Physics Ncert Solutions Class 12th

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer – (a, d)

Explanation – magnetic field lines for magnetic induction B form continuous lines. So lines of B are necessarily continuous across S.

Also H varies inside and outside. So lines of H cannot all be continuous across S

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer- (b)

Explanation- as curie law explains B $\propto \frac{\mathit{B}\left(\mathit{m}\mathit{a}\mathit{g}\mathit{n}\mathit{e}\mathit{t}\mathit{i}\mathit{c}\mathit{}\mathit{f}\mathit{i}\mathit{e}\mathit{l}\mathit{d}\mathit{}\mathit{i}\mathit{n}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\right)}{\mathit{t}\left(\mathit{t}\mathit{e}\mathit{m}\mathit{p}\mathit{}\mathit{i}\mathit{n}\mathit{}\mathit{k}\mathit{e}\mathit{l}\mathit{v}\mathit{i}\mathit{n}\right)}$

$\frac{I_2}{I_1}=\frac{B_2}{B_1}*\frac{t_1}{t_2}$

_{$I$ 1} =8Am^-1, B₁= 0.6T, t₁=4K

B₂= 0.2T, t₂=16K

So after solving we get I₂= 2/3Am^-

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer- (b)

Explanation- as gauss's law states $\int E.ds$ = q/ $?$ ₀ for electrostatic field. It does not contradicts for electrostatic fields as the electric field lines do not form continuous closed path

According to gauss's law in magnetic field

$\int E.ds$ =0

It contradicts for magnetic field, because there is a magnetic field inside the solenoid and no field outsid the solenoid carrying current but the magnetic field lines from the closed path.

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer- (d)

Explanation- the individual atoms in a ferromagnetic material possess dipole moment as in a paramagnetic material.

However they interact with one another in such a way that they spontaneously align themselves in a common direction over a microscopic volumes called domain. Thus they align perfectly.

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer- (a)

Explanation- for the earth's magnetism, the magnetic field lines of the earth resemble that of a hypothetical magnetic dipole located at the centre of earth.

The axis of the dipole not coincide with the axis of rotation of the earth but is presently tilted 11.3⁰ with respect to the later or it is either towards east or towards west

This is a Multiple Choice Questions as classified in NCERT Exemplar

Answer- (c)

Explanation- magnetic field outside the toroid is inversely proportional to distance

This is a Short Answer Type Questions as classified in NCERT Exemplar

Explanation- in the given figure let C be the amperian loop then,

 

${\int }_Q^{p}H.dl$ = ${\int }_Q^p\frac{B}{m_0}$ .dl

 between B and dl is less than 90 degree so

${\int }_Q^{p}H.dl$ = . ${\int }_Q^p\frac{B}{{\mu }_0}$ dl>0

So magnetic field from south pole to north pole inside the bar magnet

So according to ampere law

${\int }_{pQp}^{}H.dl$ =0

${\int }_{pQp}^{}H.dl$ = $\int H.dl$  +  $\int H.dl$  = 0

${\int }_Q^{p}H.dl$ >0, so ${\int }_p^{Q}H.dl$ <0,

It will be so if the angle between H and dl is more than 90 degree so cos $\theta$  is negative . it means H must run from north to south pole .

This is a Short Answer Type Question as classified in NCERT Exemplar

Explanation- I= moment of inertia of the bar magnet

m= mass of bar magnet

l= length of magnet about an any passing through its centre and perpendicular to its

length

M= magnetic moment of the magnet

B= uniform magnetic field in which magnet Is oscillating

T= 2 $\pi \sqrt[]{\frac{I}{MB}}$

I= ml²/12

When magnet is cut into two equal pieces

I¹= $\frac{m{(l/2)}^2}{2\left(12\right)}$ = ml²/96= I/8

Magnetic dipole moment M^'= M/2

Its time period oscillation is T'= 2 $\pi \sqrt[]{\frac{I}{MB}}$ = 2 $\pi \sqrt[]{\frac{I/8}{(M/2)B}}$ = $\frac{2\pi }{2}\sqrt[]{\frac{I}{MB}}$

This is a Short Answer Type Question as classified in NCERT Exemplar

Explanation- $\tau =MBsin\theta$

$\tau =pEsin\theta$

From these two we can say that

$MBsin\theta$ = $=pEsin\theta$

pE= MB

E=cB

pcB=MB

p=M/c

This is a multiple choice answer as classified in NCERT Exemplar

(c) $\varnothing$ =B.A= B₀ (2i+3j+4k)= 4B₀L²wb