Revision: Magnetism and Magnetic Effects of Electric Current Physics HSC Science Class 12 Tamil Nadu Board of Secondary Education

A magnet is a material that produces a magnetic field and can attract magnetic substances such as iron, nickel, cobalt, and some of their alloys.

The region around a magnet in which it can exert a magnetic force is called its magnetic field.

The magnetic dipole moment is defined as the product of its pole strength and magnetic length.

`vec"P" = "q"_"m"vec"d"`

Ferromagnetism is defined as the phenomenon in which substances, such as iron, cobalt and nickel, are strongly attracted by a magnetic field. Such substances are called ferromagnetic substances.

The phenomenon in which magnetisation M (or flux density B) lags behind the magnetising field H during a cycle of magnetisation.

Residual magnetisation retained by the material when H is reduced to zero after saturation.

SI Unit: A/m

The reverse magnetising field required to reduce residual magnetisation to zero.

SI Unit: A/m

The point beyond which M does not increase further with increasing H, as all domains are aligned.

The phenomenon in which a current-carrying conductor produces a magnetic field around itself is called the magnetic effect of electric current.

A current-carrying conductor is always associated with a magnetic field around it is called the magnetic effect of current. It was first discovered by Hans Christian Oersted in 1820.

Current passed through each of the two infinitely long parallel straight conductors kept at a distance of one meter apart in vacuum causes each conductor to experience a force of 2 × 10^-7 newton per meter length of the conductor.

The total force experienced by a charged particle moving with velocity \[\vec v\] in the presence of both an electric field \[\vec E\] and a magnetic field \[\vec B\] simultaneously.

\[\vec{F}=q\vec{E}+q(\vec{v}\times\vec{B})\]

Statement

The line integral \[\oint\vec{B}\cdot d\vec{l}\] taken around any closed loop equals μ₀ times the net steady current passing through the loop.

Proof (for a long straight wire)

• Consider an infinitely long straight wire carrying current I.

• By Biot–Savart law, field at distance r:
  B = \[\frac{\mu_0I}{2\pi r}\]

• Choose a circular Amperian loop of radius r, concentric with the wire.

• By symmetry, B is constant in magnitude and tangential (parallel to \[d\vec l\]) everywhere:
  \[\oint\vec{B}\cdot d\vec{l}=B\oint dl\] = B(2πr)

• Substituting B:
  \[\oint\vec{B}\cdot d\vec{l}=\frac{\mu_0I}{2\pi r}(2\pi r)\] = μ₀​I

Conclusion

\[\oint\vec{B}\cdot d\vec{l}=\mu_0I\]
The result is independent of the loop's radius, confirming the law's validity.

• Every ordinary magnet has two poles: north and south.
• Like poles repel; unlike poles attract.
• A freely suspended magnet aligns approximately along the north–south direction.
• Magnetic field lines are closed loops.
• A broken magnet always produces smaller magnets, each with both poles.
• Earth behaves approximately like a giant magnet.
• Near geographic north lies Earth’s magnetic-south region in the simple bar-magnet model.
• A compass works because its needle aligns with Earth’s magnetic field.

Motion of electrons generates a magnetic field — each electron behaves like a tiny bar magnet with a magnetic moment measured in Bohr Magneton (μ_B) = 9.27 × 10⁻²⁴ A m².