Q17 of 30 Page 128

What conclusion can you draw from the following observations on a resistor made of alloy manganin?













































Current


A



Voltage


V



Current


A



Voltage


V



0.2



3.94



3.0



59.2



0.4



7.87



4.0



78.8



0.6



11.8



5.0



98.6



0.8



15.7



6.0



118.5



1.0



19.7



7.0



138.2



2.0



39.4



8.0



158.0


From the given information shown in the table, the ratio of voltage and current is a constant value and the value is equal to 19.7 Hence manganin is a Ω ic conductor.

According to Ohm’s law the ratio of voltage and current is resistance of conductor so Resistance of manganin is 19.7 Ω .


More from this chapter

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15

A secondary cell after long use has an emf of 1.9 V and a large internal resistance of 380Ω. What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?

 16

Two wires of equal length, one of aluminium and the other of copper have the same resistance. Which of the two wires is lighter? Hence explain why aluminium wires are preferred for overhead power cables. (ρAl = 2.63 × 10–8 Ωm, ρCu = 1.72 × 10–8 Ωm, Relative density of Al = 2.7, of Cu = 8.9.)

18

Answer the following questions:

(a) A steady current flows in a metallic conductor of non-uniform cross-section. Which of these quantities is constant along the conductor: current, current density, electric field, drift speed?


(b) Is Ohm’s law universally applicable for all conducting elements? If not, give examples of elements which do not obey Ω ’s law.


(c) A low voltage supply from which one needs high currents must have very low internal resistance. Why?


(d) A high tension (HT) supply of, say, 6 kV must have a very large internal resistance. Why?

 19

Choose the correct alternative:

(a) Alloys of metals usually have (greater/less) resistivity than that of their constituent metals.


(b) Alloys usually have much (lower/higher) temperature coefficients of resistance than pure metals.


(c) The resistivity of the alloy manganin is nearly independent of/ increases rapidly with increase of temperature.


(d) The resistivity of a typical insulator (e.g., amber) is greater than that of a metal by a factor of the order of (1022/1023).