# Physics Class 12 NCERT Solutions: Chapter 3 Current Electricity Part 8

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Q: 18. Answer the following questions:

(A) A steady current flows in a metallic conductor of non-uniform cross section 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 Ohm’s law.

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

(D) A high tension (HT) supply of, say, must have a very large internal resistance

Answer:

[A] When a steady current flows in a metallic conductor of non-uniform cross-section, the current flowing through the conductor is constant. Current density, electric field, and drift speed are inversely proportional to the area of cross-section. Therefore, they are not constant.

[B]No, Ohm’s law is not universally applicable for all conducting elements. Vacuum diode semi-conductor is a non-Ohmic conductor. Ohm’s law is not valid for it.

[c] According to Ohm’s law, the relation for the potential is

Voltage (V) is directly proportional to current (I).

R is the internal resistance of the source,

If V is low, then R must be very low, so that high current can be drawn from the source.

[d] In order to prohibit the current from exceeding the safety limit, a high tension supply must have a very large internal resistance. If the internal resistance is not large, then the current drawn can exceed the safety limits in case of a short circuit.

Q: 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 manganese 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 .

Answer:

(A) Alloys of metals usually have greater resistivity than that of their constituent metals.

(B) Alloys usually have lower temperature coefficients of resistance than pure metals.

(B) The resistivity of the alloy, manganese, is nearly independent of increase of temperature.

(D) The resistivity of a typical insulator is greater than that of a metal by a factor of the order of.