Question

Question 3.18: 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 Ohm’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?

Class 12 - Physics - Current Electricity Current Electricity Page-129

Answer 1

(a) The current will be constant because it is given to be steady.

current density , it is the ratio of current to cross section area. but here in question, talking about non - uniform cords section area. hence , current density doesn't constant.

similarly electric field , E is also variable in non - uniform cross section . because current density is the product of conductivity and electric field.
e.g., $J=\sigma E},\frac{I}{A}=\sigma E$

$\implies E \propto\frac{1}{A}$

drift velocity is also variable in non- uniform cross section area. as you know, J = nev
here, J is current density, v is drift velocity , n is the number of electrons per unit volume and e is charge on electron.

(b) ohm's law is not a fundamental law in nature . it is not universally followed. in case of semiconductors, doides , transistors etc.. do not follow Ohm's law.

(c) if emf of supply battery is E and internal resistance r , then current through an external resistance R is given by I = E/(R + r)
so, internal resistance should be least to supt high currents.

(d) in high tension supply, the internal resistance is made large . because , if accidently the short circuiting take place ,the excessive current produced should not cross the safety limits.