about twice that of wood under similar conditions. Multiplying the burning rates for these two substances by their effective heats of combustion, one finds that polyurethane fires produce heat at about 4.5 times the rate of wood fires under similar conditions.

Gases

Fires can produce a number of different gases, including some that are harmless and some that are toxic. Carbon dioxide (CO2) and water vapor (H2O) are two relatively harmless gases produced by fires. Toxic gases from fires include carbon monoxide (CO), hydrogen cyanide (HCN), sulfur dioxide (SO2), and hydrogen chloride (HCl).

from: http://encarta.msn.com/encyclopedia_761563809_3/Fire.html