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#physics theory

6 public questions tagged with this topic.

Rotational EMF and AC Generator

In an AC generator, the output voltage reverses direction every half-cycle due to what?

**Rotational emf** when coil area A rotates with angular speed ω in uniform field B, flux Φ = B A cos ωt, emf e = -N dΦ/dt = N B A ω sin ωt, maximum e₀ = N B A ω, frequency = ω/2π. For square side 22 cm area 0.0484 m² N=1 ω=14 rad/s B=0.15 T, e₀=1×0.15×0.0484×14=0.1016 V, sinusoidal. The coil’s rotation causes the flux to change sign every half-cycle as it passes from one field direction to the opposite, reversing the emf. Using Φ = B A cosθ, e = -N dΦ/dt = -N A dB/dt = B l v = N B

Ref: NCERT > Physics Book > Electromagnetic Induction > Rotational EMF and AC Generator

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Energy, Intensity and Momentum of EM Waves

What is the primary source of a magnetic field as per Maxwell's generalization?

**Energy in EM wave** equally divided between electric and magnetic fields, energy density u = ½ ε₀ E² + B²/(2μ₀) = ε₀ E² = B²/μ₀, average u_avg = ½ ε₀ E₀², intensity I = c u_avg = ½ c ε₀ E₀² = E₀ B₀/(2μ₀) = c B₀²/(2μ₀), radiation pressure p = I/c for absorption, 2I/c for reflection. Maxwell generalized Ampere's circuital law by introducing displacement current. According to this, the source of a magnetic field is not only the conduction current but also the time-varying electric field (displacement current). Using c = fλ, E₀/B₀ = c, I_d = ε₀ dΦ_E/dt, and spectrum classification λ =

Ref: NCERT > Physics Book > Electromagnetic Waves > Energy, Intensity and Momentum of EM Waves

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Combination of Capacitors - Series and Parallel

Why does the potential due to an electric dipole fall off as \( 1/r^2 \) at large distances, unlike the potential of a p

**Series combination** of capacitors has same charge Q on each because connected end-to-end, single path for charge flow, induced charges equal, total voltage V = Σ V_i = Q Σ 1/C_i, so 1/C_eq = Σ 1/C_i. Different potential differences V_i = Q/C_i inversely proportional to C_i, smaller C gets larger V. The potential due to a point charge falls as 1/r because it behaves as a single source of charge. An electric dipole consists of two equal and opposite charges separated by a small distance, so their potentials partially cancel out at large distances. The net potential depends on the dipole momen

Ref: NCERT > Physics Book > Electrostatic Potential and Capacitance > Combination of Capacitors - Series and Parallel

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Electric Field and Electric Field Lines

Why does the electric field due to an electric dipole decrease with distance as \( 1/r^3 \) rather than \( 1/r^2 \) as f

**Field intensity** at distance r follows inverse-square law E = (1/4π ε₀)·q/r². At midpoint between two charges, fields superpose vectorially; if charges opposite, fields add in same direction, enhancing magnitude to E = E₁ + E₂. For a dipole, the net charge is zero, and the field results from the combined effect of two equal and opposite charges. At large distances, the fields from each charge partially cancel, leading to a faster fall-off ( 1/r³ ) compared to the 1/r² dependence of a single charge. Substituting values gives Cancellation effect, which matches expected magnitude for this elec

Ref: NCERT > Physics Book > Electric Charges and Fields > Electric Field and Electric Field Lines

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Potential Energy of Mass Systems

Why does an object need a minimum speed to escape Earth’s gravity?

Escape speed (ve = 2GMR) is the minimum speed where kinetic energy equals the gravitational potential energy’s magnitude, ensuring total energy is zero, allowing escape to infinity. As per NCERT, applying relevant law/formula with correct units and sign convention leads to To balance kinetic and potential energy. This satisfies dimensional consistency and physical conditions given, so option B is scientifically correct.

Ref: Halliday & Resnick, Fundamentals of Physics, 11th ed., Chapter 13: Gravitation.

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