Answer the following questions: (a) A steady current flows in a metallic conductor of non-uniformcross-section. Which of these quantities is constant along theconductor: 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 musthave very low internal resistance. Why? (d) A high tension (HT) supply of, say, 6 kV must have a very largeinternal resistance. Why?
a)
The current density, electric field and drift velocity are inversely proportional to the area of cross-section of a conductor. Current is the only quantity that does not depend on the area of cross section. It depends on the potential difference across the conductor.
Thus, current is constant along the conductor.
b)
The Ohm’s law is applicable only for those conductors in which the current is directly proportional to the voltage applied across the conductor. Some of the conductor does not follow this proportionality. For example; Vacuum diode semi-conductor is non-ohmic conductor. It does not follow the Ohm’s law.
Thus, Ohm’s law is not applicable for all conductors.
c)
According to ohm’s law,
V = I R I = V R
Where, V is the voltage, I is the current and R is the internal resistance.
As voltage is low therefore, we need a supply of low resistance to draw high current from the source.
d)
To disallow the flow of current from exceeding the safety limit, a high tension supply must have a large internal resistance. Because in short-circuit condition, current is very high and if internal resistance is not much high, element will blow out. Thus, to maintain the safety limit resistance should be very high.
a)
The current density, electric field and drift velocity are inversely proportional to the area of cross-section of a conductor. Current is the only quantity that does not depend on the area of cross section. It depends on the potential difference across the conductor.
Thus, current is constant along the conductor.
b)
The Ohm’s law is applicable only for those conductors in which the current is directly proportional to the voltage applied across the conductor. Some of the conductor does not follow this proportionality. For example; Vacuum diode semi-conductor is non-ohmic conductor. It does not follow the Ohm’s law.
Thus, Ohm’s law is not applicable for all conductors.
c)
According to ohm’s law,
Where,
As voltage is low therefore, we need a supply of low resistance to draw high current from the source.
d)
To disallow the flow of current from exceeding the safety limit, a high tension supply must have a large internal resistance. Because in short-circuit condition, current is very high and if internal resistance is not much high, element will blow out. Thus, to maintain the safety limit resistance should be very high.
