Tuesday, 12 May 2015

Potential Energy


In physics, the potential energy of an object is the energy which it possesses due to its position or its orientation with respect to a field of forces. [1] In the case of a system, the potential energy can depend by the arrangement of the elements that compose it. [2] It can be seen also as the potential energy capacity of an object (or system) to transform its energy into another form of energy, such as the kinetic energy.

The term "potential energy" was coined by Rankine in 1853. In the international system is measured in joules (J).

It is a scalar function of the coordinates of the object in the reference system used. Given a conservative vector field, the potential energy is its capacity to do work: the work relating to a force acting on an object is the line integral of the second kind of force evaluated on the path taken by the object, and if it is conservative value of this integral does not depend on the type of path followed. When it has to do with conservative forces can be defined as a scalar potential, which is sometimes made to coincide with the potential energy, defined in the whole space. In particular, from the mathematical point of view this potential exists only if the force is conservative, and the rest is assumed that for all the conservative forces can always define a potential energy physically.

The lightning Rod


The lightning rod is a device to attract and disperse atmospheric electrical discharges. It was invented by Benjamin Franklin, American physicist, and was applied for the first time successfully in Paris May 10, 1752.

To get to the lightning rod Franklin had made some important considerations on atmospheric electrical discharges, coming to establish that the damage caused by these were not due so much to their power, how much heat they generated on impact with any object. Also he found that when lightning, which is nothing more than an electrical discharge, striking an object, passes through only part: it was necessary then to think of something that would attract the lightning and disperse them through the force of a fixed course.

Discovering the peculiarities of metal spikes, especially those in gold, to attract the electrical discharges, thus acting as a magnet against the lightning, Franklin solved the biggest challenge: to capture him during his discharge.

 The lightning rod then consists in a long, thin metal rod coated with the tip of a noble metal (intrinsically devoid of surface layers of oxides and therefore high electrical conductivity) placed on the top of the building to be protected; from this it is derived a metal wire which is connected to earth: the electrical discharge is drawn from the tip and dispersed to the ground by means of wire. Moreover, the lightning rod, because of its shape, also has a preventive action against lightning. This consequently to the fact that the ground and the lightning conductor (connected to ground) are polarized by induction in response to the charge present on the bottom of the cloud. The lightning conductor so polarized, thanks to the dispersing power of the tips, helps to decrease the potential difference between the cloud and the ground, making it less likely that it reaches the minimum potential capable of beginning the download.

Due to the high currents and voltages that pass through it, the cable for the discharge of lightning can not be shielded. Generally copper, must have an adequate thickness to prevent current leakage into the surrounding space. For the same reasons, the link between the tip of the lightning rod and the grounding should be as short as possible, preferably in a straight line in particular avoiding sudden bends or turns that would increase the electrical impedance.

Sound Propagation


In a compressible fluid medium, usually air, the sound propagates as a pressure change created by the sound source. A speaker, for example, uses this mechanism. Compression spreads, but the air particles oscillate only a few micrometers around a stable position, in the same way that when a stone is thrown into water, the waves travel away from the point fall, but the water remains in the same place, it only move vertically and not follow the waves (a cap placed on the water stays in the same position without moving). In fluids, the sound wave is longitudinal, that is to say that the particles vibrate parallel to the wave traveling direction.

Solid, vibrating, can transmit a sound. The vibration propagates as in fluids, with a low oscillation of the atoms around their equilibrium position, resulting in a stress of the material, equivalent to the pressure in a fluid, but more difficult to measure. The rigidity of the material enables the wave transmission of transverse stresses.

Similarly, although to a lesser extent, the viscosity of a fluid may vary, especially in extreme conditions, the propagation equations derived for an ideal gas.

Propagation or sound célérité1 speed depends on the nature, temperature and pressure of the medium. In a perfect gas the sound propagation velocity is given by the equation:

c = \ frac {1} {\ sqrt {\ rho \ chi_S}} where \ \ rho is the density of gas and \, \ chi_S its isentropic compressibility.

It is seen that the sound propagation speed decreases

when the density of the gas increases (inertia effect)
when its compressibility (the ability to change the volume under the effect of pressure) increases.

In water, the speed of sound is 1482 m / s. In other settings, the vibrations can spread even faster. Thus in the steel, the vibrations they propagate 5,600 m / s to 5900 m / s. The sound does not travel in a vacuum, for lack of material whose vibration could spread into sound waves (sound insulation).