NEET Practice Questions (MCQs) with Answers & Solutions

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)

According to the NCERT text, 'In a dielectric, this free movement of charges is not possible. It turns out that the external field induces dipole moment by stretching or re-orienting molecules of the dielectric. The collective effect of all the molecular dipole moments is net charges on the surface of the dielectric which produce a field that opposes the external field. Unlike in a conductor, however, the opposing field so induced does not exactly cancel the external field. It only reduces it.' For a conductor, 'the electric field due to induced charges opposes the external field within the conductor. This happens until, in the static situation, the two fields cancel each other and the net electrostatic field in the conductor is zero.'

The NCERT states: 'In an external electric field, the positive and negative charges of a non-polar molecule are displaced in opposite directions. The displacement stops when the external force on the constituent charges of the molecule is balanced by the restoring force (due to internal fields in the molecule). The non-polar molecule thus develops an induced dipole moment.'

The text states: 'In a non-polar molecule, the centres of positive and negative charges coincide. The molecule then has no permanent (or intrinsic) dipole moment. Examples of non-polar molecules are oxygen (O$_2$) and hydrogen (H$_2$) molecules which, because of their symmetry, have no dipole moment.'

The NCERT states: 'The dipole moment per unit volume is called polarisation and is denoted by P.'

The NCERT explicitly states the formula: 'For linear isotropic dielectrics, $P = \epsilon_0 \chi_e E$ (2.37)' where $\chi_e$ is the electric susceptibility.

The NCERT explains: 'However, at the surfaces of the dielectric normal to the electric field, there is evidently a net charge density... The unbalanced charges are the induced charges due to the external field. Thus, the polarised dielectric is equivalent to two charged surfaces with induced surface charge densities... We should note that the surface charge density $\pm \sigma_p$ arises from bound (not free charges) in the dielectric.'