Magnetic Effects of Current and Magnetism MCQs for NEET — Physics Questions with Answers

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.'

From 'POINTS TO PONDER' (Point 1 on page 151): 'A satisfactory understanding of magnetic phenomenon in terms of moving charges/currents was arrived at after 1800 AD. But technological exploitation of the directional properties of magnets predates this scientific understanding by two thousand years.'

Referring to the 'POINTS TO PONDER' table (page 151), the unit for Magnetic intensity (H) is listed as A m$^{-1}$. Also, on page 146, it states that H is measured in units of A m$^{-1}$.

From 'POINTS TO PONDER' (Point 6 on page 152): 'Diamagnetism is universal. It is present in all materials. But it is weak and hard to detect if the substance is para- or ferromagnetic.'

Magnetic monopoles do not exist; field lines form closed loops, so net flux through any closed surface is zero.

$\vec F = I\vec L\times\vec B = IL\,\hat i\times(2\hat i+3\hat j-4\hat k) = IL(3\hat k + 4\hat j)$. $|\vec F| = IL\sqrt{9+16} = 5IL$.

Semicircle: $\mu_0 i/(4R)$ out of page. Two semi-infinite straight portions: each $\mu_0 i/(4\pi R)$ into page, total $\mu_0 i/(2\pi R)$. Net $= (\mu_0 i/4R)(1 - 2/\pi)$, out of page.