Description dielectric materials and their types

Dielectric Materials:

Materials that do not allow current to flow through them are called insulators or dielectrics. Dielectric materials are capable of storing electric energy. Dielectric materials do not have free electrons ( in the case of ideal dielectric) because electrons are tightly bound with the nucleus so the conductivity of the dielectric is poor and for an ideal dielectric, it is zero.

When a dielectric is placed in an external electric field, atoms or molecules of the dielectric material are polarised due to the creation of an electric dipole in the atoms or molecules, and the internal field is set up in the dielectric material which opposes the external applied electric field, thereby reducing the net electric field and hence the electric potential difference. If these dielectrics are placed between plates of a capacitor, the potential difference will be reduced without affecting the charge on the plates.

According to the band theory of solids,

A dielectric is a material in which the energy band gap between balance and conduction band is more than three electron volt.

Examples: Glass, Plastic, Mica, Rubber, Wood, Turpentine oil etc.

Types of Dielectric materials:

Dielectric materials are of two types :

1.) Non-polar dielectrics
2.) Polar dielectric

1.) Non-polar dielectrics:

In non-polar dielectric materials, the molecules that are usually diatomic and composed of the same type of two atoms have a symmetrical structure that is the positive nuclei are surrounded by a symmetrically distributed negative electron cloud. The center of gravity of positive and negative charge distribution coincide and so the molecules are electrically neutral and have zero electric dipole moment.

Examples: $H_{2}$, $O_{2}$, $CO_{2}$, $CCl_{4}$, $C_{6}H_{6}$, $C_{6}H_{12}$, $CS_{2}$ etc.

Polarisation of the non-polar dielectric materials:

When a non-polar dielectric is placed in an external electric field the positive charge of the nucleus and negative charge of the electron cloud experience electric force which causes a displacement between the positive and negative parts of the molecule from their equilibrium position in opposite directions. The distance moved is very small $(10^{-10})$ because the displacement is restricted by storing force which increases with the increase of displacement. Therefore the centre of gravity of these positive and negative charges no longer coincide and molecules are said to be polarized. The molecules does acquire an induced electric dipole moment and aligned in the direction of external field. The induced dipole moment and the polarization disappear when an electric field is removed.

2.) Polar Dielectrics:

In polar dielectric materials, the molecules which are normally composed of two or more different atoms have permanent dipole moments because the center of gravity of these positive charges and that of the negative charges in a molecule are permanently separated by a finite but small distance. This is due to the asymmetric shape of the molecule. Thus each molecule in the polar dielectric material behaves as a dipole having a permanent dipole moment. Normally these molecules are in polar dielectrics and randomly arranged such that the net dipole moment is zero and the material acts as a neutral one.

Examples: $H_{2}O$, $CHCl_{3}$, $C_{6}H_{5}Cl$, $C_{6}H_{5}NO_{2}$, $C_{2}H_{5}OH$, $NH_{3}$, $HCl$, $CO$,etc.

Polarization of the polar dielectric materials:

when polar dielectric is placed in an external electric field the molecular dipole tense to align themselves in the direction of the field and acquire a considerable amount of dipole moment. this the dielectric acid to be polarised.

In the $HCL$ molecule, the electron of the $H$ atom lies more toward the $Cl$ atom. The $H$ end of the $HCl$ molecule is positive and the $Cl$ end is negative. The molecule is therefore a dipole having dipole moment $\overrightarrow{p}$ direct from $Cl$ atom to $H$ atom.

Hence, the polarization of non-polar dielectric material is the displacement of positive and negative charge, and in the case of polar dielectric material, the polarization is the orientation of molecular dipole moment under the action of the electric field to which they are subject.

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Difference between Potentiometer and Voltmeter

There are the following differences between a potentiometer and a voltmeter given below:

Potentiometer:

1.) It is based on null method.

2.) It gives an accurate value of emf.

3.) While measuring emf, it does not draw any current from the cell.

4.) Resistance of potentiometer wire becomes infinite while measuring emf.

5.) It can be used for various experimental purposes.

6.) It can not be taken conveniently from one place to another place.

Voltmeter:

1.) It is based on the deflection method.

2.) It does not give an accurate value of emf.

3.) While measuring emf, it draws some current from the cell. Hence it reads slightly less than the actual emf.

4.) The resistance of the voltmeter is high enough but not infinite.

5.) It can be used to measure potential differences only.

6.) It can be conveniently taken from one place to another place.

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Principle Construction, Working and Angular Magnification of Simple Microscope

Principle of Simple Microscope:

The principle of the simple microscope is based on the magnification of an image by using a simple convex lens.

Construction:

A simple microscope consists of one convergent lens only. The object is placed between the lens and its focal length, and the eye is placed just behind the lens. Then the eye sees a magnified, erect, and virtual image on the same side as the object at the least distance of distinct vision $(D)$ from the eye, and the image is then seen most distinctly.

Working:

If the small object $ab$ is placed between a lens $O$ and its first focus $f$ then Its magnified virtual image $a_{1}b_{1}$ is formed at a distance $D$ from the lens. Since the eye is just behind the lens, the distance of image $a_{1}b_{1}$ from the eye is also $D$.

Angular Magnification Or Magnifying Power($M$):

The ratio of the angle subtended by the image at the eye ($\beta$) to the angle subtended by the object at the eye when placed at the least distance of distinct vision ($\alpha$) is called the angular magnification or magnifying power.

$M= \frac{Angle \: subtended \: by \: the \: image \: at \: the \: eye \: (\beta)}{Angle \: subtended \: by \: the \: object \: at \: the \: eye \: when \\ placed \: at \: least \: distance \: of \: distinct \: vision \: (\alpha)}$

$M=\frac{\beta}{\alpha} \approx \frac{tan \beta}{tan \alpha} \quad (1)$

From figure

$tan \beta = \frac{ab}{oa} $

$tan \alpha = \frac{a_{1}b_{2}}{a_{1}o}$

Here $a_{1}b_{2} = ab$

$tan \alpha = \frac{ab}{a_{1}o}$

Now substitute these values in equation $(1)$, then

$M=\frac{\frac{ab}{ao}}{\frac{ab}{a_{1}o}}$

$M=\frac{a_{1}o}{ao}$

Here $ao = u$ (Distance between object and optical center of the lens) and $a_{1}o = D$ (Least Distance of distinct vision), then the above equation can be written as

$M=\frac{D}{u} \qquad(2)$

We know that the lens formula $\frac{1}{v}-\frac{1}{u} = \frac{1}{f}$

Now put
$v=-D$ (The image $a'b'$ is being formed at a distance $D$ from lens)
$u=-u$

$\frac{1}{-D}-\frac{1}{-u} = \frac{1}{f}$

Multiply $D$ in the above equation

$-\frac{D}{D}-\frac{D}{-u} = \frac{D}{f}$

$-1-\frac{D}{-u} = \frac{D}{f}$

$\frac{D}{u} =1 + \frac{D}{f} \qquad(3)$

From equation $(2)$ and equation $(3)$, then

$M=1 + \frac{D}{f} $

If eye is kept at distance $d$ from lens then $v=-(D-d)$, and the magnifying power will be

$M=1+\frac{D-d}{f}$

To see with a relaxed eye, the image $a'b'$ should be formed at infinity. In this case, the object $ab$ will be at the focus of the lens, i.e. $u=f$ then magnifying power

$M= \frac{D}{f} $

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Light and its properties

The basic definition of Light:

Light is a form of energy that produces the sensation of vision in the eye by which we can see objects.

There are some facts about light as follows:

1. Lightwave moves along a straight line path.

2. Light waves can travel through vacuum and medium both.

3. Light is an electromagnetic wave.

4. A light wave is the transverse wave in nature.

5. Light can be dispersed.

Besides these facts, light also shows the phenomenon of interference, diffraction, polarisation photoelectric effect, etc. To explain the above facts, many principles have been given from time to time, e.g., Newton's corpuscular theory, Huygen wave theory, Maxwell's principle of electromagnetic wave, Planck's quantum principle, dual nature of light, etc.

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Origin of Biomedical Signals

The biomedical signals differ from other signals only in terms of the application — signals that are used in the biomedical field. As such, biomedical signals are produced from a variety of sources. The following is a brief description of these sources:

1. Bioelectric signals: The bioelectric signal is unique to biomedical systems. It is produced by nerve cells and muscle cells. It is produced due to the membrane potential, which under certain conditions may be excited to generate an action potential. In single-cell measurements, the specific microelectrodes are used as sensors, and the action potential itself is considered as the biomedical signal. In more gross measurements, the surface electrodes are used as sensors, and the electric field generated by the action of many cells, distributed in the electrode’s vicinity, constitutes the bioelectric signal. Bioelectric signals are probably the foremost biosignals. The fact that most biosystems use excitable cells makes it possible, to use biosignals to study and monitor the main functions of the systems. The electric field propagates through the biological medium, and thus the potential may be acquired at relatively convenient locations on the surface, eliminating the need to invade the system. The bioelectric signal is acquired by a relatively simple transducer. A transducer is required in the field of biomedical because the electric conduction in the biomedical medium is executed through ions, while the conduction in the measurement system is executed through electrons. All these lead to the fact that the bioelectric signal is broadly used in most of the fields of biomedicine.

2. Bioimpedance signals: The impedance of the tissue contains important information related to its composition, blood volume, blood distribution, endocrine activity, autonomic nervous system activity, and many more. The bioimpedance signal is usually generated by injecting into the tissue under test sinusoidal currents (frequency range of $50 kHz–1 MHz$, with low current densities of the order of $20–20 mA$). The electrode polarization problems are minimized by choosing the frequency range and the low current densities are selected to prevent tissue damage mainly due to heating effects. Bioimpedance measurements are usually performed with four electrodes. Two electrodes (known as source electrodes) are used to inject the current into the tissue and these electrodes are connected to a current source. Remaining two electrodes (known as measuring electrodes) are placed on the tissue under investigation and used to measure the voltage drop generated by the current and the tissue impedance.

3. Bioacoustic signals: Many biomedical phenomena create acoustic noise. The measurement of this acoustic noise gives information about the underlying phenomenon. The flow of blood in the heart (i.e through the heart’s valves, or through blood vessels) generates typical acoustic noise. The flow of air through the upper and lower airways and in the lungs generates acoustic sounds. These sounds are called coughs, snores, and chest and lung sounds. These sounds are used extensively in medicine. Sounds are also produced in the digestive tract and in the joints. It also has been observed that the contracting muscle generates an acoustic noise or muscle noise. Since the acoustic energy propagates through the biological medium, the bioacoustic signal may be conveniently acquired on the surface, using acoustic transducers (microphones or accelerometers).

4. Biomagnetic signals: Many organs, such as the brain, heart, and lungs, produce extremely weak magnetic fields. The measurements of these fields provide information but are not included in other biosignals (such as bioelectric signals). Due to the low level of the magnetic fields to be measured, biomagnetic signals are usually of a very low signal-to-noise ratio. Extreme precautions must be taken in designing or developing the acquisition system of these signals.

5. Biomechanical signals: The term biomechanical signals includes all signals used in the biomedicine fields that originate from some mechanical function of the biological system. These signals include motion and displacement signals, pressure and tension signals, flow signals, and others. The measurement of bio-mechanical signals requires a variety of transducers, not always simple and inexpensive. The mechanical phenomenon does not propagate in biomedical signals, as do the electric, magnetic, and acoustic fields. Hence the measurement usually has to be performed at the exact site. This very frequently complex the measurement and forces it to be an invasive one.

6. Biochemical signals: The chemical measurements from the living tissue or from samples analyzed in the clinical laboratory produce biochemical signals. Measuring the concentration of various ions inside and around a cell using specific ion electrodes. It is an example of such a signal. Partial pressures of oxygen (pO2) and carbon dioxide (pCO2) in the blood or respiratory system are other examples. Biochemical signals are often very low-frequency signals. Mostly, biochemical signals are actually DC signals.

7. Biooptical signals: Bio-optical signals are the result of optical functions of the biological system, occurring naturally or induced by the measurement. Blood oxygenation may be analyzed by measuring the transmitted and backscattered light from a tissue ( i.e.in vivo and in vitro) in several wavelengths. Important information about the fetus may be acquired by measuring the fluorescence characteristics of the amniotic fluid. Analysis of the Heart output may be performed by the dye dilution method, which requires the observation of the appearance of recirculated dye in the bloodstream. The development of fiberoptic technology has opened vast applications of bio-optical signals.

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