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Power Electronics is the application of solid-state electronics for the control and conversion of electric power.

Power electronic converters can be found wherever there is a need to modify a form of electrical energy (i.e. change its voltage, current or frequency). The power range of these converters is from some milliwatts (as in a mobile phone) to hundreds of megawatts (e.g. in a HVDC transmission system). With "classical" electronics, electrical currents and voltage are used to carry information, whereas with power electronics, they carry power. Thus, the main metric of power electronics becomes the efficiency. The first very high power electronic devices were mercury arc valves. In modern systems the conversion is performed with semiconductor switching devices such as diodes, thyristors and transistors. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry the most common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts.

The power conversion systems can be classified according to the type of the input and output power

  • AC to DC (rectifier)
  • DC to AC (inverter)
  • DC to DC (DC to DC converter)
  • AC to AC (AC to AC converter)

Introduction to physics equation power:

It is not a strange in physics to define what ‘power’ is. The power is defined to as the energy converted or the rate at which the work is performed.

Consider ΔW as the work done and Δt as the time at which the work is done, then the average power can be calculated as

pavg= ΔW/ Δt

The average power or pavg can also be called as just ‘power’. So the average power is the average of work done in unit time. The limiting value of average power is called as the instantaneous power. The work done to produce the constant power under a unit time can be calculated by using the equation

W=PT

Power has got different units depending on the situation it is being used. The SI unit is watt (W). It can be joules per second also. Other units are the ergs per seconds or the horsepower.

electricity power

Equation of Power in Mechanics

In mechanics the work done is defined using the equation

W=F. Δd

Where F is the force and Δd is the displacement. It is the force acting on a object and the displacement of the object.

Power in Electronics and Optics

In electronics:

It is the instantaneous electrical power of a component and is written as

P(t)= I(t). V(t)

Where P(t) is denoted as power in watts or in joules per second. I(t) is denoted as the current in amperes and V(t) is denoted as the potential difference across the components in volts. In the case of a resistor the power is written to as

P=V2/R

Where R=V/I which is the resistance in ohms.

In optics:

In optics the power is defined to as the average rate of energy transport in a electromagnetic radiation and it is measured in watts. The term power is also used to define the ability of a lens to focus on to light and it can also be defined as the focal length of the lens.

Introduction to scanning electron microscope image

The scanning electron microscope image lacks the resolving power obtainable with the transmission electron microscope image but has the advantage of revealing a striking three dimensional picture. The surface topography of a scanning electron microscope image can be revealed with a clarity and a depth of field not possible by any other method. Scanning electron microscope image is about 15,000 to 20,000 more magnified. For producing scanning electron microscope image, the source of illumination is an invisible beam of electrons which are generated by heating a metal filament at high voltage in a vacuum tube. Three types of electromagnets are used similar to glass lenses of compound microscope-condenser, objective, projector.

Scanning Electron Microscopy

In scanning electron microscopy the specimen is subjected to a narrow electron beam which rapidly moves over(scan) the surface of the specimen. This causes the release of a shower of secondary electrons and other types of radiations from the specimen surface. The intensity of these secondary electrons depends on the shape and the chemical composition of the irradiated object. The secondary electrons are collected by a detector which generates an electronic signal. These signals are then scanned in the manner of a television system to produce an image on a cathode ray tube.

Features for Producing a Scanning Electron Microscope Image

Some of the features for producing a scanning electron microscope image are as follows:

♣ The source of light is a beam of electrons.

♣ High power electromagnets are used for magnification.

♣ The apparatus is enclosed in a vacuum chamber.

♣ The specimen must be completely dry.

♣ The specimen used as objective must be ultrathin.

♣ Contrast is achieved by the use of heavy metals, like lead acetate and phosphotungstate.

♣ The scanning electron microscope image cannot be seen directly by eye, instead a fluorescent screen or photographic film is employed.

♣ Scanning electron microscope image is formed due to selective scattering of electrons by different molecular components of the object.

♣ An electric current of high voltage is used to generate a beam of electrons.

Distinction between Wave and Particle:

A particle occupies a well-defined position in space, which cannot be simultaneously occupied by another particle. If there is more than one particle in a given region of space, then their sum is equal to the number of individual particles. The sum can neither be more nor less. On the other hand, a wave is spread out in space. Two or more waves can co-exist in the same region. When two waves are present together, the resultant wave can be larger or smaller than the individual waves.

 

 Wave  Particle
 Wave is delocalised (spread out) in space  Particle is localised in space
 Two or more waves can exist in the same region of space  Two particles cannot simultaneously occupy the same position in space
 When two waves are present together, the resultant wave can be larger or smaller than the individual waves. In other words, two waves may interfere  If there are two or more particles in any region of space then their sum is equal to the number of individual particles. In other words, two particles do not interfere

Is electron really a particle or wave?

Upto the year 1924, electron was exclusively regarded as a particle. However, in 1924, de-Broglie suggested that an electron behaves both as a material particle and as a wave. Just like light, some experimental facts can be explained by assuming electrons as tiny particles while some other facts can be explained only on the basis of wave character of the electrons. Thus, the electron behaves as a particle as well as a wave.

The electrons are so small that they cannot be seen even with the help of a powerful microscope. But there is no doubt regarding their existence, rather they are the essential constituents of all forms of matter. Thus, they can be imagined to look like very tiny dots executing wave like motion and moving with speed 3 x 108 m/sec.

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