TS Inter 2nd Year Physics Notes Chapter 8 Magnetism and Matter

Here students can locate TS Inter 2nd Year Physics Notes 8th Lesson Magnetism and Matter to prepare for their exam.

TS Inter 2nd Year Physics Notes 8th Lesson Magnetism and Matter

→ Earth behaves as a huge magnet with magnetic field pointing approximately along geographic north and south directions.

→ Every magnet has two poles namely North pole and South pole. Magnetic monopole is an imaginary concept.

→ Like poles will repel and unlike poles will attract

→ When a magnet is divided into two parts they will behave as two separate weak magnets.

→ Magnets can be made with iron and its alloys.

→ A bar magnet consists of north pole and south pole is also called magnetic dipole. Its behaviour is similar to an electric dipole of a positive and negative charge.

→ Magnetic field lines: The path followed by a free magnetic needle will represent a magnetic line of force.

  • Magnetic field lines are closed curves.
  • The tangent to the field line at a given point represents the direction of net magnetic field (B) at that point.
  • For a strong magnet or in a strong magnetic field these lines are denser. Around a weak magnet they are less in number.
  • These lines donot intersect.

→ Bar magnet: Every bar magnet has north & south poles.

  • Magnetic field lines of a bar magnet are similar to that of a solenoid of dipole moment M = NIA.
  • Magnetic field for a far point on the axis of a bar magnet (B) = \(\frac{\mu_0}{4 \pi} \frac{2 \mathrm{M}}{\mathrm{r}^3}\).
  • On equatorial line of bar magnet
    BE = \(\frac{\mu_0}{4 \pi}\left(\frac{M}{r^3}\right)\)
  • e- For a magnetic dipole in a uniform magnetic field torque τ = m̅ x B̅ = mB sin θ.
    x = I\(\frac{\mathrm{d}^2 \theta}{\mathrm{dt}^2}\) = mB sin θ for small angles sin θ = 0.
    τ = mBθ; Time period of oscillation T = 2π\(\sqrt{\frac{\mathrm{I}}{\mathrm{mB}}}\)

→ Magnetic potential energy (Um) : It is the work done by an external field to bring the magnetic poles to the given location or configuration from infinite distance.

  • Magnetic potential energy Um = -mB cos θ = m̅. B̅
  • Magnetic potential energy Um = -mB when θ = 0°.
  • Um is minimum (stable equilibrium). Magnetic potential energy Um is maximum when 0 = 180°.
  • Um maximum = mB (unstable equilibrium).

→ Gauss’s Law for magnetism : Gauss law states that the net magnetic flux through any closed surface is zero.

→ Earth’s magnetism: The magnetic field of earth is believed to arise due to electrical currents produced by convective motion of metallic fluids in outer core of earth. This effect is also known as the dynamo effect.

  • The magnetic north pole of earth is at a latitude at 79.74° N and at a longitude of 71.8° W. It is some where in North Canada.
  • The magnetic south pole of earth is at 79.74° S and 108.22° E in the Antarctica.

→ Magnetic declination (D) :
The magnetic meridian at a given place makes some angle (D) with true geographic north and south directions.
TS Inter 2nd Year Physics Notes Chapter 8 Magnetism and Matter 1
The angle between true geographic north to the north shown by magnetic compass is called “mag¬netic declination (or) simply declination (D).”
Note: Declination is more at poles and less at equator.

→ Angle of dip (or) inclination (I) : It is the angle of total magnetic field at a given place with the surface of earth.
Note: At a given place horizontal component of earth’s magnetic field HE = BE cos I.
Vertical component of earth’s magnetic field ZE = BE sin I.
Tangent of dip tan I = \(\frac{\mathrm{Z}_{\mathrm{E}}}{\mathrm{H}_{\mathrm{E}}}\).

→ Magnetisation (I) : It is the ratio of net magnetic moment per unit volume.
I = \(\frac{m_{\text {net }}}{V}\); Where mnet = the vectorial sum of magnetic moments of atoms in bulk material and V = volume of the given material.
Magnetic intensity is a vector, dimensions L-1A.
Unit: Ampere/metre : A m-1.

→ Magnetic intensity (H) : The ratio of magnetic field (B0) to the permeability of free space (µ0) is called “magnetic intensity”.
Magnetic intensity H = \(\frac{B_0}{\mu_0}\).

→ Solenoid, magnetic intensity and magnetic field B:
For a solenoid with the interior material of zero magnetisation material B0 = µ0nl. or H = \(\frac{B_0}{\mu_0}\) = nl.

→ If solenoid is filled with a material of non¬zero magnetisation material then
B = B0 + Bm Where Bm = magnetic field due to core material. ‘
Bm = µ0g M then H = \(\frac{\mathrm{B}}{\mu_0}\) – M

→ Magnetic susceptibility (χ) : Susceptibility is a measure for the response of magnetic materials to an external field.
χ = \(\frac{\mathrm{I}}{\mathrm{H}}=\frac{\text { Magnetic intensity }}{\text { Magnetisation }}\)
It is a dimensionless quantity.
Note : Relative permeability µr = 1 + χ

→ Relation between µ, µr and χ:
In magnetism the three quantities µ, µr and χ are connected with the relation
µ = µr(1 + χ)

→ Magnetic properties of matter : All substances are magnetically divided into three types depending on the property susceptibility (χ).
I If χ is -ve ⇒ it is dia-magnetic substance.
If χ is positive and very small ⇒ it is paramagnetic substance.
I If χ is positive and large ⇒ it is ferro-magnetic substance.

→ Diamagnetic substances:

  • For these substances susceptibility x is -ve.
  • In a magnetic field they will tend to travel from strong field to weak field.
  • They seems to be repelled by magnets.
  • For diamagnetic substances the resultant angular momentum of atoms is zero.
  • Superconductors are most exotic dia-magnetic substances. For super conductors χ = -1 and µr = 0.
    Ex: Bismuth, Copper, Lead, Silicon etc. Note : The phenomenon of perfect diamagnetism in superconductors is called Meissner effect.

→ Paramagnetism:

  • These substances are feebly attracted by magnets.
  • The susceptibility (χ) of these substances is +ve and nearly equals to one.
  • In a magnetic field these substances will move from weak field to strong field.
  • Individual atoms posses permanent magnetic dipole moment. But due to random thermal motion of atoms net magnetic moment is zero.
  • Magnetisation of paramagnetic substance is given by M = C\(\frac{B_0}{T}\) and χ = C\(\frac{\mu_0}{\mathrm{~T}}\)
    where ‘C’ = Curie constant.
    Ex : Aluminium, Sodium and Calcium etc.

→ Ferromagnetism:

  • Ferromagnetic substances are strongly attracted by magnets.
  • The susceptibility (χ) is +ve and very large.
  • Individual atoms of these substances will spontaneously align in a common direction over a small volume called domain.
  • Size of domain is nearly 1 mm3 or a domain may contain nearly 1011 atoms.
  • In these substances magnetic field lines are very crowded.
  • Every ferromagnetic substance will transform into paramagnetic substance at a temperature called Curie Tempe¬rature (Tc).
    Ex: Manganese, Iron, Cobalt, Nickel etc.

→ Hysteresis loop : Magnetic hysteresis loop is a graph between magnetic field (B) and magnetic intensity (M) of a ferromagnetic substance.

→ Retentivity or Remanence : The magnetic intensity (H) of a material at applied magnetic field B = 0 is called retentivity. In hysteresis loop value of H on +ve Y-axis i.e., at B = 0 gives retentivity.

→ Coercivity: The -ve value of’magnetic field – (B) applied (i.e., in opposite direction of magnetisation) at which the magnetic intensity (H) inside the sample is zero is called “coercivity”. In hysteresis diagram the value of B on -ve X-axis gives coercivity.

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