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MUST HAVE TO KNOW THINGS IN ELECTRICAL ENGINEERING
Fleming's right-hand rule (for generators) shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field. It can be used to determine the direction of current in a generator's windings.
Fleming's left-hand rule
Whenever a current carrying conductor is placed in a magnetic field, the conductor experiences a force which is perpendicular to both the magnetic field and the direction of current.
NOTE: Fleming's left hand rule is for motors and Fleming's RIGHT hand rule hand rule is for generators.
Conductance
It is defined as a special property of a conductor which determines how easily an current can flow through it.
Equation or Formula of Electrical Conductance
Let us take a piece of conductor of length l and cross sectional area A. If length of the conductor is increased, the electrons have to drift more paths. Hence more chance of inter atomic collision. That means current gets much harder path to travel, means electrical conductance of the conductor is reduced.
Thus conductance is inversely proportional to length of the conductor.
If cross sectional area of conductor is increased then current gets more drift electrons. Hence, conductance of the conductor is increased.
From equation (1) and (2),
Where, σ = constant of proportional known as conductivity or specific conductance.
Thus conductance is inversely proportional to length of the conductor.
If cross sectional area of conductor is increased then current gets more drift electrons. Hence, conductance of the conductor is increased.From equation (1) and (2),Where, σ = constant of proportional known as conductivity or specific conductance.Definition of Electrical Conductivity
Conductivity is a material of per unit volume.
Electrical conductivity is a basic property of material. Due to this property one material can conduct electricity. Some materials are good conductor of electricity that means current can pass through them very easily; again some materials do not allow current to flow through them. The material through which current passes easily, called good conductor of electricity in other words, the electrical conductivity of these materials is high. On the other hand the materials do not allow the current to flow through them are called electrical insulators. There are some materials whose electrical conductivity is not as high as conductor and also not as poor as insulator, they have an intermediate conductivity and these type of materials are known as semiconductors.
Electrical conductivity is a basic property of material. Due to this property one material can conduct electricity. Some materials are good conductor of electricity that means current can pass through them very easily; again some materials do not allow current to flow through them. The material through which current passes easily, called good conductor of electricity in other words, the electrical conductivity of these materials is high. On the other hand the materials do not allow the current to flow through them are called electrical insulators. There are some materials whose electrical conductivity is not as high as conductor and also not as poor as insulator, they have an intermediate conductivity and these type of materials are known as semiconductors.
Unit of Conductance
As we mentioned earlier conductance is reciprocal of resistance of resistance. That is,
Unit of resistance is ohm and that is why unit of conductance is generally written as mho - the reverse spelling of ohm. A modern electrical engineering, mho is named by Siemens.
Unit of resistance is ohm and that is why unit of conductance is generally written as mho - the reverse spelling of ohm. A modern electrical engineering, mho is named by Siemens.Unit of Conductivity
The equation of conductivity, we have already deducted as,
Hence, unit of conductivity is,
Here, S is Siemens.
Hence, unit of conductivity is,Here, S is Siemens.Resistance
Electrical resistance may be defined as the basic property of any substance due to which it opposes the flow of current through it.
If one volt across a conductor produces one ampere of current through it, then the resistance of the conductor is said to be one ohm (Ω).
The resistance of any substance depends on the following factors,
- The resistance of a substance depends on its length.
- The resistance of a substance depends on its cross sectional area.
- The resistance of a substance depends on the nature of material of the substance.
- The resistance of a substance depends on the temperature of the substance.
Resistance of the conductor increases with increasing length of the conductor. This relation is also linear. Electrical resistance R of a conductor or wire is
Where, L is the length of the conductor.
Where, A is the cross-sectional area of the conductor.
Combining these two laws we get,
Electrical resistance R of a conductor or wire is
Where, ρ (rho) is the proportionality constant and known as resistivity or specific resistance of the material of the conductor or wire. Now if we put, l = 1 and a = 1 in the equation,
We get, R = ρ. That means resistance of a material of unit length having unit cross - sectional area is equal to its resistivity or specific resistance.
Resistivity of a material can be alliteratively defined as the electrical resistance between opposite faces of a unit cube of that material.
Where, L is the length of the conductor.Second Law of Resistivity
The current through any conductor depends on the numbers of electrons pass through a cross-section of conductor per unit time. So, if cross section of any conductor is larger then more electrons can cross the cross section. Passing of more electrons through a cross-section per unit time causes more current through the conductor. For fixed voltage, more current means less Electrical resistance and this relation is linear. So it can be concluded like that, resistance of any conductor is inversely proportional to its cross-sectional area.Electrical resistance R of a conductor or wire isWhere, A is the cross-sectional area of the conductor.Combining these two laws we get,
Electrical resistance R of a conductor or wire is
Where, ρ (rho) is the proportionality constant and known as resistivity or specific resistance of the material of the conductor or wire. Now if we put, l = 1 and a = 1 in the equation,We get, R = ρ. That means resistance of a material of unit length having unit cross - sectional area is equal to its resistivity or specific resistance.Resistivity of a material can be alliteratively defined as the electrical resistance between opposite faces of a unit cube of that material.
The unit of resistivity is Ω-m in MKS system and Ω-cm in CGS system.
Electric Conductor
A conductor of electricity is a material or substance which allows to flow of electric current when subjected to a potential difference. This electric current is continue to flow till the potential difference exists. For a given potential difference, the density of electric current in conductor represents how efficient a conductor is. Based on the resistivity the conductors can be classified into two categories i.e. low resistivity/high conductivity materials and high resistivity/low conductivity materials.
In equilibrium condition the conductor exhibits the following properties –
- Resistance
- Inductance
- The electric filed inside the conductor is zero
- The charge density inside the conductor is zero
- Free charge exists only on the surface of the conductor
- At the conductor surface, the electric field is normal to the surface.
Resistance of Electric Conductor
Conductors of electricity generally possessed very low resistance for flow of electricity. Ideally the resistance of a perfect conductor is zero. However, practically the resistivity of conductors varies from low to high. The conductor having low resistivity/high conductivity are used as conductor for winding of electrical machines, for transmission line, for electrical contact, earth wire etc. The conducting materials having high resistivity/low conductivity are used for making filaments incandecent lamp and heating elements for electric heaters, Ovens, furnaces.Inductance of Electric Conductor
When a conductor is used on AC supply a magnetic flux is produced. Which is consists of two parts. Internal flux and external flux. The value of internal flux is very low as compare to external flux. Due to this flux linkage to conductor itself an inductance is come into picture. This inductance results in extra voltage drop in conductor. Moreover, this inductance is also effect the current distribution over the cross-section area of conductor. Due to which, current prefers to flow through outer part of cross-sectional area. This effect is called Skin effect. This current distribution over cross-sectional area is also effected by the flux linkage to conductor due to current following through nearby conductor. This is called Proximity effect These both effects Skin effect and Proximity effect exist only for AC supply. These effects do not exist for DC supply, as the flux produced by DC supply remains constant over the time.The Electric Field Inside the Conductor is Zero
The electrical field inside a perfect conductor is zero. If the electric field exists inside the conductor, it will extract a force on electron and accelerate them. But in equilibrium condition the net force on electron is zero. Hence, electric filed does not exists inside the conductor. Means the electric field must be external to the conductor. This property of conductor make it suitable to be used for electrostatic shielding for electrical equipment.The Charge Density Inside the Conductor is Zero
This electric charge does not exists inside the conductor. The mutual electrostatic repulsion force, between like charges i.e. electrons, demands that the electrons must be as far as possible. This electrostatic repulsion force pushes the electrons to the surface on conductor. Due to which there is no electric charge exists inside the conductor results in zero charge density inside the conductor.Free Charge Exists Only on the Surface of the Conductor
As discussed above, the charge particle does not exist inside the conductor. Due to electrostatic repulsion force, the electrons move to outer surface of the conductor. Due to which there is no electric charge exists inside the conductor. Hence, free electric charge exits only on the surface of the conductor.The Conductor Surface, The Electric Field is Normal to The Surface
If we go through the boundary condition of dielectric to conductor, the electric field is normal to the surface of conductor and tangent part of electric field to surface is zero. Means, the electric field intensity is normal to the surface of conductor and the tangential part of electric field intensity is zero.Magnetic Field
The space around a magnet within which the influence of the magnet can be predicted, is called magnetic field.
We can visualize or map a magnetic field with a small free to rotate magnetic needle. If we place the said magnetic needle in front of the North Pole of a bar magnet, the north end of the needle will face outwards from the North Pole of the bar magnet. If we push the magnetic needle towards the direction of the north end of the needle, the alignment of the needle will change further towards the South Pole of the magnetic bar. If the needle further proceeds along the alignment of north head of the needle, again the alignment of the needle is further changed towards the magnetic South Pole of the bar.
In this way if we move the magnetic needle and follow the alignment of needle head at each position of the needle, we will find that the needle will ultimately comes to the south pole of the magnetic bar following a typical curvature path.
This path of travelling a magnetic needle following the alignment of needle itself, from North to South Pole of a bar magnet is referred as magnetic lines of force. There are possibilities of number of such magnetic paths in a magnetic field as shown below.
This path of travelling a magnetic needle following the alignment of needle itself, from North to South Pole of a bar magnet is referred as magnetic lines of force. There are possibilities of number of such magnetic paths in a magnetic field as shown below.
We can map a magnetic field by using iron filings. Here, we keep a bar magnet below a horizontally placed card board. Now we will sprayed fine iron flings on the card. We will find that, the iron filings set themselves in the form of curve chains between north and south pole of the bar magnet. This curve chains of iron filings indicate the lines of force of the magnetic field of the bar magnet.
The lines of force is the line on which if we place a magnetic north pole it will experience the magnetic force in the direction along the tangent drawn on curve (lines of force) on that point. We can also define the line of force as a curve, along which a free to move unit north pole travels from north pole to south pole of a magnet due to influence of magnetic field.
The lines of force is the line on which if we place a magnetic north pole it will experience the magnetic force in the direction along the tangent drawn on curve (lines of force) on that point. We can also define the line of force as a curve, along which a free to move unit north pole travels from north pole to south pole of a magnet due to influence of magnetic field.
FLUX
Flux can be used in various concepts, such as
Magnetic Flux
It means the number magnetic field lines passing through a closed surface. Its SI unit is – Weber and in CGS is – Maxwell. It is denoted as Φm.
If we place an imaginary isolated unit north pole in a magnetic field it will experience a repulsive force from north pole and an attractive force from south pole of the magnet which has created the field. Due to these both forces, acting on the isolated unit north pole, the north pole will move along a particular path in the field if the pole is free to do so. If we place the same isolated unit north pole at different distance from the magnet in the field, it may follow a different path of travelling.
We call these paths of travelling of the unit north pole in the field, as lines of force. As we can place this imaginary isolated unit north pole at infinite number of points in the field, there may be infinite numbers of lines of force in the field. But visualize a magnetic field with infinite number of lines of force is useless for any scientific calculation. So we have to develop some unique concept, so that we can represent a magnetic field according to its entire strength. We take the unit of magnetic flux as weber. If a field has &hi; weber flux, it means the field has total φ number of lines of force. Like isolated north pole, the concept of lines of force in a magnetic field is also imaginary. It does not has any physical existence. This is only used for different magnetic calculation and for explaining different magnetic properties.
Properties of Magnetic Flux
- Magnetic flux of a filed is considered as the total number of magnetic lines of force in the field. These are also called magnetic flux lines.
- Each magnetic flux line is closed loop.
- Each magnetic flux line starts from north pole of a magnet and comes to the south pole through the field and continues from south pole to north pole in the body of the magnet.
- No two flux lines cross each other.
- Two similar lines of force travel side by side but repeal each other.
- The lines of force are stretched like elastic cord.
Magnetic Flux Density
The number of magnetic lines of force passing through a unit area surface perpendicular to the magnetic field is called magnetic flux density. If total φ Weber flux perpendicularly through a surface of area A m2, Magnetic flux density of the field would be,
We generally represent magnetic flux density by capital letter B.Electric Charge
Every matter in this universe is made of atoms. The atoms are electrically neutral. This is because, each atom has equal number of protons and electrons. Protons have positive charge. In an atom, protons sit in the central nucleus along with electrically neutral neutrons. The protons are strongly bounded in the nucleus. So, protons cannot be detached from the nucleus by any normal process. Each electron revolves round the nucleus in definite orbit in the atom. Electrons have negative charge. The quantity of electric charge of an electron is exactly equal to that of a proton but in opposite in nature. The electrons are negative and protons are positive. So, a piece of matter normally electrically neutral, since it is made of electrically neutral atoms.
The electrons are also bounded in the atoms but not all. Few of the electrons which are farthest from the nucleus may be detached by any means. If some of these detachable electrons of neutral atoms of a body, are removed, there will be a deficit of electrons in the body. After, removal of some of the detachable electrons from the neutral body, the total number of protons in the body becomes more than total number of electrons in the body. As a result the body will become positively charged.
Not only a body can give away electrons, it may also absorb some extra electrons, supplied from outside. In that case, the body becomes negatively charged.
So, deficit or excess of electrons in a body of matter is called electric charge.
Charge of an electron is very small and it is equal to
. So, total 
number of electrons have electric charge of 1 Coulomb. So, if a body deficits 
number of excess electrons, the body will be of 1 coulomb negative electric charge. number of electrons, the body will be of 1 coulomb positive electric charge. On the other hand, if a body has number of excess electrons, the body will be of 1 coulomb negative electric charge.
Not only a body can give away electrons, it may also absorb some extra electrons, supplied from outside. In that case, the body becomes negatively charged.
So, deficit or excess of electrons in a body of matter is called electric charge.
Charge of an electron is very small and it is equal to
. So, total number of electrons have electric charge of 1 Coulomb. So, if a body deficits number of excess electrons, the body will be of 1 coulomb negative electric charge. number of electrons, the body will be of 1 coulomb positive electric charge. On the other hand, if a body has number of excess electrons, the body will be of 1 coulomb negative electric charge.
Charged body is an example of static electricity. This is because, the electric charge is confined in the body itself. Here, the charge is not in motion.
But when the electric charge is in motion, it causes electric current. Electric charge has the potential of doing work. That means it has potential to either attract opposite nature of charge or repulse same nature of charge. A charge is the result of separating electrons and protons.
But when the electric charge is in motion, it causes electric current. Electric charge has the potential of doing work. That means it has potential to either attract opposite nature of charge or repulse same nature of charge. A charge is the result of separating electrons and protons.
Statement of Coulomb’s Law
First Law
Like charge particles repel each other and unlike charge particles attract each other.
Second Law
The force of attraction or repulsion between two electrically charged particles is directly proportional to the magnitude of their charges and inversely proportional to the square of the distance between them.
Formulas of Coulomb’s Law
According to the Coulomb’s second law,
Where,
Where,- ‘F’ is the repulsion or attraction force between two charged bodies.
- ‘Q1’ and ‘Q2’ are the electrical charged of the bodies.
- ‘d’ is distance between the two charged particles.
- ‘k’ is a constant that depends on the medium in which charged bodies are presented. In S.I. system, as well as M.K.S.A. system k=1/4πε. Hence, the above equation becomes.
- Coulomb’s law is valid, if the average number of solvent molecules between the two interesting charge particles should be large.
- Coulomb’s law is valid, if the point charges are at rest.
- It is difficult to apply the Coulomb’s law when the charges are in arbitrary shape. Hence, we cannot determine the value of distance ‘d’ between the charges when they are in arbitrary shape.
The value of ε0 = 8.854 × 10-12 C2/Nm2.
Hence, Coulomb’s law can be written for medium as,
Then, in air or vacuum εr = 1. Hence, Coulomb’s law can be written for air medium as,
The value of εr would change depends on the medium. The expression for relative permittivity εr is as follows;
Principle of Coulomb’s Law
Suppose if we have two charged bodies one is positively charged and one is negatively charged, then they will attract each other if they are kept at a certain distance from each other. Now if we increase the charge of one body keeping other unchanged, the attraction force is obviously increased. Similarly if we increase the charge of second body keeping first one unchanged, the attraction force between them is again increased. Hence, force between the charge bodies is proportional to the charge of either bodies or both.
Now, by keeping their charge fixed at Q1 and Q2 if you bring them nearer to each other the force between them increases and if you take them away from each other the force acting between them decreases. If the distance between the two charge bodies is d, it can be proved that the force acting on them is inversely proportional to d2.
This development of force is not same for all mediums. As we discussed in the above formulas, εr would change for various medium. So, depends on the medium, creation of force can be varied.Limitation of Coulomb’s Law
Electric Field Strength or Electric Field Intensity
The force acting on a unit positive charge inside an electric field is termed as electric field strength.
Electric field strength or electric field intensity is the synonym of electric field. Electric field strength can be determined by Coulomb's law.
According to this law, the force ‘F’ between two point charges having charge Q1 and Q2Coulomb and placed at a distance d meter from each other is given by,
Here, εo is the permittivity of vacuum = 8.854 × 10-12 F/m and εr is the relative permittivity of the surrounding medium.
According to this law, the force ‘F’ between two point charges having charge Q1 and Q2Coulomb and placed at a distance d meter from each other is given by,Here, εo is the permittivity of vacuum = 8.854 × 10-12 F/m and εr is the relative permittivity of the surrounding medium.
Now, let us put Q2 = + 1 Coulomb and let us denote force F by E in the equation (1), and by doing these we get,
This equation shows the force acting the a unit positive charge placed at a distance d from charge Q1.
As per definition this is nothing but of electric field strength of charge Q1 at a distance d from that charge.
Now, we got the expression of electric field strength or intensity. Now, by combining this expression with equation (1), we get,
The above expression shows that, if we place a charge at any point in an electric field, the product of the electric field strength at that point and the charge of the body gives the force acting on the body at that point in the field. The above expression can also be rewritten as,
Depending on this expression, the electric field strength can be expressed in Newton/Coulomb. The electric field strength has direction and hence it is vector quantity.
Intensity means the magnitude or amount. Now field intensity similarly means the magnitude of the strength of the field. Finally electric field intensity or strength can be written as,
So far we have discussed about the electric field intensity at a point due to the influence of a single charge, but there may be a case where, the point is under the filed of more than one charged bodies. In that case, we first have to calculate, the electric field strength at that point for individual charges and the we have to vectorially add up all the field strengths to get resultant field strength at that point.
This equation shows the force acting the a unit positive charge placed at a distance d from charge Q1.As per definition this is nothing but of electric field strength of charge Q1 at a distance d from that charge.Now, we got the expression of electric field strength or intensity. Now, by combining this expression with equation (1), we get,
The above expression shows that, if we place a charge at any point in an electric field, the product of the electric field strength at that point and the charge of the body gives the force acting on the body at that point in the field. The above expression can also be rewritten as,Depending on this expression, the electric field strength can be expressed in Newton/Coulomb. The electric field strength has direction and hence it is vector quantity.Intensity means the magnitude or amount. Now field intensity similarly means the magnitude of the strength of the field. Finally electric field intensity or strength can be written as,
So far we have discussed about the electric field intensity at a point due to the influence of a single charge, but there may be a case where, the point is under the filed of more than one charged bodies. In that case, we first have to calculate, the electric field strength at that point for individual charges and the we have to vectorially add up all the field strengths to get resultant field strength at that point.Electric Potential
Electric potential at a point in an electric field is defined as the amount of work to be done to bring a unit positive electric charge from infinity to that point.
If two electrically charged bodies are connected by a conductor, the electrons starts flowing from lower potential body to higher potential body, that means current starts flowing from higher potential body to lower potential body depending upon the POTENTIAL DIFFERENCE of the bodies and resistance of the connecting conductor.
So, electric potential of a body is its charged condition which determines whether it will take from or give up electric charge to other body. Electric potential is graded as electrical level, and difference of two such levels, causes current to flow between them. This level must be measured from a reference zero level. The earth potential is taken as zero level. Electric potential above the earth potential is taken as positive potential and the electric potential below the earth potential is negative.
So, electric potential of a body is its charged condition which determines whether it will take from or give up electric charge to other body. Electric potential is graded as electrical level, and difference of two such levels, causes current to flow between them. This level must be measured from a reference zero level. The earth potential is taken as zero level. Electric potential above the earth potential is taken as positive potential and the electric potential below the earth potential is negative.
The unit of electric potential is volt. To bring a unit charge from one point to another, if one joule work is done, then the potential difference between the points is said to be one volt. So, we can say,
Capacitor and Capacitance
Capacitor is a passive element that stores electric charge statistically and temporarily as an static electric field. It is composed of two parallel conducting plates separated by non-conducting region that is called dielectric, such as vacuum, ceramic, air, aluminum, etc.
The capacitance formula of the capacitor is represented by,
C is the capacitance that is proportional to the area of the two conducting plates (A) and proportional with the permittivity ε of the dielectric medium. The capacitance decreases with the distance between plates (d). We get the greatest capacitance with a large area of plates separated by a small distance and located in a high permittivity material. The standard unit of capacitance is Farad, most commonly it can be found in micro-farads, pico-farads and nano-farads.
Process of making this capacitor is quite simple. Take place of aluminium foil. Cover this foil with a waxed paper. Now, cover this waxed paper with another aluminium foil. Then roll up this whole thing as a cylinder. Put two metal caps at both ends of roll. This whole assembly is then encapsulated in a case. By rolling up, we make quite a large cross-sectional area of capacitor assembled in a reasonably smaller space.
The capacitance formula of the capacitor is represented by,
C is the capacitance that is proportional to the area of the two conducting plates (A) and proportional with the permittivity ε of the dielectric medium. The capacitance decreases with the distance between plates (d). We get the greatest capacitance with a large area of plates separated by a small distance and located in a high permittivity material. The standard unit of capacitance is Farad, most commonly it can be found in micro-farads, pico-farads and nano-farads.General uses of Capacitors
- Smoothing, especially in power supply applications which required converting the signal from AC to DC.
- Storing Energy.
- Signal decoupling and coupling as a capacitor coupling that blocks DC current and allow AC current to pass in circuits.
- Tuning, as in radio systems by connecting them to LC oscillator and for tuning to the desired frequency.
- Timing, due to the fixed charging and discharging time of capacitors.
- For electrical power factor correction and many more applications.
Charging a Capacitor
Capacitors are mainly categorized on the basis of dielectric used in them. During choosing a specific type of capacitors for a specific application, there are numbers of factors that get considered. The value of capacitance is one of the vital factors to be considered. Not only this, many other factors like, operating voltage, allowable tolerance stability, leakage resistance, size and prices are also very important factors to be considered during choosing specific type of capacitors.
We know that capacitance of a capacitor is given by,Hence, it is cleared that, by varying ε, A or d we can easily change the value of C. If we require higher value of capacitance (C) we have to increase the cross-sectional area of dielectric or we have to reduce the distance of separation or we have to use DIELECTRIC MATERIAL with stronger permittivity.
Hence, it is cleared that, by varying ε, A or d we can easily change the value of C. If we require higher value of capacitance (C) we have to increase the cross-sectional area of dielectric or we have to reduce the distance of separation or we have to use DIELECTRIC MATERIAL with stronger permittivity.
If we go only for the increasing area of cross-section, the rise of the capacitor may become quite large; which may not be practically acceptable. Again if we reduce only the distance of separation, the thickness of dielectric becomes very thin. But the dielectric cannot be made too thin in case its dielectric strength in exceeded.
Types of Capacitors
The various types of capacitors have been developed to overcome these problems in a number of ways.Paper Capacitor
It is one of the simple forms of capacitors. Here, a waxed paper is sandwiched between two aluminium foils.Process of making this capacitor is quite simple. Take place of aluminium foil. Cover this foil with a waxed paper. Now, cover this waxed paper with another aluminium foil. Then roll up this whole thing as a cylinder. Put two metal caps at both ends of roll. This whole assembly is then encapsulated in a case. By rolling up, we make quite a large cross-sectional area of capacitor assembled in a reasonably smaller space.
Air Capacitor
There are two sets of parallel plates. One set of plates is fixed and another set of plates is movable. When the knob connected with the capacitor is rotated, the movable set of plates rotates and overlapping area as between fixed and movable plates vary. This causes variation in effective cross-sectional areas of the capacitor. Consequently, the capacitance varies when one rotates the knob attached to the air capacitor. This type of capacitor is generally used to tune the bandwidth of a radio receiver.Plastic Capacitor
When various plastic materials are used as dielectric material, the capacitors are said to be plastic capacitors. The plastic material may be of polyester, polystyrene, polycarbonate or poly propylene. Each of these materials has slightly different electrical characteristics, which can be used to advantage, depending upon the proposed application.
This type of capacitors is constructional, more or less same as paper capacitor. That means, a thin sheet one of the earlier mentioned plastic dielectrics, is kept between two aluminium foils. That means, here the flexible thin plastic sheet is used as dielectric instead of waxed paper. Here, the plastic sheet covered by aluminium foil from two sides, is first rolled up, then fitted with metal end caps, and then the whole assembly is encapsulated in a case.
Plastic Film Capacitor
Plastic capacitor can be made also in form of film capacitor. Here, thin strips or films of plastic are kept inside metallic strips. Each metallic strip is connected to side metallic contact layer alternatively; as shown in the figure below. That means, if one metallic strip is connected to left side contact layer, then the very next is connected to right side contact layer. And there are plastic films in between these metallic strips. The terminals of this type of capacitors are also connected to side contact layer and whole assembly is covered with insulated non metallic cover.Silvered Mica Capacitor
A silvered mica capacitor is very accurate and reliable capacitor. This type of capacitors has very low tolerance. But on the other hand, cost of this capacitor is quite higher compared to other available capacitors in the market. But this high cost capacitor can easily be compensated by its high quality and performance. A small ceramic disc or cylinder is coated by silver compound. Here, electrical terminal is affixed on the silver coating and the whole assembly is encapsulated in a casing.Ceramic Capacitor
Construction of ceramic capacitor is quite simple. Here, one thin ceramic disc is placed between two metal discs and terminals are soldered to the metal discs.Whole assembly is coated with insulated protection coating.Mixed Dielectric Capacitor
The way of constructing this capacitor is same as paper capacitor. Here, instead of moving waxed paper as dielectric, paper impregnated with polyester is used as dielectric between two conductive aluminium foils.Electrolyte Capacitor
Very large value of capacitance can be achieved by this type of capacitor. But working voltage level of this electrolyte capacitor is low and it also suffers from high leakage current. The main disadvantage of this capacitor is that, due to the use of electrolyte, the capacitor is polarized. The polarities are marked against the terminals with + and – sign and the capacitor must be connected to the circuit in proper polarity.
A few micro meter thick aluminium oxide or tantalum oxide film is used as dielectric of electrolyte capacitor. As this dielectric is so thin, the capacitance of this type of capacitor is very high. This is because; the capacitance is inversely proportional to thickness of the dielectric. Thin dielectric obviously increases the capacitance value but at the same time, it reduces working voltage of the device. Tantalum type capacitors are usually much smaller in size than the aluminium type capacitors of same capacitance value. That is why, for very high value of capacitance, aluminium type electrolyte capacitors do not get used generally. In that case, tantalum type electrolyte capacitors get used.
Aluminium electrolyte capacitor is formed by a paper impregnated with an electrolyte and two sheets of aluminium. These two sheets of aluminium are separated by the paper impregnated with electrolyte. The whole assembly is then rolled up in a cylindrical form, just like a simple paper capacitor. This roll is then placed inside a hermetically sealed aluminium canister. The oxide layer is formed by passing a charging current through the device, and it is the polarity of this charging process that determines the resulting terminal polarity that must be subsequently observed. If the opposite polarity is applied to the capacitor, the oxide layer is destroyed.
Energy Stored in Capacitor
While CAPACITOR is connected across a BATTERY, charges come from the battery and get stored in the capacitor plates. But this process of energy storing is step by step only.
At the very beginning, capacitor does not have any charge or potential. i.e. V = 0 volts and q = 0 C.
Now at the time of switching, full battery voltage will fall across the capacitor. A positive charge (q) will come to the positive plate of the capacitor, but there is no work done for this first charge (q) to come to the positive plate of the capacitor from the battery. It is because of the capacitor does not have own voltage across its plates, rather the initial voltage is due to the battery.
At the very beginning, capacitor does not have any charge or potential. i.e. V = 0 volts and q = 0 C.
Now at the time of switching, full battery voltage will fall across the capacitor. A positive charge (q) will come to the positive plate of the capacitor, but there is no work done for this first charge (q) to come to the positive plate of the capacitor from the battery. It is because of the capacitor does not have own voltage across its plates, rather the initial voltage is due to the battery.
First charge grows little amount of voltage across the capacitor plates, and then second positive charge will come to the positive plate of the capacitor, but gets repealed by the first charge. As the battery voltage is more than the capacitor voltage then this second charge will be stored in the positive plate. At that condition a little amount of work is to be done to store second charge in the capacitor. Again for the third charge, same phenomenon will appear. Gradually charges will come to be stored in the capacitor against pre-stored charges and their little amount of work done grows up.
It can’t be said that the capacitor voltage is fixed. It is because of the capacitor voltage is not fixed from the very beginning. It will be at its maximum limit when potency of capacitor will be equal to that of the battery.
As storage of charges increases, the voltage of the capacitor increases and also energy of the capacitor increases.
So at that point of discussion the energy equation for the capacitor can’t be written as energy (E) = V.q
As the voltage increases the electric field (E) inside the capacitor dielectric increases gradually but in opposite direction i.e. from positive plate to negative plate.
Here dx is the distance between two plates of the capacitor.
Charge will flow from battery to the capacitor plate until the capacitor gains as same potency as the battery.
So, we have to calculate the energy of the capacitor from the very begging to the last moment of charge getting full.
It can’t be said that the capacitor voltage is fixed. It is because of the capacitor voltage is not fixed from the very beginning. It will be at its maximum limit when potency of capacitor will be equal to that of the battery.As storage of charges increases, the voltage of the capacitor increases and also energy of the capacitor increases.
So at that point of discussion the energy equation for the capacitor can’t be written as energy (E) = V.q
As the voltage increases the electric field (E) inside the capacitor dielectric increases gradually but in opposite direction i.e. from positive plate to negative plate.
Here dx is the distance between two plates of the capacitor.Charge will flow from battery to the capacitor plate until the capacitor gains as same potency as the battery.So, we have to calculate the energy of the capacitor from the very begging to the last moment of charge getting full.
Suppose, a small charge q is stored in the positive plate of the capacitor with respect to the battery voltage V and a small work done is dW.
Then considering the total charging time, we can write that,
Now we go for the energy loss during the charging time of a capacitor by a battery.
As the battery is in the fixed voltage the energy loss by the battery always follows the equation, W = V.q, this equation is not applicable for the capacitor as it does not have the fixed voltage from the very beginning of charging by the battery.
Now, the charge collected by the capacitor from the battery is
Now charge lost by the battery is
This half energy from total amount of energy goes to the capacitor and rest half of energy automatically gets lost from the battery and it should be kept in mind always.
Then considering the total charging time, we can write that,
Now we go for the energy loss during the charging time of a capacitor by a battery.As the battery is in the fixed voltage the energy loss by the battery always follows the equation, W = V.q, this equation is not applicable for the capacitor as it does not have the fixed voltage from the very beginning of charging by the battery.
Now, the charge collected by the capacitor from the battery is
Now charge lost by the battery isThis half energy from total amount of energy goes to the capacitor and rest half of energy automatically gets lost from the battery and it should be kept in mind always.Construction of Plate Capacitor
Capacitor is constructed by using two conducting surfaces or plates and an insulating material (i.e. Dielectric like mica, paper, air etc) between these two surfaces.Working of Capacitor
As a capacitor is passive component, it does not generate energy. But it is able to store energy from an energy source like a battery or another charged capacitor. When a battery(DC Source) is connected across a capacitor, one surface, named plate I gets positive end of the battery and another surface, named plate II gets negative end of the battery. When battery is connected, the full voltage of that battery is applied across that capacitor. At that situation, plate I is in positive potency with respect to the plate II.
Current from the battery tries to flow through this capacitor from its positive plate (plate I) to negative plate (plate II) but cannot flow at max value due to separation of these plates with an insulating material. Rather a very small current will flow through this insulating material (dielectric) from Positive to Negative plate depending upon the value of strength of this dielectric.
An electric field will form inside the capacitor dielectric from positive to negative plate. As time goes on, positive plate (plate I) will accumulate positive charge from the battery and negative plate (plate II) will accumulate negative charge from negative end of the battery. After a certain time, the capacitor holds maximum amount of charge as per its capacitance with respect to this voltage. This time span is called charging time of this capacitor.
Now, after removing this battery from this capacitor, these two plates will hold positive and negative charges with respect to a certain voltage level for long time. Thus this capacitor acts as energy source.
If two ends (plate I and plate II) get shorted through a load, a current will flow through this load from plate I to plate II up to all charges get vanished from both plates. This time span is known as discharging time of the capacitor.
An electric field will form inside the capacitor dielectric from positive to negative plate. As time goes on, positive plate (plate I) will accumulate positive charge from the battery and negative plate (plate II) will accumulate negative charge from negative end of the battery. After a certain time, the capacitor holds maximum amount of charge as per its capacitance with respect to this voltage. This time span is called charging time of this capacitor.
Now, after removing this battery from this capacitor, these two plates will hold positive and negative charges with respect to a certain voltage level for long time. Thus this capacitor acts as energy source.If two ends (plate I and plate II) get shorted through a load, a current will flow through this load from plate I to plate II up to all charges get vanished from both plates. This time span is known as discharging time of the capacitor.How does a Capacitor Respond in DC?
Suppose a capacitor is connected across a battery through a switch. When switch is ON, i.e. t = 0+, a certain value of current will flow through this capacitor. After a certain time (i.e. charging time) capacitor never allow current to flow through it further. It is because of maximum number of charges are accumulated on both surfaces and capacitor acts as a battery which has positive end connected to the positive end of the battery and negative end connected to the negative end of the battery with same potency. Due to zero potential difference between battery and capacitor, no current will flow through it. So, it can be said that, initially a capacitor is short circuited and finally open circuited when it gets connected across a battery.How does a Capacitor Respond in AC?
Suppose a capacitor is connected across an AC source. Consider, at a certain moment of positive half of this alternating voltage, plate I gets positive polarity and plate II negative polarity. Just at that moment plate I accumulates positive charges and plate II accumulates negative charges. But at the negative half of this applied AC voltage, plate I gets negative charges and plate II positive charges. And so on. There is no flow of electron between these two plates as they change their polarity with the change of source polarity. The capacitor plates get charged and discharged alternatively by the AC.Types of Capacitor
The types of capacitor are as follows:Polarized Capacitor
Polarized Capacitors are broadly Classified into following catagories.Electrolytic Capacitor
Aluminum Electrolytic Capacitor
- Non Solid
- Hybrid Polymer
- Solid Polymer
- Non Solid
- Solid MnO2
- Solid Polymer
- Solid MnO2
- Solid Polymer
- Class I
- Class II
- Class III
- Class IV
- Class I
- Class II
- Class III
- Class IV
Non Polarized Capacitor
- Metal Insulated Semiconductor Capacitor
- Ceramic Capacitor
- Class I
- Class II
- Film Capacitor
- Metalized (Paper as Dielectric)
- Film/Foil (PP Film, PET Film, PEN Film, PPS Film, PTFE Film)
RMS or Root Mean Square Value of AC Signal
Why rms values are used in AC system?
What does an average and rms value mean?
Why all the ratings of AC systems are in rms not in average value?
What is the difference between rms and average value?
These are the questions which come in our minds every time when we are dealing with AC circuits.
Suppose, we have a simple DC circuits (figure - 1) and we want to replicate it in an AC circuit. We got every thing same, except supply voltage which is now to be an AC supply voltage. Now, the question is what should be the value of AC supply voltage so that our circuit works exactly same as that of DC.
Let us put same value of AC supply voltage (AC Vpeak = 10 volt) which is in our DC circuits. By doing that we can see (figure 3) for a half cycle how the AC voltage signal is not covering up the whole area (blue area) of constant DC voltage, which means our AC signal can not supply the same amount of power as our DC supply.
What does an average and rms value mean?
Why all the ratings of AC systems are in rms not in average value?
What is the difference between rms and average value?
These are the questions which come in our minds every time when we are dealing with AC circuits.
Suppose, we have a simple DC circuits (figure - 1) and we want to replicate it in an AC circuit. We got every thing same, except supply voltage which is now to be an AC supply voltage. Now, the question is what should be the value of AC supply voltage so that our circuit works exactly same as that of DC.Let us put same value of AC supply voltage (AC Vpeak = 10 volt) which is in our DC circuits. By doing that we can see (figure 3) for a half cycle how the AC voltage signal is not covering up the whole area (blue area) of constant DC voltage, which means our AC signal can not supply the same amount of power as our DC supply.
Which means we must increase the AC voltage to cover the same area and see if it is supplying the same amount of power or not.
We found that (figure 4) by increasing the peak voltage Vpeak up to (π/2) times of DC supply voltage we can actually cover the whole area of DC in AC. When the AC voltage signal completely represents the DC voltage signal then that value of DC signal is called the "average value" of AC signal.
Now our AC voltage should supply the same amount of power. But when we switched-on the supply surprisingly, we found that AC voltage is supplying more power than the DC. Because an average value of AC supplies same amount of charges but not the same amount of power. So, to get same amount of power from our AC supply we must decrease our AC supply voltage.
We found that by decreasing the peak voltage Vpeak up to √2 times DC voltage we get same amount of power flowing in both the circuits. When the AC voltage signal supply same amount of power as in DC then that value of DC voltage is called root mean square or rms value of AC.
We are always concerned about how much power is flowing through our circuits irrespective of how much electrons are needed to supply that power and that is the reason why we always use the rms value of AC supply instead of average value everywhere in AC system.
We found that (figure 4) by increasing the peak voltage Vpeak up to (π/2) times of DC supply voltage we can actually cover the whole area of DC in AC. When the AC voltage signal completely represents the DC voltage signal then that value of DC signal is called the "average value" of AC signal.Now our AC voltage should supply the same amount of power. But when we switched-on the supply surprisingly, we found that AC voltage is supplying more power than the DC. Because an average value of AC supplies same amount of charges but not the same amount of power. So, to get same amount of power from our AC supply we must decrease our AC supply voltage.We found that by decreasing the peak voltage Vpeak up to √2 times DC voltage we get same amount of power flowing in both the circuits. When the AC voltage signal supply same amount of power as in DC then that value of DC voltage is called root mean square or rms value of AC.We are always concerned about how much power is flowing through our circuits irrespective of how much electrons are needed to supply that power and that is the reason why we always use the rms value of AC supply instead of average value everywhere in AC system.
Conclusion
Average value of an AC current represent the equal amount of charges in DC current.
RMS value of an AC current represent the equal amount of power in DC current
Average value of an AC current represent the equal amount of charges in DC current.
RMS value of an AC current represent the equal amount of power in DC current
AC current takes less amount of charges to supply the same amount of DC power.
Active and Passive Elements
Active elements of an electrical circuit are those elements which can continuously give as well as take energy to and from the circuit, respectively.eg Voltage source and Current source.
Passive elements of an electric circuit are those elements which cannot deliver or absorb energy continuously. As per this definition, resistance, inductance and capacitance are taken as basic passive elements of an electric circuit.
Resistor:
Resistor is taken as passive element since it can dissipate energy as heat as long as current flows through it but in any situation a resistor cannot deliver energy to the circuit. Resistance is the property of any substance by which it can resist the flow of current through it. An electrical conductor has very low resistance, whereas an insulator has very high electrical resistance. The unit of resistance is ohm, which can be represented by the symbol Ω.
Inductor:
An inductor is also considered as passive element of circuit, because it can store energy in it as magnetic field, and can deliver it to the circuit, but not in continuous basis. The energy absorbing and delivering capacity of an inductor are limited and transient in nature. That is why, an inductor is taken as passive element of a circuit.
Capacitor:
For same reason, a capacitor is considered as passive element, because it can store energy in it as electric field and deliver it to the circuit, but not in continuous basis. The energy dealing capacity of a capacitor is limited and transient too.
Resistor is taken as passive element since it can dissipate energy as heat as long as current flows through it but in any situation a resistor cannot deliver energy to the circuit. Resistance is the property of any substance by which it can resist the flow of current through it. An electrical conductor has very low resistance, whereas an insulator has very high electrical resistance. The unit of resistance is ohm, which can be represented by the symbol Ω.
Inductor:
An inductor is also considered as passive element of circuit, because it can store energy in it as magnetic field, and can deliver it to the circuit, but not in continuous basis. The energy absorbing and delivering capacity of an inductor are limited and transient in nature. That is why, an inductor is taken as passive element of a circuit.
Capacitor:
For same reason, a capacitor is considered as passive element, because it can store energy in it as electric field and deliver it to the circuit, but not in continuous basis. The energy dealing capacity of a capacitor is limited and transient too.
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