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 say `6 kV` must have a very large internal resistance. why ?

Explanation

(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 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 `V = IR`
Voltage (V) is directly proportional to current `(I)`
R is the internal resistance of the source
`I=(V)/(R)`
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 safely limit, a high tension supply must have a very large internal resistance. If the inernal resistance is not large, then the current drawn can exceed the safety limits in case a short circuit.