Class 12 Physics for NEET Exam: Important Formulas and Questions

Class 12 Physics is an important part of NEET 2026 preparation, covering topics such as Electrostatics, Current Electricity, Capacitors, Magnetic Fields, Wave Optics, Modern Physics and Ray Optics. Understanding the core concepts and remembering the relevant formulas can help students solve numerical and conceptual questions more effectively.

Revising important formulas, physical laws and frequently tested concepts is essential for strengthening Physics preparation. The key topics covered here include formula-based concepts, circuit behaviour, electromagnetic waves, optics and semiconductors, along with important questions to support revision for NEET 2026.

Electrostatics: Important Formulas and Concepts

Electrostatics focuses on electric charges, fields, potential and their interactions. Revise the key formulas and properties of charge distributions, conductors and electric dipoles for NEET.

1. Coulomb's Law

The force (F) between two point charges (q₁, q₂) separated by distance r is given by F = k q₁ q₂ / r². In a medium with dielectric constant 'K', this force reduces to (1/K) times the force in vacuum.

2. Electric Field and Potential

• Electric Field (E) at a distance r from a point charge q: E = k q / r²

• Electric Potential (V) at a distance r from a point charge q: V = k q / r

3. Electric Field and Potential due to Continuous Charge Distributions

Infinite Wire (Line Charge Density λ)

• Electric Field (E) at distance r: E = 2 k λ / r

• Potential Difference (ΔV) between r₁ and r₂: ΔV = 2 k λ ln(r₂ / r₁)

Infinite Sheet (Surface Charge Density σ)

• Electric Field (E): E = σ / (2ε₀)

Charged Ring

• Electric Potential (V) on the axis at distance x from the center, for radius R and charge q: V = k q / √(R² + x²). Maximum potential is at the origin.

• Electric Field (E) on the axis at distance x from the center: E = k q x / (R² + x²)^(3/2). Maximum electric field occurs at x = ±R/√2.

4. Electric Field due to a Circular Arc

The electric field at the center of a circular arc with linear charge density λ, subtending an angle 2θ, and radius R: E = (2 k λ / R) sin(θ). This formula must be remembered.

5. Electric Field and Potential due to Spherical Charge Distributions

Hollow Sphere / Conductor

• Outside (r > R): E = k q / r², V = k q / r

• On the Surface (r = R): E = k q / R², V = k q / R

• Inside (r < R): E = 0, V = k q / R (constant)

Solid Sphere (Uniformly Charged Insulator)

• Outside (r > R): E = k q / r², V = k q / r

• On the Surface (r = R): E = k q / R², V = k q / R

• Inside (r < R): E = k q r / R³, V = (k q / (2R³)) * (3R² - r²).

• Potential at Center (r=0): V = (3/2) k q / R. This point is important to remember.

Cavity in a Solid Sphere

The electric field within a cavity inside a uniformly charged solid sphere is uniform. Its magnitude and direction remain constant, given by E = ρ d / (3ε₀), where ρ is the charge density and d is the vector connecting the sphere's center to the cavity's center.

6. Potential Energy

• Potential Energy of two point charges (q₁, q₂ at distance r): U = k q₁ q₂ / r. Remember to substitute charges with their algebraic signs.

• Potential Energy in terms of Potential: U = q V.

7. Electric Dipole

• Dipole Moment (p): p = q d (from negative to positive charge).

• Electric Field due to a Dipole:

• On the Axial Line: E_axial = 2 k p / r³ (direction of dipole moment).

• On the Equatorial Line: E_equatorial = k p / r³ (opposite to dipole moment).

• Potential at Equatorial Point: V = 0.

• Dipole in a Uniform Electric Field:

• Torque (τ): τ = p x E or τ = p E sinθ.

• Potential Energy (U): U = -p ⋅ E or U = -p E cosθ. Do not forget the negative sign.

• Net Force: F_net = 0.

8. Conductors

Any excess charge on a conductor resides entirely on its surface. The electric field inside a conductor is always zero.

Gauss's Law

Statement: Φ = ∫ E ⋅ dA = Q_inside / ε₀.

• Q_inside: Refers only to the charge enclosed within the Gaussian surface.

• Flux through a Cube due to a Point Charge Q:

• At center: Φ = Q / ε₀

• At center of a face: Φ = Q / (2ε₀)

• At center of an edge: Φ = Q / (4ε₀)

• At a corner: Φ = Q / (8ε₀) (Flux through the three faces meeting at that corner is zero).

9. Electric Field Just Outside a Conductor

The magnitude of the electric field just outside the surface of a conductor is given by: E = σ / ε₀.

10. Self Potential Energy of Charged Objects

• Self potential energy of a charged conducting spherical shell: U = KQ² / 2R.

• Self potential energy of a uniformly charged solid sphere: U = (3/5) KQ² / R.

11. Electric Field due to a Uniformly Charged Disc

The magnitude of the electric field on the axis of a uniformly charged disc (with surface charge density σ) at a distance x from its center is given by: E = (σ / 2ε₀) * [1 - x / √(R² + x²)].

12. Potential of Combined Charged Drops

When N identical small drops, each with charge q and radius r, combine to form a single large drop, the potential of the large drop V_big = N^(2/3) * V_small.

(Memory Tip: The potential of a large drop formed by N small identical drops is N^(2/3) times the potential of a single small drop.)

Current Electricity: Important Formulas and Concepts

Current Electricity deals with the flow of electric charge, resistance, electrical power and circuit analysis. Focus on Ohm's law, drift velocity, Kirchhoff's laws and galvanometer conversions.

Core Concepts and Formulas

• Ohm's Law: V = IR.

• Power Dissipation (P): P = I²R or P = V²/R.

• Joule's Law of Heating: H = I²RT.

Ideal vs. Non-Ideal Ammeter/Voltmeter

• Ideal Ammeter: Has zero resistance. Replaced by a plain wire in circuits.

• Ideal Voltmeter: Has infinite resistance. Removed from circuits for simplification.

Microscopic View of Current and Resistance

• Drift Velocity (v_d): v_d = (eE / m) * τ.

• Current (I): I = n A e v_d.

• Current Density (J): J = σE.

• Resistance (R): R = ρL / A.

• Resistivity (ρ) and Conductivity (σ): ρ = 1/σ.

Percentage Change in Resistance with Length

For a wire stretched or compressed such that its volume remains constant, Resistance (R) is proportional to the square of the Length (L) (R ∝ L²). For large percentage changes, use the fundamental formula for percentage change.

Current in Electrolytes (Multiple Charge Carriers)

The total current in an electrolyte is the sum of the currents due to each type of charge carrier, as both positive and negative ion movements contribute to conventional current in the same direction.

Current Density and Electric Field (Microscopic Ohm's Law)

• Current Density (J): J = σE or E = Jρ.

• Current (I) related to J: I = J ⋅ A.

Nodal Analysis (Kirchhoff's Current Law)

The algebraic sum of currents entering a node is zero. This method helps solve for unknown node potentials. Current from node X to node Y through resistor R is (X - Y) / R.

Galvanometer Conversion

• To make an ammeter, connect a shunt (low resistance) in parallel with the galvanometer.

• To make a voltmeter, connect a large resistance in series with the galvanometer.

LR Circuit Transient Analysis: T=0 and T=infinity

• At t = 0 (immediately after closing the switch): The inductor acts as an open circuit (infinite resistance). No current flows through it.

• At t = ∞ (steady state): The inductor acts as a short circuit (zero resistance). It carries steady current.

Capacitors: Fundamental Concepts and Formulae

Capacitance questions involve charge storage, energy, dielectric materials and capacitor combinations. Revise the standard formulas and understand how capacitors behave during charging, discharging and charge sharing.

Charge-Voltage Relationship

• Q = C * V.

• Energy Stored in a Capacitor: U = ½ C V² or U = ½ Q² / C.

Capacitance of a Parallel Plate Capacitor

• Vacuum/Air: C = ε₀ A / D.

• With Dielectric (K): C' = K * C = K * ε₀ A / D.

• With Partial Dielectric Filling (thickness t, constant K, plate separation d): C = ε₀ A / [d - t (1 - 1/K)]. This formula is highly important.

Charging and Discharging

• Charging: Q(t) = Q_max [1 - e^(-t/RC)], I(t) = I_max e^(-t/RC)].

• Discharging: Q(t) = Q₀ e^(-t/RC)].

RC Circuit Behavior at Time = 0 and Time = ∞

• At t = 0, an uncharged capacitor acts like a short circuit.

• At t = ∞ (steady state), a capacitor acts like an open circuit.

Capacitors in Series and Parallel

• Series: 1/C_eq = Σ(1/C_i), Charge (Q) is same.

• Parallel: C_eq = ΣC_i, Potential Difference (V) is same.

Sharing Charges Between Capacitors

When two capacitors (C₁, V₁) and (C₂, V₂) are connected:

• Common Potential (V_common) = (C₁V₁ + C₂V₂) / (C₁ + C₂).

• Heat Loss (Energy Loss) = ½ [C₁C₂ / (C₁ + C₂)] (V₁ - V₂)².

• For a charged (C, V) connected to an uncharged identical capacitor (C, 0): V_common = V/2, Heat Loss = ¼ C V².

Magnetic Fields: Formulas and Forces

Magnetism covers magnetic fields produced by currents, forces on charged particles and conductors, and electromagnetic induction. Pay special attention to field formulas, circular motion and Faraday's and Lenz's laws.

Magnetic Field Formulas (Biot-Savart Law Derivatives)

• Finite Straight Wire: B = (μ₀I / 4πr) (sin θ₁ + sin θ₂).

• Infinite Straight Wire: B = μ₀I / 2πr.

• Circular Coil (at center): B = μ₀I / 2r.

• Circular Arc (at center): B = (μ₀I / 4πr) * θ (θ in radians).

• Ideal Solenoid (inside): B = μ₀nI (n = turns per unit length).

• Axis of a Circular Ring: B = (μ₀NIR²) / (2(R² + x²)^(3/2)).

Magnetic Field Formulas (Cylinders and Cavities)

• Hollow Cylinder (current along axis): B = 0 (inside), B = μ₀I / 2πr (outside).

• Solid Cylinder (uniform current): B = (μ₀Ir) / (2πR²) (inside), B = μ₀I / 2πr (outside).

• Cavity within an Infinite Long Cylinder: B = μ₀ (J × d) / 2.

Ampere's Circuital Law

∮ B ⋅ dL = μ₀I_enclosed.

Magnetic Force on a Moving Charge

• Lorentz Force: F = Q(v × B).

• Circular Motion (v ⊥ B): r = mv / QB, T = 2πm / QB. The time period (T) is independent of the velocity (v).

• Helical Path: r = m(v sinθ) / qB, Pitch = (v cosθ) * T.

Condition for Undeviated Motion (Velocity Selector)

If E + v x B = 0, then the particle moves undeviated with E = -(v x B).

Force on a Current-Carrying Conductor

• Segment dL: dF = I (dL x B).

• Straight Conductor L: F = I (L x B).

• Force Between Two Parallel Wires: F/L = (μ₀ / 2π) * (I₁I₂ / R). Same direction currents attract, opposite direction currents repel.

• Force on a Closed Loop in a Uniform Field: Net force is zero.

Magnetic Dipole Moment (M)

• M = I * A (for a loop), M = N * I * A (for N turns).

• Torque (τ): τ = M x B.

• Potential Energy (U): U = - M ⋅ B.

• Work Done (W): W = MB (cosθ₁ - cosθ₂).

Moving Coil Galvanometer

• Deflecting Torque: τ_deflecting = N I B A (in radial field).

• Current Sensitivity (I_s): I_s = N B A / C.

Bar Magnet and its Fields

• Axial Field: B_axial ∝ M / r³.

• Equatorial Field: B_equatorial ∝ M / r³.

• Cutting a Magnet (perpendicular to length): Each piece has M' = M/n.

• Time Period of Oscillation: T = 2π√(I/MB).

Magnetic Flux

Φ_B = ∫ B ⋅ dA. For uniform B, Φ_B = BAcosθ.

Faraday's Law and Lenz's Law

• Faraday's Law: ε = -dΦ_B/dt.

• Lenz's Law: Induced current opposes the change in magnetic flux. (If approaching, repel; if receding, attract).

Motional EMF

• Translational: ε = B L v (when B, L, v are mutually perpendicular).

• Rotational: ε = (1/2) B ω L².

Induced Electric Field

A time-varying magnetic field produces an induced electric field. ∮ E ⋅ dL = -dΦ_B/dt.

Self-Inductance

• Φ_self = L I.

• Solenoid: L = μ₀N²A/l.

• Self-Induced EMF: ε_L = -L (dI/dt).

LR Circuits and Energy in Inductors

• Inductor Behavior at t=0: Open circuit.

• Inductor Behavior at t=∞: Short circuit.

• Energy Stored: U = (1/2) L I².

Electromagnetic Waves: Fundamental Properties

Electromagnetic waves consist of oscillating electric and magnetic fields that propagate through space. Revise their speed, field relationships, energy density, intensity and electromagnetic spectrum.

Speed of Light (c)

c = 1 / √(μ₀ε₀). E₀ = B₀ * c.

Properties of EM Waves

• E, B, and v are mutually perpendicular.

• E and B fields are in phase.

• Ê = B̂ × Ĉ (E-field, B-field, and direction of propagation C).

Energy Density in EM Waves

Energy is equally divided between electric and magnetic fields. u_avg_E = u_avg_B.

Intensity of EM Waves

I = ½ ε₀ E₀² c. I = P / (4πr²) for a point source.

Electromagnetic Spectrum

Order by decreasing wavelength: Radio > Micro > IR > Visible > UV > X-ray > Gamma.

Wave Optics: Important Concepts and Formulas

Wave Optics explains light through wavefronts, interference and path differences. Young's Double-Slit Experiment is particularly important for questions involving fringe width, fringe positions and changes in the experimental setup.

Wavefronts

Locus of points in the same phase. Spherical for a point source, Cylindrical for a linear source.

Path Difference (Δx) and Phase Difference (Δφ)

Δφ / (2π) = Δx / λ.

Interference of Waves

I_net = I₁ + I₂ + 2√(I₁I₂) cos(Δφ).

• Constructive (Maxima): Δx = nλ, I_max = (√I₁ + √I₂)² = (A₁ + A₂)².

• Destructive (Minima): Δx = (2n+1)λ/2, I_min = (√I₁ - √I₂)² = (A₁ - A₂)².

• I_max / I_min = (A₁ + A₂)² / (A₁ - A₂)².

Young's Double-Slit Experiment (YDSE)

• Path Difference: Δx ≈ d (y/D).

• Bright Fringes: y_n_bright = nλD/d.

• Dark Fringes: y_n_dark = (2n+1)λD/(2d).

• Fringe Width (β): β = λD/d.

• Effect of Slab (thickness t, refractive index μ): y_shift = (μ - 1)t * (D/d).

• Effect of Medium: β' = β / μ'.

• Fringe Location relative to CM: n-th bright at nβ; n-th dark at (n - ½)β.

Optical Path

Optical Path Length = μt.

Modern Physics: Photons, Photoelectric Effect, Bohr's Model

Modern Physics explains the particle nature of light, photoelectric emission and the energy levels of atoms. Revise the equations connecting photon energy, momentum, stopping potential and atomic transitions.

Photons

• Energy (E): E = hf = hc / λ.

• Momentum (p): p = h / λ.

• Number of Photons (N) per second: N = P / (hc/λ).

• Numerical Shortcuts for Energy: E = 1240 / λ (nm) for E in eV.

Photoelectric Effect

• Einstein's Equation: E = Φ + KE_max.

• Threshold Frequency (f₀) and Wavelength (λ₀): Φ = hf₀ = hc/λ₀.

• Stopping Potential (V₀): KE_max = eV₀.

• Effect of Intensity: Increases saturation current, does not affect KE_max or V₀.

• Effect of Frequency: Increases KE_max and V₀.

• V₀ vs 1/λ Graph: A straight line with slope hc/e and y-intercept -Φ/e.

Bohr's Model of the Atom

• Angular Momentum (L): L = n (h / 2π).

• Energy of Electron (E_n): E_n = -13.6 * Z² / n² eV.

• Energy of Photon (ΔE): ΔE = 13.6 * Z² * [ (1 / n_small²) - (1 / n_large²) ] eV.

• Spectral Series: Lyman (n=1), Balmer (n=2), Paschen (n=3), etc.

• Ground State (n=1), First Excited State (n=2).

De Broglie Wavelength

λ = h / mv = h / p = h / √(2m * KE) = h / √(2m * qΔV).

For same KE, λ ∝ 1/√m. Smallest mass = largest λ.

Semiconductors

Semiconductor questions focus on energy bands, doping, PN junctions, diodes and logic gates. Understand the behaviour of different semiconductor types and the applications of special-purpose diodes.

Logic Gates

OR, AND, NOT, NOR, NAND, XOR, XNOR. Boolean algebra rules apply.

Band Theory of Solids

• Valence Band (bound electrons), Conduction Band (free electrons).

• Forbidden Energy Gap (E_g): Conductors (E_g ≈ 0), Semiconductors (E_g ≈ 1 eV), Insulators (E_g > 3 eV).

• Hole: Deficiency of an electron; behaves as a positive charge.

Doping of Semiconductors

• N-type: Pentavalent impurities (e.g., P), majority electrons.

• P-type: Trivalent impurities (e.g., B), majority holes.

• Both are electrically neutral.

PN Junction Diode Biasing

• Forward Bias: P-type to positive, N-type to negative. Depletion layer narrows. Current increases sharply after knee voltage (0.3V for Ge, 0.7V for Si). Acts as a short circuit (ideal).

• Reverse Bias: P-type to negative, N-type to positive. Depletion layer widens. Small leakage current. Acts as an open circuit (ideal).

Rectifiers

• Half-wave: One diode, passes one half-cycle.

• Full-wave: Two/four diodes, passes both half-cycles.

Special Purpose Diodes

• LED: Forward bias, emits light.

• Photodiode: Reverse bias, current increases with light intensity.

• Solar Cell: No external bias, generates power.

• Zener Diode: Reverse bias, used for voltage regulation.

Ray Optics: Mirrors, Lenses, Prisms and Refraction

Ray Optics deals with image formation and the behaviour of light at reflecting and refracting surfaces. Focus on sign conventions, optical formulas, prism deviation and total internal reflection.

Plane Mirrors

• Angle of incidence = Angle of reflection.

• Image: Virtual, erect, same size.

• Rotation of Mirror: Reflected ray rotates by 2θ in the same direction.

Spherical Mirrors (Concave/Convex)

• Mirror Formula: 1/v + 1/u = 1/f.

• Magnification (m): m = -v/u.

• Concave: f is negative.

• Convex: f is positive.

Lenses (Convex/Concave)

• Lens Maker's Formula: 1/f = (μ - 1) * (1/R1 - 1/R2).

• Lens Formula: 1/v - 1/u = 1/f.

• Magnification (m): m = v/u.

• Power (P): P = 1/f. P_net = P1 + P2 + … (for lenses in contact).

• Cutting a Lens along the principal axis: f' = 2f, P' = P/2.

Refraction at Spherical Surfaces

μ2/v - μ1/u = (μ2 - μ1) / R.

Prisms

• Deviation (δ): δ = i + e - A.

• Minimum Deviation (δ_min): Occurs when i = e and R1 = R2 = A/2.

• μ = sin((δ_min + A) / 2) / sin(A / 2).

• Thin Prism: δ = (μ - 1)A.

Refraction at Plane Surfaces

• Apparent Depth: h_apparent = h_real / μ_relative.

• Shift: Shift = h_real - h_apparent.

Total Internal Reflection (TIR)

sinθc = μ_rarer / μ_denser.

Lens-Mirror Combination

Solve step-by-step: Lens → Mirror → Lens.

Important Questions for Class 12 Physics NEET 2027

Practising NEET-level questions helps you apply formulas, identify common question patterns and improve accuracy. Revise the concepts covered above and solve numerical problems regularly to strengthen your exam preparation.

Q. If a dipole P = 2i + 3j is placed at (1, 2) in a non-uniform electric field E = 3xi + 4yj, find the force on the dipole.

Q. A particle of mass m and charge q is attached to a light rod of length L. The rod can rotate freely in the plane of the paper about the other end, which is hinged at P. The entire assembly lies in a uniform electric field E, also acting in the plane of the paper, as shown. The rod is released from rest when it makes an angle θ with the direction of the electric field. Determine the speed of the particle when the rod is parallel to the electric field.

(A) √[2qEL(1 − cos θ)/m]

(B) √[2qEL(1 − sin θ)/m]

(C) √[qEL(1 − cos θ)/(2m)]

(D) √[2qEL cos θ/m]

Q. A copper wire is stretched to increase its length by 20%. The percentage increase in the resistance of the wire is ______ %.

Q. A galvanometer gives full-scale deflection with a current of 0.006 A. By connecting it to a 4990 Ω resistance, it can be converted into a voltmeter with a range of 0–30 V. If it is connected to a resistance of (2n/249) Ω, it becomes an ammeter with a range of 0–1.5 A. Find the value of n.

Q. A parallel-plate capacitor has a plate area of 40 cm² and a plate separation of 2 mm. The space between the plates is filled with a dielectric medium of thickness 1 mm and dielectric constant 5. Calculate the capacitance of the system.

(1) 240 F

(2) (3/10) ε₀ F

(3) (10/3) ε₀ F

(4) 100 F

Key Tips to Master 12th Grade Physics for NEET 2027

Class 12 Physics carries a large share of NEET marks, and most of it comes from formula-based, direction-based and concept-based questions across Electrostatics, Current Electricity, Magnetism, Optics and Modern Physics. Use these five tips to revise faster, avoid common slips and apply formulas correctly under exam pressure.

• Master standard results and use algebraic signs. Learn the field and potential results for rings, spheres, discs and dipoles, such as E_axial = 2kp/r³ and E_equatorial = kp/r³. Always substitute charges with their signs in U = kq₁q₂/r and U = −pE cosθ.

• Remember circuit behaviour at t = 0 and t = ∞. An uncharged capacitor acts like a short circuit at t = 0 and an open circuit at t = ∞. An inductor does the opposite. This one idea solves many RC and LR circuit questions quickly.

• Use proportionality shortcuts to save time. For a stretched wire of constant volume, R ∝ L². For identical drops combining, V_big = N^(2/3) × V_small. For a cut lens, f′ = 2f. These shortcuts avoid long calculations.

• Link magnetism formulas to their conditions. For circular motion (v ⊥ B), use r = mv/qB and T = 2πm/qB, and note that T does not depend on speed. For velocity selectors, remember the undeviated condition E + v × B = 0. For parallel wires, same-direction currents attract.

• Revise optics and modern physics through setup changes. In YDSE, fringe width is β = λD/d, and it becomes β/μ in a medium. For the photoelectric effect, intensity changes only the saturation current, while frequency changes KE_max and V₀. In ray optics, stick to one sign convention throughout a problem.

Revising Class 12 Physics formulas, understanding core concepts and practising important questions can help you strengthen your NEET 2026 preparation. Focus on topics such as Electrostatics, Current Electricity, Magnetism, Optics and Modern Physics, and regularly solve numerical problems to improve accuracy and time management. Combine these resources with regular self-practice to build confidence for the exam with PW NEET Batches.

Related Resources

• Moving Charges and Magnetism-Magnetic Fields

• Faraday’s Law and Lenz’s Law-Magnetic Fields

• Young’s Double-Slit Experiment-Wave Optics

• Photoelectric Effect-Modern Physics

• Lens Maker’s Formula-Ray Optics