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Basic Electrical Engineering

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Basic Electrical Engineering BASIC ELECTRICAL ENGINEERING BY: KUMAR AVINASH CHANDRA GUEST FACULTY Email: [email protected] Contact: +91 7837 11 4644 DR. A.P.J.A.K. W. I. T., DARBHANGA ELECTRIC CIRC U IT • An electric circuit is a closed path consisting of active and passive elements all interconnected and the current flow is confined to the closed path. • An Active element is one which supplies energy to the circuit where as a Passive element is one which receives energy and then this energy is converted into heat (resistor) or stores it in an electric (capacitor) or magnetic field (inductor). ELECTRIC CURRENT • The basic unit of charge is the charge on an electron. The mks unit of charge is coulomb. An electron has a charge of 1 .062 x l0-19 C. • An electric current is defined as the time rate of flow of charge through a certain section. Its unit is ampere. • A current is said to be of one ampere when a charge of 1 coulomb flows through a section per second. • Mathematically, i = 푑푞 푑푡 ELECTRIC POTENTIAL • The movement of charges contribute to current and the amount of work done per unit charge is the potential difference between the two points. • The electronic charges flow from a lower potential to a higher potential and these contribute to electronic current, whereas the conventional current is considered to flow from higher potential to lower potential. • If a differential charge dq is given a differential energy dw, the rise in potential of the charge. Mathematically, v = 푑푤. 푑푞 • The units of potential or potential difference as derived from equation are joule/coulomb or s termed as volts. RESISTANCE • A insulator is one which opposes the flow of current. • The derived units for resistance is ohm, its quantity symbol is R and the unit symbol is Q. The unit for electric resistance is defined as R = 푉 (r = 푉 ) 퐼 퐴 • If the potential difference between point a and b is say 1 V when the current in the circuit is 1 A, the resistance between points a and b is said to he of 1 Q. • A circuit element designed to have property of resistances is known as a resistor and its circuit symbol is shown in Fig RESISTIVITY • The value of resistances R of a conductor is directly proportional to its lengths i.e. R oc l 푙 Also, if the cross sectional area of the cylindrical wire is varied, the resistance is found to be inversely proportional to the cross-sectional area A i.e. R oc Τ퐴 These two proportions can be combined and we have R oc 푙 퐴 and by using a constant of proportionality p the proportion becomes the equation and R = p 푙 Ω 퐴 In order to find the units for p we substitute the units of other quantities Ohm = p 푙푒푛푡ℎ = p 푚푒푡푟푒 퐴푟푒푎 푚푒푡푟푒2 or p = Ohm-metre Thus p, the resistivity of the material or specific resistance is the resistance of a material measured between opposite forces. CONDUCTIVITY AND CONDUCTANCE • The specific resistance or resistivity p of a material represents its opposition to the flow of electric current. However the reciprocal of p represents a unit describing the material in terms of its ease to allow flow of electric current. y = 1/p known as the conductivity of the material. Similarly, the reciprocal of resistance R is the conductance G i.e. G = 1/R. The SI derived units for conductance from Table i. 1 is the Siemens. The quantity symbol is G and the unit symbol is S. The Siemens is derived as G A = V , S, where A is amperes. Conductance also given in mho and its symbol . However, the use of unit mho must be avoided and Siemens should be used. POWER If potential is multiplied by current 푑푞, we have 푑푡 V x I = 푑푊 * 푑푞 = 푑푊 푑푞 푑푡 푑푡 which gives rate of change of energy with time and is equal to power. The SI derived unit for power (P) is watt expressed as Joule per sec. ENERGY We have, P = 푑푊 푑푡 dW = Pdt = VI dt W = VI dt and energy of a device is defined as the capacity of doing the work and its derived unit is watts-sec. THANKYOU Email: [email protected] Contact: +91 7837 11 4644 Next Topic: Ohm’s Law.
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