Question

Answer the following question:

(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 Ohms law universally applicable for all conducting elements?If not, give examples of elements which do not obey Ohms 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?

Answer 1

(a) Since current is given to be steady, it is constant. The current density, electric field and drift speed are inversely proportional to area of cross section.

(b) No, examples of non-ohmic elements are vacuum diode,semiconductor diode, etc.

(c) By Ohms Law, $I=V/R$. Now, if current required is high, the voltage should be high and the resistance should be low. Hence, a low voltage supply from which one needs high currents must have very low internal resistance.

(d) Any high tension supply must have a large internal resistance, because, if the circuit is shorted (accidentally), the current drawn will exceed safety limits. This can be dangerous for human life and can cause fatal accidents.