Magnetism

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Magnetism is a fundamental topic in NEET Physics that appears consistently in the examination. The key formulas and concepts for quick revision include the following. The horizontal component of Earth’s magnetic field is given by B_H = B cosθ, where θ is the angle of inclination or dip. The angle of dip δ satisfies the relation δ = tan⁻¹(B_V/B_H), where B_V is the vertical component. Magnetic materials are classified into three categories: diamagnetic, paramagnetic, and ferromagnetic substances. The Curie law relates magnetic susceptibility χ to temperature T through the equation χ = C/T, where C is the Curie constant. Above the Curie temperature, ferromagnetic materials become paramagnetic. Soft iron has high permeability and low coercivity, making it ideal for temporary magnets and electromagnets, whereas steel has high coercivity and is preferred for permanent magnets. The magnetic moment of an electron is given by μ_B = eh/4πm, where e is the electronic charge, h is Planck’s constant, and m is the electron mass. In the examination, students frequently confuse the various magnetic quantities, so it is essential to remember that B represents magnetic flux density, H is the magnetic field intensity, M is the magnetisation, and χ is the magnetic susceptibility. The relationships B = μ₀(H + M) and μ = μ₀(1 + χ) are fundamental and should be memorised. Remember to use British English spelling throughout your answers, including words such as “magnetisation” (not “magnetization”) and “demagnetisation” (not “demagnetization”). The SI unit of magnetic field strength A/m is equivalent to Oe in the CGS system.

🟡 Standard — Regular Study (2d–2mo)

For students who want genuine understanding…

The hysteresis loop is one of the most important concepts in magnetism and is frequently tested in NEET examinations. When a ferromagnetic material is magnetised by an increasing magnetic field H, the magnetic flux density B increases along the curve OA, reaching saturation at point A. When H is reduced to zero, B does not return to zero but retains a value B_R called remanent magnetism or retentivity. This property makes permanent magnets possible. To demagnetise the material completely, a reverse field H_C called coercivity must be applied. The hysteresis loss is proportional to the area of the B-H loop and represents the energy lost per unit volume per cycle during magnetisation and demagnetisation. Soft iron has a narrow hysteresis loop with low coercivity and low energy loss, making it suitable for transformer cores and electromagnets. Steel has a wide hysteresis loop with high coercivity, making it suitable for permanent magnets because it retains its magnetisation well.

The magnetic susceptibility χ distinguishes the three types of magnetic materials. Diamagnetic substances have χ ≈ −10⁻⁵, which is small and negative, and is essentially independent of temperature. Paramagnetic substances have χ ∝ 1/T, following the Curie law, meaning susceptibility decreases as temperature increases. Ferromagnetic substances exhibit spontaneous magnetisation below their Curie temperature T_C, and their susceptibility follows the Curie-Weiss law χ = C/(T − T_C) above T_C. The domain theory explains ferromagnetism: within each domain, atomic magnetic moments are aligned in one direction, but different domains have different directions of magnetisation. When an external field is applied, domains aligned with the field grow at the expense of others.

The magnetic circuit analogy with Ohm’s law is useful: magnetic flux Φ is analogous to current, magnetomotive force (MMF = NI) is analogous to electromotive force, and reluctance is analogous to resistance. The equation Φ = MMF/Reluctance parallels V = IR. The tan galvanometer and deflection magnetometer are instruments used to measure the horizontal component of Earth’s magnetic field. The neutral point is where the magnetic field due to a magnet exactly balances Earth’s horizontal magnetic field. The magnetic meridian is the vertical plane containing Earth’s magnetic axis, whereas the geographical meridian is the vertical plane containing Earth’s geographical axis. The angle between these two planes is called the magnetic declination. The angle of dip is measured in the magnetic meridian.

Ewing’s domain theory explains how ferromagnetic materials become magnetised when subjected to an external magnetic field. Magnetic shielding uses soft iron shells to divert magnetic field lines, protecting sensitive equipment from external magnetic interference. In NEET numerical problems, students must identify which magnetic elements are given and apply the appropriate formulas. The relationship between B, H, M, and χ must be clear: B = μ₀(H + M), M = χH, and μ_r = 1 + χ. Remember that relative permeability μ_r varies with H, being high at low field strengths and decreasing as saturation is approached.

🔴 Extended — Deep Study (3mo+)

Comprehensive theory…

The hysteresis loop deserves detailed study for NEET success. Starting from the demagnetised state, as H increases from zero, B increases along the initial magnetisation curve OA. At point A, further increase in H produces minimal increase in B; this is magnetic saturation. Upon reducing H to zero, B decreases along AB, retaining residual magnetisation B_R at point B. This retentivity or remanence is the ability to remain magnetised after the magnetising field is removed. The value B_R depends on the material and is important for permanent magnet design. Increasing H in the reverse direction, B decreases and eventually reaches zero at point C, where the reverse field magnitude equals H_C, the coercivity. Coercivity measures how strongly a material resists demagnetisation. Continuing the reverse cycle to saturation at D, then reversing again to reach saturation at A completes the loop.

The energy loss per cycle equals the area of the hysteresis loop measured in joules per cubic metre per cycle. In AC transformers, hysteresis loss in the core is one of two major losses; the other is eddy current loss. Eddy currents are induced currents in the conductive core due to changing magnetic flux, and they cause heating. The standard remedy is laminating the core—using thin sheets of transformer steel insulated from each other by varnish—which breaks up the conducting paths and dramatically reduces eddy current losses. The power loss is given by P = k f B_maxⁿ V for hysteresis, where k and n (typically 1.6) are material constants, f is frequency, B_max is maximum flux density, and V is volume.

Above the Curie temperature, a ferromagnetic material becomes paramagnetic and the susceptibility follows the Curie-Weiss law χ = C/(T − T_C). This law can be derived from the Weiss molecular field theory, which assumes each atom experiences an internal molecular field proportional to the magnetisation: H_m = λM, where λ is the molecular field constant. The Curie constant C = Nμ₀μ_B²g²J(J+1)/(3k_B). The total field experienced by an atom is H_total = H_external + λM. Below T_C, spontaneous magnetisation occurs even without an external field due to the molecular field overcoming thermal agitation.

The BH curve is non-linear for ferromagnetic materials: relative permeability μ_r is not constant but varies with H, being high at low H, reaching a maximum, and then decreasing as saturation is approached. In magnetic circuits with an air gap, the effective reluctance increases significantly because air has much higher reluctance than ferromagnetic material, reducing the flux for a given MMF. The stored magnetic field energy density is u = B²/(2μ₀) in free space, or u = ½ BHV per unit volume in materials. The force on a ferromagnetic material in a magnetic field gradient is F = m_grad B, where m is the magnetic moment.

Different magnetic materials serve different purposes. Alnico (an alloy of aluminium, nickel, cobalt, and iron) has high remanence B_R and high coercivity H_C, making it excellent for permanent magnets. Mu-metal (an alloy of nickel and iron) has very high relative permeability (up to 100,000) and is used for magnetic shielding. Ferrite cores (ceramic magnetic materials) have high electrical resistance, reducing eddy current losses at radio frequencies, and are widely used in RF transformers and inductor cores. The squegger circuit (or blocking oscillator) uses a saturable reactor to generate oscillations. NMR (nuclear magnetic resonance) and MRI (magnetic resonance imaging) are advanced applications of magnetic resonance, where nuclei precess in a strong static field and absorb radiofrequency energy. At high frequencies, magnetic materials exhibit complex permeability with real and imaginary parts; tan δ = μ″/μ′ represents the loss angle.

Common traps in NEET examinations include confusing B (magnetic flux density) with H (magnetic field intensity); they are related by B = μ₀μ_r H but are not the same physical quantity. Another trap is confusing coercivity with retentivity—they are opposite concepts: retentivity (remanence) is the leftover magnetisation when H = 0, while coercivity is the reverse field needed to reduce B to zero. Domain walls, particularly Bloch walls where magnetisation rotates perpendicular to the wall plane, are sometimes tested in advanced questions. Superparamagnetism occurs in nanoscale ferromagnetic particles below a certain size; each particle behaves as a single domain with thermal agitation preventing stable remanence, resulting in the material behaving like a paramagnet with a very large effective moment.