| Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Mark the correct choice as: |
| Assertion (A): Current flows in a conductor only when there is an external electric field within the conductor. |
| Reason (R): The drift velocity of the electrons is directly proportional to the electric field. |
A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A)
B) Both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A)
C) Assertion (A) is true but Reason (R) is false
D) Assertion (A) is false and Reason (R) is also false
Correct Answer: A
Solution :
(a) In the absence of electric field, the electrons move randomly in all directions. Hence, there is no net motion of electrons and no current flow in the conductor. But in the presence of electric field, electrons moves in a particular direction. Each electron experiences a force in the direction opposite to that of the electric field and moves from the negative end to the positive end of the conductor. Thus there is a current flow. The drift velocity, \[\overrightarrow{v\,}d=-\frac{e}{m}\overrightarrow{E}\,\tau \,i.e.,\overrightarrow{v}\,d\propto \mu \overrightarrow{E}\].
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