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

Diamagnetic substance are weekly magnetised in a direction opposite to that of applied magnetic field. These are repelled in an external magnetic field i.e., have a tendency to move from high to low field region. 

The work done in rotating a magnetic dipole against the torque acting on it, when placed in magnetic field is stored inside it in the form of potential energy.

When magnetic dipole is rotated from initial position $\theta ={\theta }_1$ to final position $\theta ={\theta }_2,$ then work done =MB$\left(\cos {\theta }_1-\cos {\theta }_2\right)$

=MB$\left(1-\frac{1}{2}\right)$

=$\frac{2\times {10}^4\times 6\times {10}^{-4}}{2}=6J$

Ferromagnetism decreases with rise in temperature. If we heat a ferromagnetic substance, then at a definite temperature the ferromagnetic property of the substance suddenly disappears and the substance becomes paramagnetic. The temperature above which a ferromagnetic substance becomes paramagnetic is called the curie temperature of the substance. 
Note: The Curie temperature of Iron is ${770}^{°}C$ and that of Nickel in ${358}^{°}C$.

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.