Rubber

August 11, 2011 by · Leave a Comment 

sci“Rubber” means the natural rubber which comes from the latex contained by some trees and other plants – as opposed to synthetic rubber (elastomer) which is generally an oil byproduct. In this description we use the terms elastomer and rubber as synonyms.

Elastomers are a class of materials which differ quite obviously from all other solid materials in that they can be stretched, easily and almost completely reversibly, to high extensions. An ordinary postal rubber band illustrates this behavior. It will generally be made from natural rubber, and can be stretched perhaps 600% (i.e. to seven times its original length), after which – before reaching its ultimate breaking elongation – it can be released and will rapidly recover to almost exactly the original length it had before stretching. The material is said to be elastic.

Most synthetic elastomers are not as elastic as natural rubber, but all can be stretched (or otherwise deformed) in a reversible manner to an extent which easily distinguishes them from all other solid materials. (n.b. a metal spring exhibits high reversible elasticity, but this is a feature of its wound shape. The actual metal itself of which the spring is made only deforms slightly, by twisting locally, at any particular point – nothing like the high deformations of which elastomers are capable.)

Elastomers are a special case of the wider group of materials known as polymers. Polymers are not made up of discrete compact molecules like most materials, but are made of long, flexible, chain-like or string-like, molecules. At this scale the inside of a piece of rubber can be thought of as resembling a pile of cooked spaghetti. In spaghetti, however, the chains, though intertwined, are all separate. But in most practical elastomers each chain will be joined together occasionally along its length to one or more nearby chains with just a very few chemical bridges, known as crosslinks. So the whole structure forms a coherent network which stops the chains from sliding past one another indefinitely – although leaving the long sections of chain between crosslinks free to move. The process by which crosslinks are added is known as vulcanization. To achieve vulcanization the raw rubber is mechanically mixed with a number of compounding ingredients carefully chosen to give the properties required for the particular application. The reason why elastomers behave as they do is associated with the type of molecular structure described above.

Against this background the reason why rubber can stretch so much is that, at normal temperatures, each long chain-like molecule (like any molecule) is in a constant state of agitation (thermal motion). For these flexible long-chain molecules the movement is considerable, and the molecule is agitated so much that it can take up a highly kinked shape. Because of this kinking, the distance between the two ends of the chain is very much less than its fully stretched length. This gives the rubber its flexibility. When a rubber band is stretched some of the highly kinked chains are simply being stretched out. Stretching can then continue until many of the chains are fully extended, or until the rubber breaks.

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The Pulse

July 14, 2011 by · Leave a Comment 

science 07-11-11Feeling the pulse is one of the hallmarks of the medical profession, and has been for many a century. As well as being informative, this action can give the doctor something physical to do while he takes time to think.

The pulse is most commonly felt where the radial artery lies near the surface on the thumb side of the wrist. It is made palpable by the ‘pulse pressure wave’ — initiated by each heart beat — reaching and expanding the artery. This wave is transmitted to the wrist at about 10 yards per second around forty times faster than the speed of the blood flow itself.

The information obtained from feeling the pulse is limited but important. The feel of the artery itself may suggest whether its wall has normal resilience, or is hardened and thickened by arteriosclerosis.

The pulse may feel, at one extreme, ‘strong’ and ‘full’ or, at the other, ‘weak’ or ‘thready’. These are indirect indications of the stroke volume of the heart. The impulse felt in the radial artery is related to the rise in arterial blood pressure generated by the heart at each beat — the pulse pressure. For any given stroke volume, this rise in pressure depends on the elasticity of the arteries: the more compliant they are the less the pressure rises; the stiffer they are with age and arteriosclerosis, the more sharply the pressure rises. These subtleties may be recognized by an experienced observer.

The rate may be faster or slower than normally expected in the circumstances. In healthy adults the rate at rest, although typically 60–70, can be anything from 40 per minute, say in an elite long-distance swimmer, to about 80 per minute. Even so the rate can, for example, be used to distinguish a simple faint (slow) from loss of consciousness caused by haemorrhage (fast).

The rhythm may be regular or irregular. In a person at rest an absolutely regular pulse is in fact unusual because of the phenomenon of respiratory sinus arrhythmia — an increase when breathing in and a decrease when breathing out.

An exaggerated sensation of the beating of the heart — palpitation — may or may not be associated with a faster than normal pulse rate; it is also a normal accompaniment of the increase in strength and rate of the heart-beat induced by strenuous exercise, or by the sympathetic nervous systems in stressful conditions, and can be a component of abnormal anxiety states.

Awareness of pulsation within ourselves, particularly when emotions are heightened — and even at the earliest in our mother’s womb — may well be inextricably related to the creation and appreciation of music.

In these areas, an artery passes close to the skin.

To measure the pulse at the wrist, place the index and middle finger over the underside of the opposite wrist, below the base of the thumb. Press firmly with flat fingers until you feel the pulse.

To measure the pulse on the neck, place the index and middle finger just to the side of the Adam’s apple, in the soft, hollow area. Press firmly until you locate the pulse.

Once you find the pulse, count the beats for 1 full minute, or for 30 seconds and multiply by 2. This will give the beats per minute.

To determine the resting heart rate, you must have been resting for at least 10 minutes. Take the exercise heart rate while you are exercising.

Measuring the pulse can give very important information about your health. Any change from normal heart rate can indicate a medical condition. Fast pulse may signal an infection or dehydration. In emergency situations, the pulse rate can help determine if the patient’s heart is pumping.

The pulse measurement has other uses as well. During exercise or immediately after exercise, the pulse rate can give information about your fitness level and health.

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