NCERT Class XII Chapter
Current Electricity
Questions With Solutions
© examsnet.com
Question : 18
Total: 24
Answer the following questions:
(a) A steady current flows in a metallic conductor of non-uniform crosssection.
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 large internal resistance. Why?
(a) A steady current flows in a metallic conductor of non-uniform crosssection.
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 large internal resistance. Why?
Solution:
(a) The current will be constant because it is given to be steady.
Current density, J =
The electric field E is also variable in non uniform cross-section.
J = σE
Drift speed also changes in non uniform cross-section.
J =n e v d
v d
v d
(b) Ohm’s law is not a fundamental law in nature. It is not universally followed. Semiconductor diodes, transistors, thermistors, vacuum tubes etc. do not follow Ohm’s law.
(c) If emf of supply battery is E and internal resistance r, then current through an external resistance R is given by
I =
so, internal resistance r should be least to supply high currents.
(d) In high tension supply, the internal resistance is made large. Because, if accidently the short circuiting take place, the excessive current produced should not cross the safety limits.
Current density, J =
The electric field E is also variable in non uniform cross-section.
J = σE
Drift speed also changes in non uniform cross-section.
J =
(b) Ohm’s law is not a fundamental law in nature. It is not universally followed. Semiconductor diodes, transistors, thermistors, vacuum tubes etc. do not follow Ohm’s law.
(c) If emf of supply battery is E and internal resistance r, then current through an external resistance R is given by
I =
so, internal resistance r should be least to supply high currents.
(d) In high tension supply, the internal resistance is made large. Because, if accidently the short circuiting take place, the excessive current produced should not cross the safety limits.
© examsnet.com
Go to Question: