Class 12 Physics
CBSE and UP Board
Current Electricity
Additional Exercise
Chapter-3 Exercise 3.18
NCERT Solutions Class 12 Physics Chapter 3 Question 18
Answer the following questions: (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 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 very low internal resistance. Why? (d) A high tension (HT) supply of, say, 6 kV must have a very large internal resistance. Why?
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Ans (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.
Ans (b).
No, Ohm’s law is not universally applicable for all conducting elements. Vacuum diode semi-conductor is a non-ohmic conductor. Ohm’s law is not valid for it.
Ans (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.
Ans (d).
In order to prohibit the current from exceeding the safety limit, a high tension supply must have a very large internal resistance. If the internal resistance is not large, then the current drawn can exceed the safety limits in case of a short circuit.