Physics MCQs for NEET — Practice Questions with Answers

From Example 14.3, 'No! Any slab, howsoever flat, will have roughness much larger than the inter -atomic crystal spacing (~2 to 3 Ã…) and hence continuous contact at the atomic level will not be possible. The junction will behave as a discontinuity for the flowing charge carriers.'

Under 'POINTS TO PONDER', point 12 mentions, 'When such a junction is made, a ‘depletion layer’ is formed consisting of immobile ion-cores devoid of their electrons or holes. This is responsible for a junction potential barrier.'

The NCERT text states, 'The thickness of depletion region is of the order of one-tenth of a micrometre.'

The text explains, 'Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side. The motion of charge carriers due to the electric field is called drift.'

The NCERT states, 'The n-material has lost electrons, and p material has acquired electrons. The n material is thus positive relative to the p material.'

According to the NCERT text, 'The path taken by a fluid particle under a steady flow is a streamline. It is defined as a curve whose tangent at any point is in the direction of the fluid velocity at that point.' Option o1 is false because 'No two streamlines can cross, for if they do, an oncoming fluid particle can go either one way or the other and the flow would not be steady.' Option o2 is false because 'The velocity of a particular particle may change as it moves from one point to another.' Option o4 is false; streamlines are closer together where velocity is higher and can converge/diverge.

The equation of continuity for incompressible fluids states $Av = ext{constant}$. Here, $Av$ represents the volume flux or flow rate, which remains constant throughout the pipe of flow. While the density of an incompressible fluid is constant, the question asks what remains constant in the flow, and volume flux is directly derived from the continuity equation applied to the flow.

The NCERT text explicitly states, 'Equation (9.10) ... is called the equation of continuity and it is a statement of conservation of mass in flow of incompressible fluids.'

From the equation of continuity, $Av = ext{constant}$. If A decreases, then v must increase to maintain the constant product. The text says, 'Thus, at narrower portions where the streamlines are closely spaced, velocity increases and its vice versa.'

The NCERT text states, 'No two streamlines can cross, for if they do, an oncoming fluid particle can go either one way or the other and the flow would not be steady. Hence, in steady flow, the map of flow is stationary in time.' This precisely defines steady flow.