NEET Practice Questions (MCQs) with Answers & Solutions

According to the NCERT text 'Thus, Gauss’s law for magnetism is: The net magnetic flux through any closed surface is zero.' This law reflects the non-existence of isolated magnetic poles (monopoles).

The NCERT text states: '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.' Since magnetic monopoles don't exist, magnetic field lines always form closed loops, implying no net flux through a closed surface. Electrostatic field lines originate from positive charges and end on negative charges.

Gauss's law for electrostatics states that $\oint \vec{E} \cdot d\vec{A} = q/\epsilon_0$, where $q$ is the enclosed electric charge. If magnetic monopoles existed, they would act as sources or sinks for magnetic field lines, similar to electric charges. Therefore, Gauss's law for magnetism would become $\oint \vec{B} \cdot d\vec{A} = \mu_0 q_m$, where $q_m$ is the enclosed magnetic monopole charge.

Magnetic field lines form closed loops, originate from the North pole and end at the South pole (outside the magnet, and continue inside from South to North), and do not intersect. Magnetic fields can pass through conductors; for example, a current-carrying wire produces a magnetic field in the surrounding space, including within the wire itself if considered.

The NCERT text explicitly states: 'Thus, Gauss’s law for magnetism is: The net magnetic flux through any closed surface is zero.' Mathematically, magnetic flux is given by the surface integral of the magnetic field, so $\oint \vec{B} \cdot d\vec{A} = 0$.

The NCERT text states: 'There are no sources or sinks of B; the simplest magnetic element is a dipole or a current loop.'

The text explains this: '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.' Since magnetic field lines don't begin or end at points (like electric field lines do on charges), they must form continuous closed loops. Any line entering a closed surface must also exit it, resulting in a net flux of zero.

Since the net magnetic flux through any closed surface is zero, any magnetic flux emerging from the North pole (considered positive) must be balanced by an equal amount of magnetic flux entering the South pole (considered negative) within the same closed surface. This is a direct consequence of magnetic field lines forming closed loops.

The NCERT text states: 'Ampere’s law is to Biot-Savart law, what Gauss’s law is to Coulomb’s law. Both, Ampere’s and Gauss’s law relate a physical quantity on the periphery or boundary (magnetic or electric field) to another physical quantity, namely, the source, in the interior (current or charge).' Here, current acts as the source for the magnetic field in Ampere's law, similar to how electric charge acts as the source for the electric field in Gauss's law for electrostatics.

For an electric dipole, the total enclosed charge is $q + (-q) = 0$. According to Gauss's law for electrostatics ($\oint \vec{E} \cdot d\vec{A} = q_{\text{enclosed}}/\epsilon_0$), if the net enclosed charge is zero, the net electric flux through the closed surface is also zero.