Question

Answer the following question:
(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 voltages supply from which one needs high currents must have low internal resistances, why?
(D) A high tension (HT) supply of say 6KV must have a very large internal resistance. Why?

Answer 1

Hint: The ohm law states that for any circuit the electric current is directly proportional to the voltage and is inversely proportional to the resistances. Therefore, one ohm is equal to the one volt/ampere. The device which does not obey ohm's law is also stated as non-ohmic devices.

Step-By-Step answer:
(A) According to the question it is given:
Current$ \to $ constant
$
  I = neA{V_d} \\ \Rightarrow {V_d} = \dfrac{I}{{neA}} \\
 $
Where n is the number of electrons moving through the conductor, e is the electrons and A is the area of cross section.
As it is given non uniform cross section so A and ${V_d}$ both will vary
In case of, current density, electric field and drift speed will vary because all three quantities are inversely proportional to area. Let us assume alpha we get, $\alpha = \dfrac{I}{A}$

(B) No, the ohm’s law is universally not applicable for all conducting elements moreover, examples of elements which do not obey ohm’s law are semiconductors, transistors, diodes. The device which does not obey ohm's law also stated as non-ohmic devices.
(C) As we all are aware of general formula i-e;
$V = IR{\text{ }}and{\text{ I = }}\dfrac{V}{R}$
Always remember for low voltage we required low resistance.

(D) A high tension (HT) supply of say 6KV must have a very large internal resistance so that just to be on a safe side internal resistance is kept at higher or high tension supplies. If accidentally the wire gets short it makes sure that it causes less damage.

Note: High tension is always twisted because it makes a conductor more flexible and more exposed to air moreover it has a feature that helps to cool the conductor and therefore it increases its conductivity.