Answer the following questions : (a) A steady current flows in a metallic conductor of non-uniform cross-section. Explain which of these quantities is constant along the conductor : current, current density, electric field and 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 low internal resistance, why ?
(d) A high tension `(HT)` supply of say `6 kV` must have a very large internal resistance. why ?

Explanation

(a) Only current through the conductor of non-uniform are of cross-section is constant as the remaining quantities vary inversely with the area of cross-section of the conductor.
(b) Ohm's law is not applicable for non-ohmic elements. For example, vaccum tubes, semi-conducting diode, liquid electroyle etc.
(c) As, `I_(max)` = e.m.f./internal resistance, so for maximum current, internal resistance should be least.
(d) A high tension supply must have a large internal resistance otherwise, if accidently the circuit is shorted, the current drawn will exceed safety limit and will cause damage to circuit.