Physics MCQs for NEET — Practice Questions with Answers

The drift speed of electrons is much smaller, about $10^{-5}$ times the typical thermal speed at ordinary temperatures. Thermal speeds are typically of the order of $2 \times 10^2$ m/s, while drift speeds are around $1.1 \times 10^{-3}$ m/s. (NCERT, Example 3.1 (b) (i), page 87-88)

Electric field is established throughout the circuit, almost instantly (with the speed of light) causing at every point a local electron drift. Establishment of a current does not have to wait for electrons from one end of the conductor traveling to the other end. (NCERT, Example 3.2 (a), page 88)

The current density $j$ gives the amount of charge flowing per second per unit area normal to the flow, and is related to drift velocity by $j = nq v_d$. For electrons, $q = -e$, so the magnitude is $|j| = n e v_d$. (NCERT, Point 7 in Summary, page 103)

Simple, because the electron number density is enormous, $\sim 10^{29} \text{ m}^{-3}$. A vast number of charge carriers, even moving slowly, can transport significant charge. (NCERT, Example 3.2 (c), page 88)

In the absence of an electric field, the paths are straight lines; in the presence of an electric field, the paths are, in general, curved. This is due to the acceleration caused by the electric field between collisions. (NCERT, Example 3.2 (e), page 88)

Comparing the vector form of Ohm's law, $\vec{j} = \sigma \vec{E}$ (Eq. 3.13), with the derived equation $\vec{j} = \frac{ne^2\tau}{m} \vec{E}$ (Eq. 3.22), we find that $\sigma = \frac{ne^2\tau}{m}$ (Eq. 3.23). (NCERT, Eq. 3.22 and 3.23, page 86-87)

The electric field E is directed towards the left. Since electrons carry negative charge, their drift velocity is opposite to the electric field. Therefore, the net charge transported (due to negative electrons moving right) is towards the right. (NCERT, page 86, paragraph starting 'Because of the drift...')

This result is surprising. It tells us that the electrons move with an average velocity which is independent of time, although electrons are accelerated. This is the phenomenon of drift... (NCERT, page 86, and also Example 3.2 (b) explains that collisions cause loss of acquired speed).

An important quantity is the mobility $\mu$ defined as the magnitude of the drift velocity per unit electric field: $\mu = |v_d| / E$. (NCERT, Eq. 3.24, page 89)

The current I is given by $I = neA|v_d|$. If I, n, and e are constant, then $A|v_d|$ must be constant. Therefore, if A increases, $|v_d|$ must decrease to maintain the constant product. (Derived from NCERT, Eq. 3.18, page 86)