Page 39 - Electrician - TT (Volume 1)
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ELECTRICIAN - CITS




           Potential difference
           The difference of electrical potential between two points in an electric circuit. It’s always less than EMF. Its symbol
           is V and is also measured in volts. The instrument that measures the P.D. is called ‘VOLTMETER ’. It is always
           connected in parallel with the circuit whose voltage is to be measured.
           Conductance
           The property of a conductor to conduct electricity is called conductance. Its symbol is G It unit is ‘mho’. Good
           conductors have good conductance.
           Resistance
           The property of a material to oppose the flow of electric current through them is called resistance. The resistance
           of each material is depending upon its composition, length, area and temperature. Its unit is ohm (Ω).
           Representation of emf, current & resistance
                Quantity           Symbol         Unit of Measurement     Unit Abbreviation
                Current             1              Ampere (“Amp”)               A

                Voltage             E or V         Volt                         V
                Resistance          R              Ohm                          Ω

           Ohm’s law (Fig 6)
           G .S. Ohm established this law which gives the relation between current, voltage and resistance in a closed
           electric circuit.
           OHM’s law states that in a closed circuit the current t(I) flowing through a conductor at constant temperature
           is directly proportional to the voltage(V) across the conductor and inversely proportional to its resistance(R).
           I α V

           I=V/R

              Fig 6








           Laws of resistance
           The resistance, R offered by conductors depends on the following factors
           1  The resistance of the conductor varies directly with its length(l).

           2  The resistance of the conductor varies inversely proportional to cross sectional area(a).
           3  The resistance of the conductor depends on the material with which it is made of(ρ)
           It also depends upon temperature of the conductor. Ignoring the last factor for the time being we can say that
           R=ρ l/a

           Specific resistance (Resistivity) (Fig 7): It may be defined as the resistance offered to a current if passed
           between  the  opposite  faces  of  the  unit  cube  of  the  material.  Specific  resistance  (resistivity)  is  measured  in
           ohm-centimeter.

           Problem: Q. If a10m of Eureka wire 0.14 cm in diameter has the resistance of 2.5Ω. find the specific resistance.
           Solution
           Length of wire, L=1000 cm
           Diameter, d=.14cm

           Resistance=2.5Ω

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                                    CITS : Power - Electrician & Wireman - Lesson 5 - 12
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