ELECTROSTATIC INDUCTION

ELECTROSTATIC INDUCTION

Electrostatic Induction - Electric induction is the phenomenon in which positive and negative charges are accumulated or separated in a substance, when a charged body is brought nearer to it. For example when a negatively charged body A is brought nearer to a conductor BC the near end B acquires positive charge and the far end C acquires negative charge (Fig. 2.4). That means the negative charges in A repelled the electrons in BC to the far end. The amount of  charge in A and in BC are equal in magnitude. If A is positively charged, BC will have the opposite effect. This method of charging a body is known as electrostatic induction and is used in electrostatic machines and capacitors. Electrophorus, lighting arrester and vandegraaff generator are few examples for electrostatic machines.This is known as electrostatic induction

TRANSFORMER LOSSES

In practice the output power is always lesser then the input power and hence the efficiency of the transformer is always less than 100 percent. this implies that some amount of energy is lost in the form of heat. The energy loss can be considered as copper losses, eddy current losses, hysteresis losses and flux leakage

Copper losses – when a current I flow through resistance R, an amount of power equal to i² R t watts is converted into heat. this arise in both copper coils and iron core. The copper coils possess resistance. When  current flow through this coil, electrical energy equal to i² R t is converted into heat. To reduce the loss, the current cannot be reduced because the normal operation of the transformer of the transformer will be affected. Instead the resistance of the coil must be minimized by using the wire of low resistivity wire. Therefore, thicker wire should be used as transformer coil. the optimum thickness will be decide by comparing the cost, space and saving of power. Copper is the best coil material available today and therefore is commonly used.

Eddy current losses – the iron core consisting of concentric layers of iron each acting as circuited single turn coil. hence, emf will be induced in the core. This emf will be produce currents called eddy currents. the eddy currents will give rise to i² R t losses. These eddy currents can be eliminated by making the iron core in the form of thin sheet of metal, and each sheet is insulted from its neighbor by thin layer of paper .this type of core is known as laminated core .the core is usually made up of stelloy ,an alloy of steel.

Hysteresis losses – the transformer core is the magnetic material. The core is magnetized twice in each cycle of the alternating voltage. when the direction of ac changes, the magnetization also gets reversed. During this reversal some energy is lost due to the molecular friction and the energy appears as heat. The loss of energy by molecular friction is called hysteresis loss. This can be reduced in practice by choosing a suitable magnetic material such as mumetal which has low hysteresis loss. mumetal is a ferromagnetic alloy containing 78 percent nickel,17% percent iron and 5% copper. its has high permeability.

Flux leakage  -  all the flux linked with the primary is not linked with the secondary due to leakage. this result in the loss of the energy. This can be minimized by using a shell type of core

FLUORESCENCE

FLUORESCENCE

When electromagnetic radiation falls on the certain crystalline substance, visible or ultraviolet light is emitted from the crystal. This phenomenon is called fluorescence. Fluorescent is a form of light emission within  10 ^-8 of x-ray exposure. If the emission of light delayed beyond   10 ^-8 is called phosphorescence .actually the crystal absorbs the radiation of shorter wavelength which produces excitation in the crystal.this is called FLUORESCENCE

CASSETTE

cassette

The caste is the rigid holder that contains the screens and film. the front surface, the side facing the tube , should be made of material with low atomic number, such ads plastic or cardboard or aluminum and should be thin yet sturdy. Attached to the inside of front cover is the front screen and attached to the back cover is the back screen. The radiographic film is loaded between these two screens. These days most cassettes are loaded with identical screen for front and back. Between each screen and the cassette cover will be compression device such as felt or rubber that maintains close screen film contact when the cassette is closed and latched. The back cover is usually made up of heavy metal such as lead to minimize backscatter. The x-rays pass through the film screen combination to back cover will be absorbed photoelectrically more readily in high z material then in low z material.this called cassette.

HOW X-RAY PRODUCTION TAKES PLACE?

SOURCE FOR XRAY PRODUCTION

The production of x-ray needs the following requirements

·         Electron source cathode

·         Target to stop the electron(anode)

·         High voltage power supply to accelerate electrons

·         Vacuum

·         And tube insert and housing.

·         Electrons can be produced by ionization in gas or by thermionic emission

·         The electron source act as cathode and target act as anode. The high voltage is applied between anode and cathode. this voltage  accelerate the electrons to a high velocity as a result the electron will possess a high kinetic energy, when the electrons are stopped by the target ,the electron kinetic energy is converted into x-rays energy thus x-rays are produced

·         A high vacuum is maintained between the anode and cathode. this is necessary

·         To avoid collision between the electrons and gas molecules and

·         To avoid oxidation of tungsten filament in the cathode they require vacuum is less than 10^-5mmhg

·         The housing absorbs the x-ray emerging in undesired direction.

·         Maintains the require vacuum

·         And act  as an electrical insulator

·         And also contain cooling system which remove the heat from the target the equipment having all above requirements is called an x-ray tube

CAPACITANCE OR CAPACITOR

CAPACITANCE OR CAPACITOR

CAPACITANCE

the property of a conductor to store electric charge is know as capacitance.it is defend as the ratio between the charge and its potential.if the q is the charge in a conductor of potential v then the capacitance c is given by:- c = q/v

the unit of capacitance is farad(F).one farad is the capacitance of a capacitor which require one coulomb electric charge to raise its potential by one volt, in practice, micro farad and pico farad is useds

1farad= 1 columb/volt

CAPACITOR

a capacitor is a device which increase the capacitance of the conductor.it usually consist of two conductors , one charged and other is earth connected.the space between the plate is filled with some insulating material called dielectric. 

the capacitance of capacitor depends on several factors such as

• area of over lap between the plates
• distance between the plates
• nature of dielectric medium

capacitor uses:

• used to store electric charge
• used to measure potential difference and small currents
• it is useful in reducing voltage fluctuation,generating oscillations,for providing time delay in various electric circuits
• with use of capacitor require electric field can be obtained

THERMIONIC EMISSION

THERMIONIC EMISSION

When a metal is heated, its molecules (Atom) take thermal energy. Some of its electrons take up so much energy that it can go away from the surface of the metal. These are called Thermions. Without this energy, electrons can only move inside the metal and cannot come out.

Thus, the arrival of electrons with thermal energy in this way is called Thermionic Emission. In this way, the accumulation of electron flakes on the metal surface in the form of electron clouds is called Edison Effect.

These negatively charged electrons come out of the metal to form a space charge. This space charge no longer allows other electrons to flow out of the metal surface until they are given enough thermal energy to On resistance to space charge. By this space charge, preventing other electrons from coming out of the metal surface is called Space Charge Effect.
When the electrons come out of the filament, the filament absorbs positive charge, so that the filament attracts some electrons again from this space charge. In this way, when the filament is heated to this Emission Temperature, a equilibrium is formed in which the number of electrons emitted from the filament is the same as the number of electrons absorbed by the filament. Due to which the number of electrons in the space charge is fixed, the number of  which depends on the temperature of the filament. Thus a very large number of electrons can be obtained by accelerating a large number of electrons from the cathode to the anode. These electrons can be obtained. Beams always flow in the same direction (from cathode to anode) in the x-ray tube. In this electron beam, the repulsion force acts due to the same charge between the electrons of the electrons, due to which the electron beam spreads in its width, as a result it collides on a very large part of the x-ray tube. The foxing cups are used to prevent this dispersion of the electron beam and to limit the collision area of ​​the electron beam at the anode. This forcing cup is given a minus potential equal to the filament| It is designed in such a way that it converts the electron beam to the desired shape and size on the target. This forcing cup is usually made of nickel. The x-ray tube usually has one filament and the modern x-ray tube has two filaments. Dual filament X-ray tubes have two filaments arranged in a Side by Side or One Above Other arrangement. They have one filament larger than the other. They are used to generate only one filament electron beam at a time. Large filament is for long exposure. You can see the filament by removing the filter from the beam exit port of the x-ray tube as the filter appears red after heating. The modern x-ray machine also has more than two (3) filaments and the filaments in the stereoscopic angiographic tube are in a different arrangement, in which two filaments are located at a distance of 4 cm, in which a stereoscopic film foot is removed from the exposure.