Electrics and Electrical Phenomenon - Essay Example

Electrics A. Definitions i. Coulomb (C d after a famous French Physicist, it is the unit of electrical charge. One coulomb is defined as the amount of charge transferred by a current of 1 ampere in 1 second. ii. Ampere (A or Amp): It is the measure of electrical current. One ampere is the flow of one coulomb of charge per second in an electric conductor. iii. Volt (V): It is the measure of potential difference between two points. It is the equivalent of the electromotive force applied to a conductor that will produce a current in the conductor. The difference in potential sets the direction of current flow in the conductor. As per the prevailing standards most of homes around the world use 110 volt or 220 volt electrical connections. Accordingly the electrical appliances are manufactured by the industry. iv. Watt (W): This is the unit of power or energy. One unit of power is i.e. one watt is equivalent to 1 joule of energy per second. It is basically a measure of the rate of energy consumption. All the electrical appliances like bulb, refrigerator, television sets, computer, printer, iron consume some amount of electricity which decides their wattage. As a thumb rule, more the wattage more will be electrical consumption by that product. v. Farad (F): It is the SI unit of capacitive charge. An electric capacitor is made up of two parallel plates. As a result of electrical current some charge is stored on the plates of the capacitor. This charge is measured in Farad. One Farad is the capacitance having an equal and opposite charge of 1 coulomb on each plate and a voltage difference of 1 voltage between the plates. If the charge stored is less in quantity, it is measured in Micro-Farad (F) or Pico-Farad (pF). vi. Henry (H): It is the measure of inductive force produced in an inductor. Electromotive force is produced when we vary the current in an inductor. One volt of electromotive force (emf) is produced when the current is varied at the rate of one ampere per second. The unit is named after Joseph Henry, an American scientist of 18th century, who discovered electromagnetic induction. B. Electrical Phenomenon a. Temperature Coefficient of Resistance: This coefficient depicts the effect of temperature on the resistance of an electrical conductor. Increase or decrease of temperature affects the movement of the molecules within an atom. As temperature rises the movement increases, which in turn results in more collision amongst the molecules, thus impacting the specific resistance of the material. Therefore, in general the value of resistance (measured in ohms) of a material will depend upon the temperature coefficient of the resistance for the conductor. The total resistance for a material can be defined as; R = Rref [1 + (T-Tref)] Ohms or Where, R = Resistance of the conductor at a temperature, 'T' Rref = Resistance of conductor at a reference temperature of Tref. The Tref is usually 200C and sometimes for experimental purposes it is 00C. = Temperature coefficient of resistance for conductor material. T = Conductor temperature in degrees Celsius T = Reference temperature at which the Temp. Coefficient for the material is defined b. Relative Permittivity of Substance (r): It is the Ratio of the electric field strength in vacuum to that in a given medium. i.e. r = / o Where is the permittivity of the substance and o is the permittivity of the free space. The relative permittivity is also known as the dielectric strength of the substance. The dielectric constant is a complex constant with the real part giving reflective surface properties. The relative permittivity values affect the magnetic and electric behaviour of a conductor. The value of relative permittivity of a given substance keeps varying depending upon the electrical frequency, temperature etc. c. Magnetic Hysteresis: The Hysteresis is a magnetic property of a substance. This phenomenon is generally observed in ferromagnetic substances like Iron, nickel, cobalt etc. This group is called ferromagnetic as iron or 'ferric' is an important component of this group. When the ferromagnetic material is magnetized in one direction by applying electric field strength, the substance is magnetized to a certain level. And when we remove the electric field strength, ideally the magnetic field level should come to zero, but in reality it doesn't become zero. So once again an electric field is applied in the opposite direction, this brings the magnetic field level to zero. Subsequently when the electric field is again brought to zero, the substance starts showing magnetic properties. Gradually this takes the shape of a loop, known as hysteresis loop. This property of retaining the magnetic property in the form of a 'memory' is used for manufacturing magnetic memories for audio tapes and computers. In general the magnetic flux generated is proportional to the magnetic induction. i.e. B = H = r o Where, B = Magnetic flux density, H = Magnetic Field strength, r = relative permeability of the substance, o.= permeability of free space or magnetic constant d. Flemings Right Hand Rule: This is a very important rule determining the direction of the induced electromotive force of a conductor moving in a magnetic field. To illustrate the rule, the Thumb, first finger and second finger of the right hand are held in such a position that each one is at right angles to each other. If the thumb is supposed to indicate the direction of motion of the electric conductor, and the first finger (index finger) indicates the magnetic field then the second finger (middle finger) will point towards the induced emf in the conductor. This rule is also known as the 'Generator Rule', as this very rule is used while generating voltage from an electric generator. There's another similar rule for motors. This is called Fleming's left hand rule, which illustrates the functioning of motors. Both these rules were named after their inventor John Ambrose Fleming, a British engineer. e. Time Constant: Generally denoted by the Greek letter tau (), the time constant is a measure of the frequency response of signal processing systems, magnetic tapes and memories. The time period it takes for the charge of an electric circuit to decay or reach a predefined level is known as the time constant. In general, the time constant is defined as the time required to for the charge to rise from zero to 1-1/e i.e. (1-1/2.72) or 63.2 percent of the full charge. In can also be defined during the discharging process of a fully charged component as well. It is the time for the charge reach a value of 1/e i.e. 1/2.72 or 36.8 percent of the original (full charge) value. Therefore, for an RC circuit it will be defined as the time required for charging of the capacitor (C) to 63.2 percent of full charge or to discharge a fully charged capacitor to 36.8 percent of its initial voltage. Similarly for an RL (i.e. resistance-inductance circuit), the time constant is defined as, time required for the current to reach a value of 63.2 percent of full value from zero. In other words it can be defined as the time for the RL circuit to reach the current from full value to a value of 36.8 percent. For an RC circuit, the time constant is, = RC For an RL circuit the time constant is, = R/L f. Resonance in R-L-C circuits: In an electric circuit containing Resistance (R), Inductance (L) and Capacitance (C), the net impedance is the vector sum of all the three components. But there are certain frequencies at which a circuit acts as a purely resistive circuit. This situation is termed as Resonance. Ideally, while the current passes through an inductor, it leads in phase by an angle of 900 from voltage in the circuit. On the other hand, the voltage across a capacitor leads the current by 900. This property of these components is used in designing a resonant circuit. At resonance the net impedance of the circuit becomes Resistive, because XL and XR, the resistive components of Inductance and Capacitance nullify each other. At resonance the circuit thus becomes a tuned circuit. The frequency of the circuit at series resonance is; f0 = 1 / (2LC) Hz g. Power Factor Improvement: Power factor plays a crucial role in the effective generation and distribution of power supply. Power factor in general is defined as the ratio of working power to the apparent power. Because it the ratio of the powers, therefore it is a unit-less quantity. PF = KW/KVA An ideal circuit is the one when the values of both the numerator and the denominator are the same i.e. the total amount of power being supplied to the circuit is reaching to the output without any loss in the reactive components. Therefore the maximum value of PF is unity i.e. 1 (for a purely resistive circuit). The total KVA of a circuit or component comprises of the active power as well as the reactive power. It is the inductive power, which causes harm to the efficiency levels during transmission and distribution. Therefore efforts are made to minimise the inductive power by using adequate amount of capacitive compensation, which in turn results in improvement of the power factor. h. Transformer Short Circuit Test: Transformers in general are used to step up or step down a voltage. The number of winding turns in primary and secondary prove of crucial importance in the functioning of the transformer. For the purpose of calculations and drawing the characteristics of a transformer, we perform two types of tests, namely short-circuit test and open circuit test. In the short-circuit test, the low voltage winding of the transformer is short circuited. This way, the impendence from the low voltage side is transformed to the high voltage side. This way we can determine; The Equivalent transformer impedance (ZP) The Equivalent winding resistance (RP) The Equivalent leakage reactance (XP) Short circuit in fact means that we need only a very small primary voltage (VSC) for the secondary rated (ISC) current to flow. i. Relative Permeability of a Substance: This is a measure of the resistance offered by a substance to the applied magnetic field strength. The magnetic permeability in general is defined as the ratio of magnetic flux density in a medium to the magnetizing force. i.e. = B/H But in Engineering we generally define the permeability in relative terms instead of absolute terms. The relative permeability is therefore defined as the ratio of the permeability of a substance to the permeability of a vacuum at the same magnetic field strength. i.e. r = /0 Where 0 is the permeability of free space (or magnetic constant), 0 = 4 107 The value of relative permeability of a material tells us about the magnetic properties of the material. Diamagnetic materials have a relative permeability less than 1. These materials have least amount of magnetic properties. Paramagnetic materials have relative permeability of about 1. These materials can pick up some magnetic properties under certain conditions. Ferromagnetic materials with relative permeability much more than 1 are known to have strong magnetic properties. j. Transformer Open Circuit Test: Like the short circuit test, the open circuit test of a transformer also helps in determining its parameters by transferring the impedances on one side. For the open circuit test the secondary winding is left open circuited. Thereafter we connect a wattmeter, a voltmeter, and an ammeter on the primary side of the circuit. The primary is then fed with the primary current and we measure the secondary output voltage. With the help of an open circuit test, we can determine; Equivalent core resistance (RM) Equivalent core reactance (XM) Turns Ratio (a) Since no load is connected across the secondary therefore during the calculations we neglect the loss in resistance and reactance. Read More