CBSE Class 12 Physics 2019 Delhi Set 1 Paper

Question : 20

Total: 27

(a) State Gauss's law for magnetism. Explain its significance.
(b) Write the four important properties of the magnetic field lines due to a bar magnet.
OR
Write three points of differences between para, dia and ferro- magnetic materials, giving one example for each.

Solution:

(a)Statement of Gauss's law in magnetism
Its significance
(b) Four Important properties
(a) Gauss's law for magnetism states that "The total flux of the magnetic field, through any closed surface, is always zero.

Alternatively,

∮

→

⋅

→

=0
This law implies that magnetic monopoles do not exist / magnetic field lines form closed loops.
(b) Four properties of magnetic field lines.

(i) Magnetic field lines always form continuous closed loops.

(ii) The tangent to the magnetic field line at a given point represents the direction of the net magnetic field at that point.

(iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field.

(iv) Magnetic field lines do not intersect.
OR
Three points of difference
One example of each

	Diamagnetic	Paramagnetic	Ferromagnetic
1	−1≤χ<0	−1<χ<ε	χ>>1
2	0≤µr<1	0≤µr<(1+ε)	µr>>1
3	µ<µ0	µ>µ0	µ>>µ0

Where ε is any positive constant.
Examples (Any one example of each type)
Diamagnetic materials : Bi,Cu,Pb,Si, water, NaCl, Nitrogen (at STP)
Paramagnetic materials : Al,Na,Ca, Oxygen (at STP), Copper chloride
Ferromagnetic materials : Fe,Ni,Co,Alnico

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