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High – Pressure Physics

Posted on 12 April 2010 by yizhan

High-pressure physics, the study of the effect on matter of extraordinary pressure and techniques required to achieve and measure such pressure.  The development of high-pressure physics affords a striking example of how science, by continually building on its achievements of the past,  progress at continually accelerating rate.  The beginning of high-pressure physics as subject of serious scientific endeavor go back nearly two hundred years to experiments by Jon Canton (1718- -1772) published by the royal society of London  in which he established that water is measurably compressible, not incompressible as had been supposed.  The next important experiments  were not made for nearly fifty years.  After this the rate of the progress gradually picked up, and by the end of the nineteenth century there where important center of activities in England, France and Germany.  Early the twentieth century important activities spread to the United States, and now are being conducted in many laboratories in both academic and industrial all over the world, under an increasingly wide range of pressure and with increasingly important results.  We should bear in minds that pressure is defined as force per unit area, and is not the same as force itself.  A very high pressure maybe produced by a relatively small force if it is applied in a very small surface area.

Problem of Techniques. There are many difficulties in high-pressure experimenting, and the initial slowness of progress  was in large part due to the slowness with which the problems of techniques were solved. There are at least three types of this problems:  the  problem of  preventing leakage of the liquid in which pressure is generated and by which it is transmitted; the problem of preventing the vessel containing the pressure from bursting under the action of pressure; and the problem of accurately measuring the pressure and its various effects.  These problems have by now been solved to a large extent, and even on those case where present solutions are inadequate, the principles are understood on winch a more precise solution depends.

Effect of High-pressure. Extensive investigation of the effect of pressure on various physical properties of matter have been made by a writer in the years 1908 – 1956.  In the range up to 30,000 atmosphere it was posible to study nearly all the physical properties of many substances in the general classification of room temperature.  In the range of 30,000 up to 100,000 atmosphere and sometimes beyond, the measurements were more limited in scope, but it was possible to study the effect of pressure on the volume, electrical resistance, and phase changes of nearly all the elements and many other substances, all temperature within a few hundred degrees of room temperature.  Most of  the phase changes induce by pressure are reversible, so that when pressure is released, the material reverts to its original condition.  There are however , a few instance of irreversible changes produce by pressure.  One example is a  phosphorus, which maybe permanently altered by pressure a few tens of thousands of atmosphere, becoming black and conductor of electricity, with a density 50% greater than its original yellow form.

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Hydraulic's Principles

Posted on 11 April 2010 by yizhan

Most of the construction equipment today, uses Hydraulics.  Well for starters, the fundamentals of hydraulics is basically pressure and fluids, by applying pressure to a fluid from one point to another.

hydraulic log splitter, is an example of an equipment that uses the technology of hydraulics.  It is composed of a small 4 stroke engine and pumps that provide pressure to the process, a small tank use as storage of the fluids, a cylinder with a piston and a rod and a spool valve that control the push/pull of  the hydraulic cylinder.

Hydraulic press, is a machine   use to insert parts that needs to be forced fit such as bearings, bushings, sleeves and dowels.  Just like a log splitter they both use the same principles, by which the application of a small force in one point can produce a very large force in another point.  This device is consist of two piston fitting into two cylinder, the two cylinder are different in size, the first one is small and the other is larger in diameter.   The fluid inside is in communication with the two cylinders.  By the principles of  hydraulics, the intensity of pressure, or the pressure per unit area, such as pounds per square inch, is the same every point throughout the system which is at the same elevation.  In  addition, an increase of pressure in any part of a confined fluid causes  an equal increase throughout the liquid. tis statement  is also known as the Pascal’s Principle, after the French philosopher  and scientist , Blaise Pascal (1623-1662).  What ever difference there maybe in pressure due to difference in elevation of two piston is negligible compare to the pressure involve.

As illustrated in the figure below,  the accumulated force from the small piston would be F1 while the accumulated force from the large piston would be F2.  And the area of the small piston would be A1 and for the large piston A2.  By applying the force F1 to the small piston an intensity of pressure will be produced in the fluid which i F1/A1 , and same as the intensity from the large piston.

F1/A1 = F2/A2

But since A2 is much Larger than A1, it is then that F2 is larger than F1. v This is the principle of the hydraulics press that is widely used in the industry.

However, neglecting friction, the process done by F1 must be equal to the process done by F2, then if s represents distance moved by the pistons,    F1 s1 = F2 s2 or s2 = F1/F2 s1.   It is then that the large piston moves only a small distance than the small one.

Hydraulic ram, a device that uses the flow of water through a gradual incline to lift a small portion of the flow of the elevation usually  higher than the source.  The principle of this operation is like a water hammer.  Water is permitted to flow through a long drive pipe with a moderate drop in elevation.  When the flow reach its normal velocity, for the installation for the velocity head (kinetic energy per unit volume) of the water is sufficient to close a waste valve at the lower part of the pipe.   The inertia of the moving water and its relative incompressibility produce a water hammer- a momentary rise of pressure at the lower end of the pipe.  This pressure is sufficient to force a small part of the water through a discharge valve to a higher elevation.  The waste valve opens again, the flow in the pipes is re-established, and the cycle is repeated.

Unlike the log splitter and the hydraulic press, the hydraulic ram is only powered by the momentum of the moving water no prime mover or engine needed to complete its operation, but, the usefulness of this is only limited for special cases.

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