Physics MCQs for NEET — Practice Questions with Answers

From the 'MOVING_CHARGES_AND_MAGNETISM' section, it's mentioned: 'These lines called magnetic field lines form closed loops. This is unlike the electrostatic field lines which originate from positive charges and end at negative charges.'

The equation $\oint \vec{B} \cdot d\vec{A} = 0$ means that the net magnetic flux passing through any closed surface is zero. This is a direct consequence of the non-existence of magnetic monopoles, implying that magnetic field lines have no starting or ending points (sources or sinks) within a closed surface, and thus must form closed loops, entering and leaving the surface in equal amounts.

Gauss's Law for Magnetism states, 'The net magnetic flux through any closed surface is zero.' This holds true irrespective of what is enclosed by the surface (assuming no magnetic monopoles). It's distinct from Ampere's Law, which relates the line integral of B to the enclosed current.

The text states: 'Ampere’s circuital law is not new in content from Biot-Savart law... Ampere’s law is to Biot-Savart law, what Gauss’s law is to Coulomb’s law.'

The text highlights this key difference: 'The difference between the Gauss’s law of magnetism and that for electrostatics is a reflection of the fact that isolated magnetic poles (also called monopoles) are not known to exist... These lines called magnetic field lines form closed loops. This is unlike the electrostatic field lines which originate from positive charges and end at negative charges.'

In the absence of an electric field, electrons move randomly. If we consider all the electrons, their average velocity will be zero since their directions are random. (NCERT, Section 3.5, page 85)

From the NCERT text, the average velocity $v_d$ is given by $v_d = -\frac{eE\tau}{m}$. This is derived from averaging the acceleration experienced by electrons between collisions. (NCERT, Eq. 3.17, page 86)

The amount of charge crossing area A in time $\Delta t$ is $I\Delta t$. This is also equal to $neA|v_d|\Delta t$. Therefore, $I = neA|v_d|$. (NCERT, Eq. 3.18, page 86)

The relaxation time ($\tau$) is defined as the average time between successive collisions of an electron with the positive ions in the conductor. The NCERT text states, 'The average value of $t_i$ then is $\tau$ (known as relaxation time).' (NCERT, page 86)

Each 'free' electron does accelerate, increasing its drift speed until it collides with a positive ion of the metal. It loses its drift speed after collision but starts to accelerate and increases its drift speed again only to suffer a collision again and so on. On the average, therefore, electrons acquire only a drift speed. (NCERT, Example 3.2 (b), page 88)