English Text (Science)

Topic 1

DEFIBRILLATOR 

            One device in which capacitors have an important role is the defibrillator. Up to 360 J is stored in the electric field of a large capacitor in a defibrillator when it is fully charged. The defibrillator can deliver all this energy to a patient in about 2 ms. (this is roughly equivalent to 3000 times the power delivered to a 60-W lightbulb!) Under the proper conditions, the defibrillator can be used to stop cardiac fibrillation (random contractions) in heart attack victims. When fibrillation occurs, the heart produces a rapid, irregular pattern of beats. A fast discharge of energy through the heart can return the organ to its normal beat pattern. Emergency medical teams use portable defibrillator that contain batteries capable of charging a capacitor to a high voltage. (the circuitry actually permits the capacitor to be charged to a much higher voltage than that of the battery). The stored energy is released through the heart by conducting electrodes, called paddles, that are placed on both sides of the victim’s chest. The paramedics must wait between applications of the energy due to the time necessary for the capacitors to become fully charged. In this case and others (e.g., camera flash units and lasers used for fusion experiments), capacitors serve as energy reservoirs which can be slowly charged and then discharged quickly to provide large amounts of energy in short pulse.

A camera’s flash unit also uses a capacitor, although the total amount of energy stored is much less than that stored in a defibrillator. After the flash unit’s capacitor is charged, tripping the camera’s shutter causes the stored energy to be sent through a special lightbulb that briefly illuminates the subject being photographed.

Topic 2

Xerography and Laser Printers

            The basic idea of xerography was developed by Chester Carlson, who was granted a patent for the xerographic process in 1940. The unique feature of this process is the use of a photoconductive material to form an image. (A photoconductor is a material that is a poor electrical conductor in the dark but becomes a good electrical conductor when exposed to light).

            The xerographic process is illustrated in figure 25.31. First, the surface of a plate or drum that has been coated with a thin film of photoconductive material (usually selenium or some compound of selenium) is given a positive electrostatic charge in the dark. An image of the page to be copied is then focused by a lens onto the charged surface. The photoconducting surface becomes conducting only in areas where light strikes it. In these areas, the light produces charge carries in the photoconductor that move the positive charge off the drum. However, positive charges remain on those areas of the photoconductor not exposed to light, leaving a latent image of the object in the form of a positive surface charge distribution.

Next, a negatively charged powder called a toner is dusted onto the photoconducting surface. The charged powder adheres only to those areas of the surface that contain the positively charged image. At this point, the image becomes visible. The toner (and hence the image) is then transferred to the surface of a sheet of positively charged paper.

Finally, the toner is “fixed” to the surface of the paper as the toner melts while passing through high temperature rollers. This results in a permanent copy of the original.

A laser printer operates by the same principle, with the exception that a computer-directed laser beam is used to illuminate the photoconductor instead of a lens. 

 

Topic 3

GALILEO GALILEI

            Galileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the scientific revolution. His achievements include the first systematic studies of uniformly accelerated motion, improvements to telescope and consequent astronomical observations, and support for Copernicanism. Galileo’s empirical work was a significant break from the abstract Aristotelian approach of his time. Galileo has been called the “father of modern observational astronomy”, “the father of modern physics”, and the “father of modern science”.

            One of the most common examples of uniformly accelerated motion is that of an object allowed to fall freely near the earth’s surface. It was widely believed until the time of Galileo that heavier objects fall faster than lighter objects and that the speed of fall is proportional to how heavy the object is.

            Aristotle believed heavier objects fall to the ground faster than lighter objects. As legend has it, Galileo dropped two objects from the Leaning Tower of Pisa to disprove Aristotle theory. These objects were the same size, but different weights. They fell to the ground at the same speed. This disproved Aristotle’s theory and led Galileo to discover the Law of Falling Bodies.

            To support his claim that falling objects increase in speed as they fall, Galileo made use of a clever argument: a heavy stone dropped from a height of 2 m will drive a stake into the ground much further than will the same stone dropped from a height of only 0,2 m. Clearly, the stone must be moving faster in the former case.

            As we saw, Galileo also claimed that all object, light or heavy, fall with the same acceleration, at least in the absence of air. If you hold a piece of paper horizontally in one hand and a heavier object in the other and release them at the same time, the heavier object will reach the ground first. But if you repeat the experiment, this time crumpling the paper into a small wad, you will find that the two objects reach the floor at nearly the same time.   

            

Topic 4                

Use of Sound by Animal

            Human use sound to communicate, of course, as do many other animals. But some animals have refined the use of sound in specialized ways. In 1793, Italian physiologist Lazzaro Spallanzani did some experiments with bats and established that they use sound to locate their prey. He took bats that lived in the cathedral tower in Pavia, blinded them, and then turned them loose. Weeks later, those bats had fresh insects in their stomachs, proving that they didn’t locate food by sight. Similar experiments with bats that were make deaf, however, showed that they could neither fly nor locate insects.

            Today, we understand that bats navigate by emitting high-pitched sound waves and then listening for the reflection of those waves off of other objects. By measuring in the time it takes for a pulse of sound waves to go out, be reflected, and come back, the bat can determine the distance to surrounding objects, particularly the flying insects that make up its diet. Typically, a bat can detect the presence of an insect up to 10 meters away.

            In an interesting application of the principle of natural selection, some species of moths have developed sophisticated sense organs to hear the sound emitted by bats. Using ears on their thorax or abdomen, these moths can hear the high-pitched sounds emitted by bats and thus can tell when they are being “seen”. When they hear the sound, the moths take immediate evasive action. In few cases, moths have developed an even more sophisticated defense. When a bat approaches, they emit a series of high-pitched clicks that “jam” the bat’s detection system.

            At the opposite end of the sound spectrum, when confronted by very low frequency sounds, we often don’t so much hear sound waves as feel them. We sense the vibrations in our bodies. You may have experienced this sensation when hearing very low notes an organ. Some animals (elephants, for example) routinely use sound in the 20-40 Hz range to communicate with each other over long distance. The mating call of the female elephant, for example, is experienced as a vibration by humans, but attracts bull elephants from many miles away.

            Whales, dolphins, and porpoises use low frequency sound echoes as a navigation tool in the ocean, much as bats do in air. Sometimes, however, the sounds that they emit are in the audible range for humans. Perhaps the most famous examples of sophisticated use of sound by animals are songs of humpback whales, which have appeared on a number of commercial recordings. The functions of these songs are not clear. It appears, however, that all of the whales in wide area of ocean (the south Atlantic, for example) sing the same song, although some individuals may leave out parts. The songs change every year, but the whales in a given area change their songs together.

Topic 5

Animal Insulation : Fur and Feathers

            Houses aren’t the only place where insulation can be seen in our world. Two kinds of animals-birds and mammals-maintain a constant body temperature despite the temperature of their surroundings, and both have evolved methods to control the flow of heat into and out of their bodies. Part of these strategies involves the use of insulating materials    furs, feathers, and fat    that serve to slow down the heat flow. Because most of the time an animal’s body is warmer than environment. The most common situation is one in which the insulation works to keep heat in.

            Whales, walruses, and seals are examples of animals that have thick layers of fat to insulate them from the cold arctic waters in which they swim. Fat is a poor conductor of heat and plays much the same role in their bodies as the fiberglass insulation in your attic. Feathers are another kind of insulation; in fact, many biologists suspect that feathers evolved first as a kind of insulation to help birds maintain their body temperature, and only later were adapted for flight. Feathers are made of light, hollow tubes connected to each other by an array of small interlocking spikes. They have some insulating properties themselves, but their main effect comes from the fact that they trap air next to the body, and, as we have pointed out, stationary air is a rather good insulator. Birds often react to extreme cold by contracting muscles in their skin so that the feathers fluff out. This has the effect of increasing the thickness (and hence the insulating power) of the layer of trapped air. (incidentally, birds need insulation more than we do because their normal body temperature is 41^oC or 106^oF).

            Hair (or fur) is actually made up of dead cells similar to those in the outer layer of skin. Like feathers, hair serves as an insulator in its own right and traps a layer of air near the body. In some animals (for example, polar bears), the insulating power of the hair is increased because each hair contains tiny bubbles of trapped air. The reflection of light from these bubbles makes polar bear fur appear white       the strands of hair are actually translucent.

            Hair grows from follicles in the skin, and small muscles allow animals to make their hair stand up to increase its insulating power. Human beings, who evolved in a warm climate, have lost much of their body hair as well as the ability to make most of it stand up. We have a reminder of our mammalian nature, however, in the phenomenon of “goose-bumps,” which is the attempt by muscles in the skin to make the hair stand up.