# Physics Class 12 NCERT Solutions: Chapter 3 Current Electricity Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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

[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 .