#superconductivity

Wheatstone Bridge and Meter Bridge

Why does a superconductor exhibit zero resistance below its critical temperature?

**Wheatstone bridge balance** condition R₁/R₂ = R₃/R₄, R₄ = R₂ R₃/R₁, when galvanometer current zero, potentials at midpoints equal. At balance, no current through galvanometer, enabling precise resistance measurement independent of source voltage. Below the critical temperature, electrons in a superconductor form Cooper pairs, which move without scattering off lattice ions, eliminating resistance as there’s no energy loss to collisions. Applying I = n e A v_d, R = ρ l/A, R_t = R₀[1+αΔT], Kirchhoff's ΣI=0, ΣV=0, R_eq series/parallel, V = ε - I r and P = I²R, evaluation yields No collisions with lattice,

Ref: NCERT > Physics Book > Current Electricity > Wheatstone Bridge and Meter Bridge

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Earth's Magnetism and Magnetic Declination

A material’s susceptibility becomes negative and large when:

**Elements of Earth's field** include declination D, inclination I, horizontal component B_H, total field B = √(B_H² + B_V²). B_H provides compass direction, declination varies with location, important for navigation, inclination 0° at magnetic equator, 90° at poles. A superconductor has a susceptibility of chi = -1 (large and negative) when it transitions to the superconducting state below its critical temperature, expelling all magnetic fields via the Meissner effect, a unique property among materials. Substituting values gives It becomes superconducting, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on m/r³ and torque relation τ = m B sinθ.

Ref: NCERT > Physics Book > Magnetism and Matter > Earth's Magnetism and Magnetic Declination

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Torque on Magnetic Dipole and Potential Energy

A material that conducts electricity perfectly while expelling magnetic fields relies on:

**Magnetic dipole in uniform field** experiences torque τ = m B sinθ and potential energy U = -m·B = -m B cosθ, minimum -mB when aligned (θ=0°), maximum +mB at anti-alignment (θ=180°). Work done rotating from θ₁ to θ₂ equals ΔU = mB(cosθ₁ - cosθ₂). Superconductors achieve perfect conductivity and expel magnetic fields (Meissner effect) below their critical temperature, where induced surface currents counteract the external field, a phenomenon tied to zero resistance and perfect diamagnetism. Substituting values gives Induced surface currents, which matches expected magnitude for this magnetic configuration, confirming dipole field dependence on m/r³ and torque relation τ = m B sinθ.

Ref: NCERT > Physics Book > Magnetism and Matter > Torque on Magnetic Dipole and Potential Energy

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PHYSICS

Why does a superconductor exhibit zero resistance below its critical temperature?

Below the critical temperature, electrons in a superconductor form Cooper pairs, which move without scattering off lattice ions, eliminating resistance as there’s no energy loss to collisions.

Ref: NCERT Physics Textbook - Latest Edition for Academic Session 2026-27 (Rationalized Textbook for Class XI and XII, continuing as per NCERT advisory for 2026-27), Chapter: Kinetic Theory (Latest NCERT 2026-27), Topic: RMS speed v_rms ∝ √T, temperature dependence, ratio v₂/v₁ = √(T₂/T₁) and calculation. The section explains governing laws, formulas like μ₀ = 4π × 10⁻⁷ T·m/A, SI units and illustrative examples. Page.

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