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Monday, 30 March 2020

Facts about Chemical Reactions

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Facts about Chemical Bonding

The force that holds together the different atoms in a molecule is called Chemical Bonding.

Types of Bonding

The bond formed by the transfer of electrons from one atom to another is called ionic bond or electrovalent bond.

Ionic bond was proposed by Kossel.

In covalent bonds the atoms are linked together by the sharing of electrons between the atoms.

The number of electrons contributed by an atom for sharing is known as its covalency.

van der Waals bonds act between molecules which are brought close together. They are the weakest bonds.

The attractive force which binds the hydrogen atom of one molecule with the electronegative atom of another molecule is called hydrogen bond.

In co-ordinate bond (dative bond) the elec-trons are shared and the shared pair of electrons are derived from only one atom.

Co-ordinate bond was proposed by Sidgwick.

The attractive force between mobile electrons and the metallic kernels is referred to as metallic bonds. 

Bond Characteristics

Bond length: The average distance between the centres of nuclei of two bonded atoms.

Bond energy or Bond strength: The amount of energy required to break one mole of the bond and separate the bonded atoms in the gaseous state.

Bond angles: The average angle between the bonded orbitals is known as bond angle.

Bond order: It is the number of bonds between two atoms.

Types of Bonds

1. Ionic or Covalent - Formed by transfer of electrons
2. Co-ordinate - Mutual sharing of electrons
3. Electrovalent - One-sided sharing of electrons

Facts about Types of Reactions

Chemical reaction is a process by which one or more substances get transformed into new substances.

The substance which gets transformed or which undergo chemical reaction is called reactants.

The new substance formed as a result of chemical reaction is called products.

A chemical reaction in which reactant react to produce product and the product in turn react with one another to produce the reactant is called reversible reaction.

Difference between exothermic and endothermic reaction ?

Ans: A reaction in which heat is evolved is called the exothermic reaction. A reaction in which heat is absorbed is called endothermic reaction.

The chemical reaction in which reactant react to produce product and the products does not reversibly produce reactant are called irreversible reaction.

Chemical reaction in which light energy is either absorbed or released is called photochemical reaction.

Chemical reaction in which electric energy is either absorbed or released is called electro-chemical reaction.

Combination reaction: In this type of reaction an element or compound combines with another element or compound to form a new compound.

eg: Mercury combines with oxygen to form mercuric oxide.

Decomposition reaction: In this type of re-action a compound breaks down into simpler substances.

eg: Water can be broken down by electricity to form hydrogen and oxygen.

Simple displacement or Substitution reaction: In this type of reaction one element or radical replaces another element or radical in a compound.

eg: Zinc can replace the hydrogen in hydrochloric acid to form zinc chloride and hydrogen gas.

Double displacement reaction: In this type of reaction an element or radical of one compound changes place with an element or radical of another compound.

eg: Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

Isomerisation reaction: In this type of reaction rearrangement of atoms in a given substance takes place and new substance is formed.

eg: On heating ammonium cyanate (NH4 CNO) changes to Urea (H2NCO NH2)

Redox Reactions

Oxidation involves the addition of oxygen or the removal of hydrogen.

Oxidation involves the loss of one or more electrons.

Reduction involves the removal of oxygen or the addition of hydrogen.

Reduction, involves the gain of one or more electrons.

Oxidising agent (oxidant): Any substance which is capable of oxidising other substances and is capable of accepting or gaining electrons lost during oxidation is called oxidising agent.

Reducing agent (Reductant): Any substance which is capable of reducing other substances and is capable of releasing or losing electrons gained during reduction is called reducing agent.

The reaction in which oxidation and reduction takes place simultaneously are called redox reaction.

Oxidation number of an element indicates the number of electrons lost, gained or shared as a result of chemical bonding.

In oxidation, oxidation number increases.

In reduction, oxidation number decreases.

Oxidation number of an atom in free state is zero.

Oxidation number of alkali metals in molecule is always +1

Oxidation number of alkaline earth metals in a molecule is always +2

Sum of oxidation number of atoms in a molecule is equal to zero.

Oxidation -> Addition of oxygen -> Removal of hydrogen -> Removal of electron

Reduction -> Removal of oxygen -> Addition of hydrogen -> Addition of electron

Facts about Periodic Table

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Periodic Table

Periodic table is an arrangement of elements with similar properties placed together.

 Lavoisier classified the elements into metals and non-metals.

Dobereiner classified the elements as triads, in which the atomic weight of the middle element was the arithmetic mean of the other two.

The law of octaves was proposed by Newland.

Mendeleev's periodic table is based on atomic weights.

Mendeleev's periodic table consists of 7 periods and 9 groups.

Modern periodic table is based on atomic number of elements.

Modern periodic table contains 7 horizontal rows called periods and 18 vertical columns called groups and 118 elements.

Elements of group 1 are called Alkali metals.

The periodic table is divided into four main blocks

1. s-block - Elements of group 1 and 2
2. p-block - Elements of group 13 to group 18
3. d-block - Elements of group 3 to group 12
4. f-block - Lanthanides and Actinides

Elements of group 2 are called Alkaline earth metals.

Elements of group 17 are called Halogens.

There are 118 elements known today, out of which 90 are naturally existing.

First period contains only 2 elements, this is the shortest period.

The sixth period consists of 32 elements, this is the longest period.

Elements of group 18 show little reactivity and are termed noble gases.

Elements from atomic number 58 to 71 are lanthanides or rare earths.

Elements from atomic number 90 to 103 are termed as actinides.

The atomic number was first calculated by Moseley.

Man made elements are known as transuranics.

Elements from atomic number 93 onwards do not occur in nature. So they are called synthetic or transuranic elements. 

s and p-block elements are called representative elements.

d-block elements are called transition elements.

f-block elements are called inner transition elements.

In a group, from top to bottom atomic size increases.

What is electro negativity ?

Ans: The power of atom to attract electron pairs. The order of electro negativity is F (4.00) > O (3.50) > Cl (3.10) > N (3.00). The electro negativity scale was introduced by Linus Pauling.

Atomic size decreases from left to right in a period.

Ionization Energy is the energy required to remove an electron completely from an isolated gaseous atom or ion of an element.

Ionization energy generally increases from left to right in a period with increase in atomic number or decrease in atomic size.

In a group, from top to bottom ionization energy decreases.

Electron Affinity is the energy change when an electron is gained by a neutral gaseous atom converting it into a negatively charged ion.

In a group electron affinity, decreases from top to bottom and in a period it increases as we move from left to right.

Classification of Elements 

1. Metals and non metals - Lavoisier  
2. Triads – Dobereiner
3. Law of octaves – Newland
4. Based on atomic weight – Mendeleev
5. Based on atomic number – Moseley

Sunday, 29 March 2020

Facts about Atoms and Molecules

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Facts about Atoms and Molecules

The term 'atom' was introduced by Ostwald.

Atom was derived from a greek word ‘atomos' which means indivisible.

Atoms are the smallest units of matter that have all the characteristics of an element. Atoms are the building blocks of everything in the universe.

Atoms cannot exist free. They exist in combination with the atoms of the same element or another element.

Atoms of noble gases like Helium, Neon, Argon and Krypton are unreactive and exist in the free state.

Atoms of metals are also exist free in nature. (Iron, Copper, Gold,Silver....)

An atom can take part in chemical combination but does not occur free in nature.

A molecule is the smallest particle of an element or compound that can have a stable and independent existence.

All substances are made of tiny particles called molecules.

The term 'molecule' was introduced by Avogadro.

Facts about an Atomic Structure

On the basis of discharge tube experiments it was stated that atom is comprised of three fundamental particles called protons, neutrons and electrons.

The central part of atom is called nucleus. Protons and neutrons are present in nucleus and electrons are revolving around the nucleus in definite path called orbits.

Nucleus was discovered by Rutherford. It has positive charge

Protons and neutrons are collectively known as nucleons.

The radius of nucleus of an atom varies in the order 1.5 x 10-13 cm to 6.5 x 10-13 cm

Atom - Atom theory (John Dalton) - Structure (Niels Bohr) -  Plum pudding model (J.J. Thomson) - Wave mechanical model (Max Planck )

The smallest atom: Helium (He)

The simplest atom: Hydrogen (H)

Biggest known atom: Francium (Fr)

Fundamental Particles

1. Electron

Electron was discovered by Cathode ray experiment in 1897.

The name 'electron' was proposed by Stoney.

The charge of an electron(-1.602x 10-'9C) was determined by Millikan in his oil drop experiment.

The actual mass of an electron (9.1x10-31 kg) was calculated by J.J. Thomson

The dual nature of electron was proposed by Victer de-Broglie.

The dual nature of electrons was experimentally proved by I.H.Germer.

Electron is the lightest fundamental particle of an atom.

Electron is the significant particle of atom/ matter/nature .

2. Proton

Presence of proton was observed by Goldstein in 1896 in a discharge tube with a perforated cathode.

Experiments carried out by Lord Rutherford showed that positive particles produced in a discharge tube containing hydrogen gas are same as protons.

Proton is called the identity card of an element.

3. Neutron

Neutron was discovered by James Chadwick in 1932

The reason for the late discovery of neutron was its neutral nature.

Neutron is the heaviest fundamental particle of an atom.

Of all the elementary particles present in an atom, neutron is the least stable particle.

Neutrons are bound very tightly in the atomic nucleus with the protons.

When a neurtron splits an electron and a proton are obtained, hence it is chargeless.

Fundamental Particles

Fundamental Particles – Proton, Neutron, Electron

1. Proton – Positive – Rutherford - Identity Card of an
2. Neutron - Neutral - James Chadwick - Heaviest particle
3. Electron - Negative - J.J.Thomson - Lightest Particle

Sub Atomic Particles

Positrons: Discovered by Anderson. these are positive counterpart of electrons. Highly unstable and combine with electrons producing γ- rays.

Neutrinos and Anti-neutrinos: Postulated by Fermi, these are particles of small mass and zero charge.

Pi- mesons (pions) and mu-mesons (muones): Positively and negatively charged mesons were postulated by Yukawa, these are particles having a mass intermediate between that of the electron and the proton. Neutral Mesons were postulated by Kemmer to account for the binding forces between the nucleons (protons + neutrons).

What do you mean by Green Chemists?

The term green chemistry is defined as the programme of developing new chemical products and chemical processes or making improvements. The Green chemists Yves Chauvin, Robert H. Grubbs and Richard.R.Schrock won the 2005 Nobel prize in Chemistry for work that reduces hazardous waste in creating new chemicals.

Characteristics of Atoms

Valency is the combining capacity of one atom to another.

Noble gases have zero valency.

Hydrogen shows univalency,

Sodium and Potassium show univalency.

The common valency of carbon is four.

Atomic Mass Unit (amu): One twelvth of a mass of a Carbon-12 atom: a unit used for stating atomic and formula weights

Atomic Orbitals: are regions of space around the nucleus of an atom where an electron is likely to be found.

Atomic Radius: Radius of an atom.

Atomic Weight: The average weight of an atom of an element, based on the 1/12th mass of C-12 atom.

The atomic size is about 105 times larger than the nuclear size.

Avogadro's Number: The number (6.022 x 1023) of atoms, molecules or particles found in 1 mole of any compound.

Difference between Mass number and Atomic number

Ans: Mass Number is the sum of number of protons and neutrons present in the nucleus of an atom. It is denoted by the letter A. Atomic Number is derived from nuclear charge i,f atom, hence it will be the number of protons. It is generally denoted by the letter Z.

Different types of Atomic Species

Isotopes are the different types of atoms of the same element having same atomic number and different mass numbers.

The term `isotope' was introduced by Soddy.

Isotopes differ in the no. of neutron.

Protium, Deuterium and Tritium are the isotopes of hydrogen.

Protium has no neutron (1H1).

Protium is the simplest isotope.

Deuterium has one neutron. It is called heavy hydrogen (1H2).

Tritium has two neutrons. It is the radioactive Isotope of hydrogen (1H3).

Isobars are atoms of different elements having same mass number and different atomic numbers
eg: Argon -40 (18Ar40) and Calcium -40 (20Ca40)

Isotones are atoms of different elements having different atomic numbers and mass numbers, but they have same number of neutrons.
eg: Tritium (1H3) and Helium(2He4) both contain two neutrons.

Nuclear Isomers: These are the different types of atoms of the same element having different radioactivity constants and nuclear energies.

Allotropes are different forms of the same element with different physical appearances.

eg: Red phosphorus and white phosphorus, oxygen and ozone.

Carbon exists in crystalline as well as amorphous allotropes.

Atomic Species – Isotopes, Isobars, Isotones

Isotopes - Same atomic number and different Mass number – 6C126C14

Isobars - Same mass number and different atomic number – 18Ar4020Ca40

Isotones – Different atomic number and mass number but have same number of neutrons – 1H3, 2He4

The father of periodic table is Mendeleev.

The father of modern periodic table is Moseley

Isotopes of Hydrogen – Protium, Deuterium, Tritium

1. Protium - Simplest isotope - No neutron
2. Deuterium – Heavy hydrogen - One neutron
3. Tritium - Radioactive isotope - Two neutrons

Facts about Behaviour of Gases

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Facts about Behaviour of Gases

All gases containing equal amount of substances occupy the same volume at the same temperature and pressure

Gas Laws

Boyle's Law: The pressure of a fixed amount of a gas varies inversely with the volume if the temperature is maintained constant.

Charle's Law: The volume of a fixed amount of a gas varies directly with its absolute temperature at constant pressure.

Gay-Lussac's Law: At constant volume, the pressure of given mass of gas is directly proportional to the temperature in Kelvin.

Avogadro Law: It states that at a given temperature and pressure the volume of a gas is directly proportional to the amount of gas.

Ideal Gas Equation
Boyles law, V T (n, t constant) ------------- (1)

Charles law, V T (n, P constant) ------------- (2)

Avogadro's law, V T (P, T constant) ------------- (3)

Combining the three 

V 1/P x T x n
or V = a constant x nT/P

This constant is known as universal gas constant and is denoted as 'R'

Then, V – R x n x T/P

PV= n x R x T
i.e. PV = nRT

This equation is called Ideal gas equation.

Facts about States of Matter

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Matter is anything that has mass, occupies space and offers resistance.

Matter undergoes changes which may be either chemical or physical change.

A physical change alters only the physical properties of matter and it is temporary.

eg: Melting of ice, boiling of water.

A chemical change alters the chemical proper-ties of substance and it is permanent.

eg: Corrosion of metals, curdling of milk.

Matter exists in nature as pure substances and mixtures.

Pure substances have same composition and properties.

Pure substances are divided into elements and compounds.

An element is the simplest form of matter and it cannot be decomposed by ordinary chemical reaction.

eg: Oxygen, calcium, sodium, hydrogen etc.

The term 'element' was introduced by Robert Boyle.

An element consists of only one kind of atoms.

A compound is a substance made up of elements chemically combined in a fixed proportion by weight.

eg: Oxygen and hydrogen combine to form water which is a compound.

Compounds consist of more than one type of atoms.

Elements in a compound lose their individual chemical characteristics.

A material obtained by mixing two or more sub-stances in any proportion is known as mixtures.

eg: Mixture of sand and sugar.

In a mixture, the components do not lose their identity.

The components of a mixture can be easily separated by ordinary methods.

Mixtures are of two types. Homogeneous mixtures and heterogeneous mixtures

Homogeneous mixtures have the same com-position throughout the sample.

eg: Salt solution, sugar solution etc.

Heterogeneous mixtures have different compositions and the components are distinguishable.

eg: Carbon, nitre and sulphur mixture (gun powder)

Changes of Matter

1. Physical - Temporary - Melting of Ice
2. Chemical - Permanent - Corrosion of metals

Matter is divided into Pure substances and Mixtures

1. Pure substances are divided into Elements and Compounds
2. Mixtures are divided into Homogeneous and Heterogeneous

Elements - Only one type of atom - Oxygen
Compounds - More than one type of atoms - Water
Homogeneous – Same composition - Salt solution
Heterogeneous - Different composition - Gun powder


There are seven stable physical states and two more meta stable physical states are known.

(i) Solid
(ii) Liquid
(iii) Gas
(iv) Plasma
(v) Bose-Einstein Condesate
(vi) Fermionic Condensate
(vii) Super Fludity

Solids are rigid and have definite volume and shape.

Kinetic energy of the particles is very small.

Liquids have definite shape but no definite volume.

Kinetic energy of particles in the liquid state will be higher than that in the solid state.

Gases have neither definite volume nor definite shape.

Gas molecules possess very high kinetic energy.

The most abundant physical state in the universe is plasma.

In plasma state matter exists as ions.

All stars and flame are in plasma state.

A Bose-Einstein condensate can also be called a "Super atom" because the cloud of many thousands of ultracold atoms behaves as if it were a single atom.

Fermionic condensate is a superfluid phase formed by fermionic particles at low temperatures.

The physical state of liquid helium is super fluidity.

Helium liquid is called Super Liquid

The physical state of Glass is super cooled liquid, since it has no sharp melting point.

The meta stable state between solid and liquid is liquid crystal

All electronic display is based on liquid crystals

The state in which matter exists depends upon temperature and

For example, water is in the liquid state at room temperature and atmospheric pressure. At 0°C and atmospheric pressure, it exists as a solid (ice). When heated to 100°C at atmospheric pressure, water is converted into a gas (steam).

Tuesday, 17 March 2020

Stepper Motor Working, Types, Applications

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Certain terminologies which are involved in the operations of stepper motors are given below:

a. Step - Angular rotation created by the output shaft all time the motor receives a step pulse.

b. Step angle - The angle measured in degrees for each rotation of the output shaft.  The step angle can be as small as 0.72° to as large as 90° in steps of 1.8°, 2.5°, 7.5° and 15°. It can be expressed in terms of rotor and stator poles (teeth) i.e., Nf and Ni

i.e. β = (Ns—Nf )/ ( Ns x Nf ) x 360
= 360/ No: of stator phase x No: of rotor teeth

c . Steps per second - The number of steps the motor takes per second.

d. Resolution - The total number of steps required for the output shaft to make one complete revolution i.e.,
No. of step / Revolution = 360°/β 

e. Holding torque - The torque required to rotate the output shaft to full step with the motor energized and in a standstill condition.

f. Step response - The time required for the motor to make one complete step. Time usually will be in milli seconds.

g. Start without error rate - The stepping at which a motor with external load inertia can start and stop without loosing a step.

h. Slew rate - The maximum step rate at which a motor will run under a rated load once the motor has started rotating. This is usually substantially greater than the start without error rate.

i. Ramping - A control method used to vary the pulse rate to accelerate from zero step per second to the running rate or from any step rate to a different rate whether it is accelerating or decelerating.


Consider a motor consisting of a four step with two pole permanent magnet rotor as shown in Fig.
The rotor of such a motor aligns with the axis of the stator field with torque being proportional to the sin θ, where θ is the angle or displacement between the rotor axis and the stator field axis. The torque angle characteristic is shown in .Fig. with 'a' and ‘b’ coils excited. The rotor has assumed a position with θ = 45o, when 'b' above is excited the rotor takes a position θ = 90°. From this it can be observed that the stable position of the rotor corresponds to that angle at which the torque is zero and is positive for smaller angles and negative for larger angles. The torque has a maximum value at θ = 90°, Thus with phase 'a' excited, the stable position is θ = 0°. But not θ = 180° (unstable).

Therefore each excitation position corresponds to a unique position of the rotor. Another unique feature of permanent magnet stepper motor is that when excited it seeks a preferred position which offers an added advantage in certain applications.
Torque Angle Characteristics Permanent Magnet Stepper Motor

The different types of stepper motors are :

a. Variable reluctance stepper motor
b. Permanent magnet stepper motor  
c. Hybrid stepper motor.


The simplest of electrical machines is variable reluctance machines generally termed as VRM. It consists of a stator with excitation winding and a magnetic rotor with salient poles. It can be either singly salient or doubly salient. In either case there are no windings or permanent magnets on their rotors. Fig. shows both the types of VRM. In Fig. (a) the cross sectional view of singly salient VRM is shown to consists of non-salient stator and two pole rotor, both constructed of high permeability magnetic material. In the figure two phase windings are shown although any number of phases is possible. The inductance of each of the stator phase winding varies with rotor position such that the inductance is maximum when the rotor axis is aligned with the magnetic axis of that phase and minimum when the two axes are perpendicular.

In Fig. (b) cross section of doubly salient two phase VRM is shown. The salient stator has four poles each with a winding. The windings on opposite poles are of the same phase and can be considered to be connected in either series or parallel. Hereto the phase inductance varies from a maximum value when the rotor axis is aligned with the axis of that phase to minimum when they are perpendicular.


Fig.1 shows a variable reluctance motor with salient poles constructed from ferromagnetic material. The stator is made from stack of steel laminations and six equally placed poles each wound with an exciting coil. The rotor may be solid or laminated and has four projecting teeth of the same width as that of the stator. There are three independent stator circuit as shown in Fig.2 each one can be energized by a d.c. pulse from the drive circuit. The coils are excited in proper sequence and each phase has its own independent switch. Diametrically opposite pairs of the stator coils are connected in series such that when one tooth becomes north, the other becomes south. In practice the switching of phase currents is done with the help of solid state control. When there is no current in the stator coils, the rotor is free to rotate but when one or more stator coils are energized the rotor takes a step forward position to form least reluctance path with magnetized stator teeth. The step angle of three phase, four rotor teeth motor is given by

P = 360/4x3 = 30°


Fig (a) & (b) shows a 2-phase 4 pole permanent magnet stepper motor. The rotor is made of ferrite or any magnetic material which can be magnetized permanently. The stator stack of phase B is staggered from that of phase A by an angle of 90°. When phase A is excited the rotor is aligned as shown in Fig (a). If now phase B is also the effective stator pole shifts counter clockwise by 22 ½o causing the rotor to move by the same angle. If at this time, phase A is de-energized with phase B energized the rotor will move by another 22 ½o. The switching arrangement is shown in Fig compared to VRM, permanent magnet stepper motors operate at larger steps upto 90o at a maximum response rates at 300 pps. Permanent magnet stepper motors has a low power requirement but possesses a high detente torque as compared to a variable reluctance stepper motor. The motor has high inertia and so slower acceleration. It produces more torque per ampere stator current than a variable reluctance motor.

It is a combination of permanent magnet stepper motor and variable reluctance stepper motor. The rotor consists of a permanent magnet that is magnetized axially to create a pair of poles as shown in Fig. Two end caps are fitted at both ends of these axial magnets which consists of equal number of teeth which are magnetized by the respective polarities of the axial magnets. The rotor teeth of one end cap are offset by a half tooth pitch so that a tooth at one end cap coincides with the slot at the other. The end views of Hybrid stepper motor shown in Fig at XX' and YY' are shown in Fig. (a) & (b).
It can be seen from Fig. (a) and (b) the stator consists of two pair of poles and the rotor has five poles. When excited at one end the rotor poles are N and at the other end S. The step angle of such a motor is,

Step angle = (5 - 4) x 360o/ (5 x 4) = 18o


When phase A is excited, the north pole of the rotor attracts the south pole as shown in Fig (a). Taking this as initial position, phase A is de-energized and simultaneously phase B is energized. The rotor would turn counter clockwise by a full step of 18o. Next phases A and B are energized one after the other to produce a further rotation of the rotor by 18° in the same direction. The truth table for such operation is shown in Figure. To reverse the direction of rotation the phase sequence should be A+, B-, A-, B+, A+ etc. Even fractional step can be obtained by suitably proportioning the excitation of the two phases. Such stepping is called micro stepping. Typical step angles for stepper motors are 15°, 7.5°, 2° and 0.72°. The choice of the angle depends upon the angular resolution required for application.


Stepper motors are a digital device, its output being determined by the number of in-pulses. The motors can position itself at any desired value accurately depending on the input command which is in the form of pulses provided it is used in the response range. Simplicity in construction and economy in operation are the main advantages of the stepper motor. It is a device which is compatible to many of the applications. Some of the applications are given below :

a. In machine tool operations when the job demands different operations by different tools one after the other, the tool port has to rotate by a specific angle and move towards the job a distance which is pre-determined for each operation. At such places these motors serve its purpose.

b. In the X-Y plotter, the pen has to move specific distance on the axis accurately. These movements of the pen are controlled by these motors.

c. In computer application, the disc drives, the printer head movement and the paper feed movements has to be precisely accurate which are controlled by these motors.

d. In the case of the line printer these motors are used for driving the paper feed mechanism.

e. In military applications these motors are used in guns to position the target precisely in relation to the radar tracking.

Besides the above there are innumerable applications such as remote indicators, as positioners, etc. which are even expanding day by day.