Ferromagnetic Substance – Magnetism & Matters | Class 12 Physics Notes

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Ferromagnetic substances

The substances which when placed in a magnetic field are strongly magnetised in the direction of the magnetising field are called ferromagnetic substances.

(1) Each atom of ferromagnetic material posseses permanent magnetic dipole moment before application of external magnetic field.

(2) The unpaired electron of one atom interacts with electron of neighbouring atom through quantum mechanical exchange interaction and align in a common direction over a small volume of material. These small volume of uniform magnetisation are called domains which are small in size $\left(10^{-18} \mathrm{m}^{3}\right)$ but contain large number of atomic dipoles $\left(\sim 10^{11}\right)$

(3) In absence of external field the domains may be randomly oriented so that their resultant magnetic dipole moment in any direction is zero.

(4) If presence of external field the magnetic dipole moment increases due to :

(a) Displacement of boundaries of domains. Here size of domain with magnetic dipole moment parallel to applied field increases while for others it decreases.

(b) Rotation of domains. Here domains rotate until their magnetic dipole moments are aligned parallel to direction of external field.

(5) In presence of weaker fields displacement of boundaries takes place while in stronger fields rotation of domains takes place.

(6) When external magnetic field is removed substance is not totally demagnetised and some magnetism is left in specimen.

(7) On heating it loses properly of ferromagnetism at certain temperature called Curie temperature and becomes paramagnetic. On cooling it becomes ferromagnetic again. The Curie temperature for iron is $770^{\circ} \mathrm{C}$.

(8) Ferromagnetic substances are paramagnetic substances which acquire very high magnetism in external magnetic field. The two differ in the order of their intensity of magnetism.

Properties of Diamagnetic, paramagnetic & Ferromagnetic Materials

Forces || Newton Laws Of motion || Class 11 Physics Notes

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# FORCE

A force is a push or a pull.

[1] since push or a pull has both magnitude and direction, so that force is a vector quantity.

[2] Forces occur in pairs. If object A exerts a force on object $\mathrm{B},$ then $\mathrm{B}$ also exerts a force on $\mathrm{A}$. For example, when a bat strikes a ball the bat exerts a force on the ball, but the ball exerts a force on the bat also.

[3] A force can cause an object to accelerate. If you kick a foot ball the ball’s velocity changes while your foot is in contact with it.

[4] A force can deform an object. As you can see from fig. The ball when hits the floor, is deformed by the contact force exerted on it by the floor. The floor is deformed too, but since it is harder than the ball, its deformation is not as noticeable.

Property $4,$ that a force causes an object to be deformed is often used to measure a force. This is the principle of a spring scale.

A spring scale indicates the amount of the spring is stretched or compressed. The magnitude of this force is proportional to the amount of the spring is stretched (or compressed), and the direction of the force is along the spring. The scale may be calibrated to read in newton (N), units of force in SI system is newton and in $\mathrm{C} . \mathrm{G} . \mathrm{S} .$ system is dyne.

1 newton $=10^{5}$ dyne.

$1 \mathrm{kg} \mathrm{wt}=\mathrm{g}$ newton.

Dimensional formula of force is $\mathrm{M}$ L $\mathrm{T}^{-2}$

Dimensional formula of force is $\mathrm{M} \mathrm{L} \mathrm{T}^{-2}$

## TYPES OF FORCES

(a) Field Forces or Range Forces:

These are the forces in which contact between two objects is not necessary.

Ex.

(i) Gravitational force between two bodies.

(ii) Electrostatic force between two charges.

(iii) Weight of a body.

(b) Contact Forces:

When a force involves direct contact between two bodies. One body exerts a force on another because of the contact between them. These forces originate with the atoms of each body – each atom

exerts a force on its neighbour (which may be an atom of another body). A contact force can be maintained only if it does not exceed the interatomic forces within either of the bodies; otherwise the binding between atoms can be overcome and the string or the surface will split into pieces. Contact forces include the pushes or pulles with your hand.

Ex.

(i) Force of rope pulling on a block to which it is tied.

(ii) Tension in string, and normal forces.

(iii) Frictional force

(c) Attachment to Another Body:

Tension (T) in a string and spring force (F=kx) comes in this group.

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