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To browse Academia. Skip to main content. By using our site, you agree to our collection of information through the use of cookies. To learn more, view our Privacy Policy. Log In Sign Up. Theraja, A. Volume I Basic Electrical Engineering.

Work, Power and Energy n. Magnetism and Electromagnetism Electromagnetic Induction Magnetic Hysteresis Electrochemical Power Sources Electrical Instruments and Measurements.. Ob- jective Tests. Value of H. Rectified A. Through Resistance, Inductance and Capacitance-A.

Complex Numbers Series A. Parallel A. Filter Networks Circle Diagrams Polyphase Circuits Value of a. Fourier Series Symmetrical Components Introduction to Electrical Energy Generation In time dt, distance travelled would be v x dt.

Obviously, all these electrons will cross the conductor cross-section in time dt. Current density i. It is denoted by the symbol J. Example 1. When a voltage is applied, a constant drift velocity of 1.

If the cross-sectional area of the material is 1 cm2,calculate the magnitude of the current. Electronic charge is 1. Electrical Engg. Aligarh Muslim University Solution. Charge Velocity and Velocity of Field Propagation The speed with which charge drifts in a conductor is called the velocity of charge. It is indepen-. Find the velocity of charge leading to 1 A current which flows in a copper conductor of cross-section 1 em2and length 10 km. How long will it take the electric charge to travel from one end of the conductor to the other.

It consists of copper plate known as anode and a zinc rod I. The chemical action taking place within the cell causes the electrons to be removed from Cu plate and to be deposited on the zinc rod at the same time.

This transfer of electrons is accomplished through the agency of the diluted H2S04 which is known as the electrolyte. The result is that zinc rod becomes negative due to the deposition of electrons on it and the Cu plate becomes positive due to the removal of electrons from it. The large number of electrons collected on the zinc rod is being attracted by anode but is prevented from returning to it by the force set up by the chemical action within the cell.

Conventional Direction of Current ,'-:' -- - -- - -- - - -- - - - - :-, The action of an electric cell is similar to that of a water pump which. Electric Current and Ohm's Law 3 It should be particularly noted that the direction of electronic current is from zinc to copper in the external circuit. However, the direction of conventional current which is given by the direction of flow of positive charge is from Cu to zinc. In the present case, there is no flow of positive charge as such from one electrode to another.

But we can look upon the arrival of electrons on copper plate with subsequent decrease in its positive charge as equivalent to an actual departure of positive charge from it. When zinc is negatively charged, it is said to be at negative potential with respect to the electrolyte, whereas anode is said to be at positive potential relative to the electrolyte. Between themselves, Cu plateis assumed to be at a higher potential than the zinc rod.

The difference in potential is continuously maintainedby the chemical action going on in the cell which supplies energy to establish this potential difference. Resistance It may be defmed as the property of a substance due to which it opposes or restricts the flow of electricity i.

Metals as a class , acids and salts solutions are good conductors of electricity. Amongst pure metals,silver, copper and aluminium are very good conductors in the given order. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat. Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.

Amongst good insulators can be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p. It is helpful to remember that electric friction is similar to friction in Mechanics. The Unit of Resistance The practical unit of resistance is ohm.

For insulators whose resistances are very high, a much bigger unit is used i. The symbol for ohm is Q. Table 1. Hence, it is used where economy of weight is more important than economy of space. Laws of Resistance The resistance R offered by a conductor depends on the following factors : i It varies directly as its length, I. If in Eq. Units of Resistivity From Eq.

It may, however, be noted that resistivity is sometimes expressed as so many ohm per m3. Although, it is incorrect to say so but it means the same thing as ohm-metre. If I is in centimetres and A in cm2,then p is in ohm-centimetre Q-cm. Values of resistivity and temperature coefficients for various materials are given in Table 1.

The resistivities of commercial materials may differ by several per cent due to impurities etc. Electric Current and Ohm's Law 5 Table 1. Resistivities and Temperature Coefficients Material Resistivity in ohm-metre Temperature coefficient at at C x JI C x Aluminium, commercial 2. A coil consists of turns of copper wire having a cross-sectional area of 0. The mean length per turn is 80 em and the resistivity of copper is 0. Find the resistance of the coil and power absorbed by the coil when connected across V d.

Pone Univ. May An aluminium wire 7. When a current of 5 A is passed through the combination, it is found that the current in the aluminium wire is 3 A. The diameter of the aluminium wire is 1 mm. Determine the diameter of the copper wire. Resistivity of copper is 0. May Solution. Let the subscript I represent aluminium and subscript 2 represent copper.

Baroda Solution. Calculate the resistance of 1 km long cable composed of 19 stands of similar copper conductors. Resistivity of copper may be taken as 1. Allowing for twist, the length of the stands. A lead wire and an iron wire are connected in parallel. Theformer carries 80 percent more current than the latter and the latter is 47 percent longer than the former. Determine the ratio of their cross sectional areas.


electrical technology by bl theraja volume 3








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