NEB Physics Viva Questions & Answers

Scalar quantities have only magnitude — no direction. They obey simple algebraic addition. Examples: mass (kg), temperature (K), speed (m/s), time (s), energy (J).

Vector quantities have both magnitude and direction. They obey the triangle or parallelogram law of addition. Examples: displacement (m), velocity (m/s), force (N), acceleration (m/s²), momentum (kg·m/s).

This distinction is critical: two 5 N forces can produce a resultant anywhere from 0 N to 10 N depending purely on their directions.

Newton's First Law: Every object stays at rest or moves with constant velocity in a straight line unless an unbalanced external force acts on it.

Inertia is matter's resistance to any change in its state of motion. Mass is the quantitative measure of inertia — greater mass means greater inertia.

Real examples: A passenger lurches forward when a bus brakes (inertia of motion). Dust flies off a carpet when beaten (inertia of rest).

Statement: The rate of change of momentum of a body equals the net external force, in the direction of the force.

Derivation: Momentum p = mv. Force F ∝ dp/dt = d(mv)/dt. For constant mass: F ∝ m(dv/dt) = ma. In SI, 1 N gives 1 kg an acceleration of 1 m/s², so F = ma.

This law gives the quantitative definition of force and is the entire foundation of classical mechanics.

Newton's Third Law: To every action there is an equal, opposite, and simultaneous reaction. The two forces always act on different bodies.

Why they don't cancel: Cancellation only happens when two forces act on the same body. Action and reaction act on two separate bodies, so each independently accelerates its own body.

Examples: Rocket ejects gas down → gas pushes rocket up. Swimmer pushes water back → water pushes swimmer forward.

Law: If no net external force acts on a system, its total linear momentum stays constant: Σp_before = Σp_after.

Derivation: For two bodies, Newton's Third Law gives F₁₂ = −F₂₁. Using F = dp/dt: dp₁/dt = −dp₂/dt → d(p₁ + p₂)/dt = 0 → p₁ + p₂ = constant.

Applications: Collisions, explosions, rocket propulsion, recoil of guns.

Simple Harmonic Motion (SHM) is periodic motion where the restoring force is directly proportional to displacement and directed toward equilibrium. Condition: F = −kx.

This gives a = −ω²x, where ω = √(k/m). The negative sign means force always opposes displacement.

Simple pendulum period: T = 2π√(L/g), valid for small angles (less than 15°). L is effective length (pivot to centre of bob), g is gravitational acceleration. Period is independent of mass and amplitude for small oscillations.

Setup: Bob mass m, string length L, small angle θ from vertical.

Restoring force: Tangential component of gravity = −mg sin θ. For small angles sin θ ≈ θ (radians), so F ≈ −mg(x/L), where x is arc length.

Comparing with F = −kx: k = mg/L. Angular frequency ω = √(k/m) = √(g/L). Period T = 2π/ω = 2π√(L/g). At large angles, motion is periodic but NOT SHM.

Systematic errors: Measuring L to wrong point (must be to centre of bob). String is not truly massless. Amplitude too large (breaks small-angle approximation). Air resistance and pivot friction cause damping.

Random errors: Timing error with stopwatch. Miscounting oscillations. Reading the ruler imprecisely.

How to minimise: Time many complete oscillations and divide. Keep amplitude less than 5°. Measure L precisely to bob centre. Release gently without pushing.

Hooke's Law: Within the elastic limit, extension of a spring is directly proportional to the applied force. F = kx.

Spring constant (k): Force per unit extension. Unit: N/m. High k = stiff spring. Found from the slope of the F-x graph.

Elastic limit: Maximum extension beyond which the spring undergoes permanent (plastic) deformation and does NOT return to its natural length. Beyond this, Hooke's Law no longer holds.

Setup: Spring constant k, extended by distance x from natural length.

Derivation: Force at extension x is F = kx. Work to extend from 0 to x = area under F-x graph (triangle) = ½ × x × kx = ½kx².

This work is stored as elastic PE: EPE = ½kx². Energy is proportional to the square of extension — doubling the extension stores four times the energy.

Coefficient of friction (μ) = ratio of frictional force to normal reaction: μ = f/N. Dimensionless. Depends only on the two surfaces in contact.

Static friction (μ_s): Opposes the tendency of motion. Can range from 0 to a maximum μ_s N. Object stays still as long as applied force is less than μ_s N.

Kinetic friction (μ_k): Opposes actual sliding motion. Always equals μ_k N. μ_k is less than μ_s always — that's why it takes more force to start something moving than to keep it moving.

Projectile motion is motion under gravity alone after launch at an angle. It has two independent components: horizontal (constant velocity u cos θ) and vertical (acceleration −g, initial velocity u sin θ).

Range derivation: Time of flight T = 2u sin θ / g. Horizontal range R = (u cos θ) × T = u² × 2 sin θ cos θ / g. Using 2 sin θ cos θ = sin 2θ: R = u² sin 2θ / g.

Maximum range occurs at θ = 45° where sin 2θ = 1. Range is independent of mass.

Free fall is motion under gravity alone with no air resistance. Every object in free fall has the same acceleration regardless of mass — proved by Galileo.

g = 9.8 m/s² at Earth's surface (standard). It varies: slightly higher at poles (Earth is oblate), decreases with altitude (g ∝ 1/r²).

Key equations (u = 0): v = gt, s = ½gt². Distance increases with the square of time — each successive second covers more distance than the last.

Newton's Law: Every two masses attract with force F = Gm₁m₂/r², where G = 6.67 × 10⁻¹¹ N·m²/kg². Force is along the line joining centres.

Deriving g: For mass m on Earth's surface (radius R, mass M): GMm/R² = mg. Therefore g = GM/R². This shows g depends on Earth's mass and radius — not on the object's mass.

Elastic collision: Both momentum AND kinetic energy are conserved. No energy lost. Examples: billiard balls, ideal gas molecules.

Inelastic collision: Momentum conserved but KE is NOT conserved. Some KE converts to heat, sound, or deformation. Examples: clay balls sticking, car crashes.

Perfectly inelastic: Maximum KE loss — bodies stick together. Key point: momentum is always conserved in all collisions.

Temperature measures the average kinetic energy of molecules in a substance. It determines the direction of heat flow — from higher to lower temperature.

Why Kelvin: No negative values — absolute zero (0 K = −273.15°C) is the theoretical minimum. Many formulas require absolute temperature: PV = nRT, Stefan-Boltzmann law, etc. Temperature ratios are only meaningful in Kelvin.

Conversion: K = °C + 273.15. Always convert to Kelvin before using in formulas.

Specific heat capacity (c) = heat needed to raise 1 kg by 1 K. Q = mcΔT → c = Q/(mΔT). Unit: J/(kg·K). Water: c = 4200 J/(kg·K).

Why water is high: Water molecules form hydrogen bonds — strong intermolecular forces. A lot of energy goes into stretching these bonds before temperature rises.

Real impact: Coastal climates are mild. Deserts have extreme day-night swings (sand has low c). Water is used in industrial cooling systems.

Latent heat of fusion (L_f): Heat per kg to change solid → liquid at melting point. Ice: 3.34 × 10⁵ J/kg. Breaks the rigid crystal lattice.

Latent heat of vaporisation (L_v): Heat per kg to change liquid → gas at boiling point. Water: 2.26 × 10⁶ J/kg. Overcomes ALL intermolecular forces to fully separate molecules.

Why L_v is greater: Melting only disrupts the crystal — molecules still stay close. Vaporisation must fully separate them against intermolecular attraction. L_v ≈ 7 × L_f for water.

Principle: In an isolated system, heat lost by the hot body = heat gained by the cold body. No heat exchanges with surroundings.

Equation: m_hot × c_hot × (T_hot − T_f) = m_cold × c_cold × (T_f − T_cold) + W × (T_f − T_cold), where W = water equivalent of calorimeter.

Water equivalent (W): Mass of water that absorbs the same heat as the calorimeter for the same temperature rise. W = m_cal × c_cal / c_water.

Conduction: Heat through contact between molecules. Fastest in solids (especially metals). No bulk movement of matter.

Convection: Heat through bulk movement of fluid. Hot fluid rises (less dense), cold sinks — creates convection currents. Examples: boiling water, sea breezes.

Radiation: Heat as electromagnetic waves. No medium needed — works through vacuum. Governed by Stefan-Boltzmann law: P = εσAT⁴. The only mode that works in space.

First Law: ΔU = Q − W. Change in internal energy = heat added to system − work done by system.

Signs: Q > 0: heat INTO system. Q less than 0: heat OUT. W > 0: system does work (expands). W less than 0: surroundings compress system. ΔU > 0: internal energy increases.

Special processes: Isothermal (ΔT = 0): ΔU = 0, Q = W. Adiabatic (Q = 0): ΔU = −W. Isochoric (ΔV = 0): W = 0, ΔU = Q.

Thermal expansion = increase in dimensions when temperature rises. Most substances expand on heating.

Linear (α): ΔL/L = αΔT. Unit: K⁻¹. For solids. Area (β): ΔA/A = βΔT. β ≈ 2α. Volume (γ): ΔV/V = γΔT. γ ≈ 3α.

Applications: Expansion gaps in bridges and railways. Bimetallic strips in thermostats. Mercury thermometers. The relationship γ ≈ 3α is proved by considering a cube expanding in all three dimensions.

Anomalous expansion: Water contracts between 0°C and 4°C — reaching maximum density at 4°C. Above 4°C it expands normally like other liquids.

Reason: At low temperatures, water molecules form an open hexagonal structure through hydrogen bonding. This occupies more volume. As ice melts (0–4°C), the structure collapses and density increases.

Why it matters for life: Ice is less dense than water → floats on top. Lakes freeze from the surface down, not solid. Liquid water at 4°C stays at the bottom, preserving aquatic life under ice in winter.

Ideal gas equation: PV = nRT. P = pressure, V = volume, n = moles, R = 8.314 J/(mol·K), T = absolute temperature in Kelvin.

Assumptions: (1) Molecules have zero volume. (2) Zero intermolecular forces. (3) Collisions with walls are perfectly elastic. (4) Molecules move randomly in all directions.

When it breaks down: At very high pressure or very low temperature. Real gases follow the van der Waals equation.

Heat (Q) is energy transferred from one body to another due to a temperature difference. Unit: Joule (J). It is a form of energy in transit — not stored in a body.

Temperature (T) is a measure of average kinetic energy of molecules. Unit: Kelvin (K). It is a state property — describes the condition of a body at a given moment.

Key difference: You can measure a body's temperature, but heat only exists during transfer. A body at high temperature doesn't necessarily have more heat than one at low temperature — it depends on mass and specific heat too.

Law 1: Angle of incidence = angle of reflection (i = r), measured from the normal. Law 2: Incident ray, reflected ray, and normal all lie in the same plane.

Wavelength dependence: No — laws of reflection are completely independent of wavelength, colour, or frequency. They apply equally to all electromagnetic radiation and sound waves.

This is why mirrors reflect all colours equally (white appearance), unlike refraction which bends different colours differently (dispersion).

Snell's Law: n₁ sin i = n₂ sin r. n₁, n₂ = refractive indices; i = angle of incidence; r = angle of refraction.

Refractive index: n = c/v (speed in vacuum / speed in medium). For air → medium: n = sin i / sin r. n ≥ 1 always. Higher n = optically denser = light slower.

Light bends toward normal entering denser medium, away entering rarer medium. Values: air ≈ 1, water = 1.33, glass = 1.5, diamond = 2.42.

Total Internal Reflection (TIR) occurs when light in a denser medium hits the interface at angle greater than critical angle. All light reflects — none refracts.

Critical angle derivation: At i_c, refracted ray goes along the surface (r = 90°). Snell's law: n sin i_c = 1 × sin 90° = 1 → sin i_c = 1/n. For glass (n=1.5): i_c = 41.8°.

Two conditions for TIR: (1) Light must go denser → rarer. (2) i must be greater than i_c. Applications: optical fibres, diamond sparkle, periscopes, mirages.

Concept: An object in a denser medium appears closer to the surface than it actually is, when viewed from a rarer medium (air).

Derivation (paraxial rays): For small angles: sin i ≈ tan i, sin r ≈ tan r. Snell's law: n ≈ tan i / tan r = (real depth)/(apparent depth). Therefore: Apparent depth = Real depth / n.

A coin in water at 3 m looks like it's at 3/1.33 = 2.26 m. Swimming pools look shallower than they are — a genuine safety issue!

Dispersion = splitting of white light into colours because different wavelengths have different refractive indices. Violet has highest n, red has lowest.

Prism: Non-parallel faces. Each colour refracts by a different angle at both surfaces — deviations add up → visible spectrum spreads out.

Glass slab: Parallel faces. Different colours refract differently inside, but they all emerge parallel to the original ray — just shifted by slightly different lateral displacements. No visible spectrum.

Setup: Thin lens, focal length f. Object distance u, image distance v from optical centre. Sign convention: distances along light direction are positive.

Result: Using ray diagrams — ray through centre goes straight; ray parallel to axis refracts through focus — and applying similar triangles gives: 1/v − 1/u = 1/f.

Convex lens: f > 0. Concave lens: f < 0. Magnification m = v/u. This single formula handles ALL thin lens problems — real/virtual images, any object position.

Refractive index (n) = c / v, where c = speed of light in vacuum, v = speed in the medium. It tells us how much slower light travels in that medium compared to vacuum.

Physical meaning: n represents how strongly a medium interacts with light and slows it down. Higher n → slower light → more bending at interfaces. It is a property of the material and wavelength (which is why dispersion occurs).

Values: Vacuum = 1 (exact). Air ≈ 1.0003. Water = 1.33. Glass = 1.5. Diamond = 2.42 — highest common value, explains its brilliance from multiple TIR.

Statement: At constant temperature, the current through a conductor is directly proportional to the potential difference across it. V = IR.

Conditions: (1) Temperature must be constant. (2) Conductor must be ohmic. (3) Physical dimensions must not change.

Limitations: Non-ohmic devices (diodes, LEDs, thermistors, filament bulbs) don't obey it. Very high currents cause heating which changes R.

Resistance (R): How much a specific conductor opposes current. Depends on material, dimensions, temperature. R = V/I. Unit: Ω. Changes if you cut wire shorter or thicker.

Resistivity (ρ): Intrinsic material property. Independent of dimensions. R = ρL/A → ρ = RA/L. Unit: Ω·m. Same for all pieces of the same material at the same temperature.

Key point: Two copper wires of different lengths have different R but same ρ. Copper (ρ = 1.7×10⁻⁸) vs nichrome (ρ = 1.1×10⁻⁶) — nichrome is 65× more resistive, which is why it's used in heaters.

Series: Same current I through each. V = V₁ + V₂ + V₃. Using V = IR: IR_eq = IR₁ + IR₂ + IR₃ → R_eq = R₁ + R₂ + R₃. Always greater than any individual R.

Parallel: Same voltage V across each. I = I₁ + I₂ + I₃ → 1/R_eq = 1/R₁ + 1/R₂ + 1/R₃. Always less than the smallest R.

Two in parallel shortcut: R_eq = R₁R₂/(R₁+R₂). Household wiring uses parallel — one device failing doesn't break the others.

Kirchhoff's Current Law (KCL): At any junction, total current in = total current out. ΣI_in = ΣI_out. This is conservation of charge — charge cannot pile up at a junction.

Kirchhoff's Voltage Law (KVL): Around any closed loop, total EMF = total voltage drops. ΣEMF = ΣIR. This is conservation of energy.

Together, these two laws can solve ANY DC circuit, no matter how complex. They are the foundation of all circuit analysis.

Balance condition: When no current flows through the galvanometer: P/Q = R/S.

Derivation: At balance, V_B = V_D. Applying KVL: I₁P = I₂R and I₁Q = I₂S. Dividing: P/Q = R/S.

This is a null method — at balance, measurement is independent of galvanometer sensitivity and battery EMF. Only the ratio of resistances matters.

Meter bridge = practical Wheatstone bridge using a 1-metre uniform wire. Known resistance R in one gap, unknown X in the other. Jockey slides to find null point at length l.

Formula: Resistance ∝ length in uniform wire → R/X = l/(100−l) → X = R(100−l)/l.

At null point, galvanometer reads zero. The measurement depends only on the ratio of lengths, not absolute resistance. Always use interchange method to cancel end resistance errors.

End resistance: Copper strips and terminals add small extra resistances not in the formula → systematic error in every reading.

Interchange method: (1) R left, X right → null at l₁ → X₁ = R(100−l₁)/l₁. (2) Swap R and X. (3) Null at l₂ → X₂ = Rl₂/(100−l₂). (4) Mean: X = (X₁+X₂)/2.

Why it works: End resistances produce equal and opposite errors in X₁ and X₂. Averaging cancels them exactly. Interchange is essential, not optional.

Reason — percentage error minimisation: If null is at 5 cm and reading error is ±1 mm: % error = 0.1/5 × 100 = 2%. If null is at 50 cm: % error = 0.1/50 × 100 = 0.2%. Ten times more accurate.

How to achieve: Choose known R ≈ X. If you suspect X ≈ 20 Ω, use R = 20 Ω. Balance falls near 50 cm.

Near the ends, small length errors become massive percentage errors in the final answer. The region 30–70 cm gives the most reliable results.

EMF (Electromotive Force): Total energy per unit charge supplied by a source. EMF = Work / charge. Unit: Volt. Property of the source itself — doesn't change with load.

Internal resistance (r): Resistance inside the battery from its chemical structure. Current through it causes voltage drop Ir.

Terminal voltage: V = EMF − Ir (during discharge). This is the voltage available to the external circuit. V is less than EMF whenever current flows. Open circuit (I = 0): V = EMF. Short circuit: V = 0.

Coulomb's Law: F = kq₁q₂/r², where k = 9 × 10⁹ N·m²/C². Attractive for unlike charges, repulsive for like charges. Force along line joining charges.

In a medium: F = kq₁q₂/(ε_r r²). Relative permittivity ε_r reduces the force. Water (ε_r = 80) weakens it to 1/80th — this is why ionic compounds dissolve in water so easily.

Coulomb's law is an inverse square law like gravitation, but unlike gravity, it can be attractive OR repulsive.

Potentiometer measures EMF using the null method. A uniform wire carries a known potential gradient (from a driver cell). The unknown EMF is connected in opposition. At balance length l: E = kl, where k = potential gradient.

Why more accurate than voltmeter: At null point (balance), zero current flows through the cell being measured. Therefore, internal resistance causes no voltage drop → we measure the true EMF. A voltmeter always draws some current, so it reads less than actual EMF.