NCERT Physics Class XII: Comprehensive Vocabulary and Key Concepts

A comprehensive vocabulary review deck covering major definitions, laws, formulas, and concepts from NCERT Class XII Physics.

Quantisation of Charge

The physical principle stating that all observable electric charges are integral multiples of a basic unit of charge denoted by $e$ ($q = ne$, where $n$ is any positive or negative integer), with $e = 1.602192 \times 10^{-19}\,C$.

Coulomb's Law

The fundamental law stating that the electrostatic force between two point charges $q_1$ and $q_2$ separated by a distance $r$ in vacuum is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them: $F = \frac{1}{4\pi\varepsilon_0}\frac{q_1 q_2}{r^2}$.

Electric Dipole Moment (\mathbf{p})

A vector quantity characterizing an electric dipole composed of charges $q$ and $-q$ separated by distance $2a$, defined as $\mathbf{p} = q \times 2a\,\mathbf{\hat{p}}$, directed along the dipole axis from the negative charge to the positive charge.

Gauss's Law (Electrostatics)

The theorem stating that the total electric flux through any closed surface $S$ is equal to $\frac{1}{\varepsilon_0}$ times the total electric charge $q$ enclosed within the surface: $\phi = \frac{q}{\varepsilon_0}$.

Electrostatic Potential ($V$)

The work done per unit positive test charge by an external force (without acceleration) in bringing a unit positive charge from infinity to a given point in an electrostatic field.

Equipotential Surface

A surface over which the electrostatic potential has a constant value at all points, with the electric field at every point directed normal to the surface in the direction of steepest potential decrease.

Dielectric Strength

The maximum electric field that a dielectric material can withstand without undergoing electric breakdown and loss of its insulating properties (approximately $3 \times 10^6\,V\,m^{-1}$ for air).

Capacitance ($C$)

The ratio of the magnitude of charge $Q$ on either conductor of a capacitor to the potential difference $V$ between them ($C = \frac{Q}{V}$), measured in farads ($1\,F = 1\,C\,V^{-1}$).

Drift Velocity ($v_d$)

The steady average velocity acquired by mobile charge carriers (electrons) inside a conductor under the influence of an external electric field $\mathbf{E}$, expressed as $v_d = -\frac{e\mathbf{E}}{m}\tau$, where $\tau$ is the relaxation time.

Mobility ($\mu$)

The magnitude of the drift velocity per unit electric field ($\mu = \frac{|v_d|}{E} = \frac{e\tau}{m}$), measured in the SI unit $\text{m}^2\,\text{V}^{-1}\,\text{s}^{-1}$.

Electromotive Force (emf)

The potential difference between the positive and negative electrodes of an electrical source or electrolytic cell in an open circuit when no current is drawn.

Kirchhoff's Junction Rule

The circuit law based on the conservation of electric charge stating that the algebraic sum of currents entering any electrical junction must equal the sum of currents leaving that junction.

Wheatstone Bridge

An electrical network of four resistors ($R_1$, $R_2$, $R_3$, and $R_4$) connected in a loop with a galvanometer and source, satisfying the null deflection balance condition $\frac{R_2}{R_1} = \frac{R_4}{R_3}$.

Lorentz Force

The total electromagnetic force acting on a point charge $q$ moving with velocity $\mathbf{v}$ in the presence of both electric and magnetic fields: $\mathbf{F} = q(\mathbf{E} + \mathbf{v} \times \mathbf{B})$.

Biot-Savart Law

The law stating that the magnetic field $d\mathbf{B}$ produced by an infinitesimal current-carrying element $I\,d\mathbf{l}$ at distance $r$ is given by $d\mathbf{B} = \frac{\mu_0}{4\pi}\frac{I\,d\mathbf{l} \times \mathbf{r}}{r^3}$, where $\frac{\mu_0}{4\pi} = 10^{-7}\,\text{T}\,\text{m}\,\text{A}^{-1}$.

Ampere's Circuital Law

The relationship stating that the line integral of magnetic field $\mathbf{B}$ around any closed loop is equal to $\mu_0$ times the net steady current $I$ passing through the area bounded by the loop: $\oint \mathbf{B}\cdot d\mathbf{l} = \mu_0 I$.

Current Sensitivity (Galvanometer)

The steady angular deflection produced in a moving coil galvanometer per unit electric current passing through its coil: $\frac{\phi}{I} = \frac{NAB}{k}$, where $k$ is the torsional spring constant.

Magnetic Flux ($\Phi_B$)

A scalar measure of the quantity of magnetic field lines passing through a given surface area $\mathbf{A}$, given by $\Phi_B = \mathbf{B}\cdot\mathbf{A} = BA\cos(\theta)$, with the SI unit weber ($\text{Wb}$).

Electromagnetic Induction

The physical phenomenon whereby an electromotive force or electric current is generated in a closed conducting loop as a result of varying magnetic fields or changing magnetic flux.

Lenz's Law

The rule stating that the polarity of an induced emf is such that it tends to produce an electric current which opposes the change in magnetic flux that produced it.

Motional Electromotive Force

The emf induced across the ends of an electrical conductor of length $l$ moving at a constant speed $v$ perpendicularly through a uniform magnetic field $B$, given by $\varepsilon = Blv$.

Self-Inductance ($L$)

The ratio of total magnetic flux linkage to the current flowing in an isolated coil ($N\Phi_B = LI$), producing a back emf $\varepsilon = -L\frac{dI}{dt}$ that opposes any change of current in the circuit.

Mutual Inductance ($M$)

The coefficient relating the magnetic flux linkage of a secondary circuit to the time-varying current in an adjacent primary circuit ($N_1\Phi_1 = M_{12}I_2$), where $M_{12} = M_{21} = M$.

Root Mean Square (rms) Current

The effective direct current equivalent that produces the identical average Joule heating power in a resistor over a cycle as the alternating current: $I = \frac{i_m}{\sqrt{2}} \approx 0.707\,i_m$.

Inductive Reactance ($X_L$)

The current-limiting opposition offered by an inductor in an alternating current circuit, given by $X_L = \omega L = 2\pi\nu L$, measured in ohms ($\Omega$).

Capacitive Reactance ($X_C$)

The current-limiting opposition offered by a capacitor in an alternating current circuit, given by $X_C = \frac{1}{\omega C} = \frac{1}{2\pi\nu C}$, measured in ohms ($\Omega$).

Impedance ($Z$)

The total effective opposition offered by an ac circuit to the flow of alternating current, defined in a series $LCR$ circuit by $Z = \sqrt{R^2 + (X_C - X_L)^2}$.

Power Factor

The dimensionless cosine of the phase angle $\phi$ between voltage and current in an ac circuit ($\cos(\phi) = \frac{R}{Z}$), governing the average power dissipated ($P = VI\cos(\phi)$).

Displacement Current ($i_d$)

An equivalent current postulated by James Clerk Maxwell arising from a time-varying electric flux ($i_d = \varepsilon_0 \frac{d\Phi_E}{dt}$), which serves as a source of magnetic field alongside conduction current.

Ampere-Maxwell Law

The generalized circuital law stating that the line integral of $\mathbf{B}$ around a closed perimeter equals $\mu_0$ times the sum of conduction current $i_c$ and displacement current: $\oint \mathbf{B}\cdot d\mathbf{l} = \mu_0 i_c + \mu_0\varepsilon_0\frac{d\Phi_E}{dt}$.

Critical Angle ($i_c$)

The angle of incidence in an optically denser medium for which the angle of refraction in the rarer medium equals $90^\circ$, defined by $\sin(i_c) = \frac{1}{n_{12}}$.

Optical Fibre

A composite glass or quartz cylindrical light pipe consisting of a central core and an outer cladding of lower refractive index that transmits optical signals via repeated total internal reflections.

Lens Maker's Formula

The relationship used to design lenses of desired focal length $f$ from a material of relative refractive index $n_{21}$ with surface radii of curvature $R_1$ and $R_2$: $\frac{1}{f} = (n_{21} - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right)$.

Power of a Lens ($P$)

The measure of convergence or divergence introduced by a lens on incident light, defined as the reciprocal of its focal length ($P = \frac{1}{f}$) and expressed in dioptres ($1\,\text{D} = 1\,\text{m}^{-1}$).

Wavefront

The continuous locus of all points in a propagating wave field that oscillate in the same phase, across which the energy travels in a perpendicular direction.

Huygens Principle

A geometrical model stating that each point on a primary wavefront acts as a source of secondary wavelets spreading out in all directions with wave velocity, whose forward tangent envelope forms the new wavefront.

Coherent Sources

Two or more light sources that produce wave disturbances maintaining a constant or zero phase difference over time, a prerequisite for stable interference fringes.

Malus' Law

The optical law stating that the transmitted light intensity $I$ through an analyser polaroid varies with the square of the cosine of the angle $\theta$ between the pass-axes of polariser and analyser: $I = I_0 \cos^2(\theta)$.

Work Function ($\phi_0$)

The minimum energy required to liberate an electron from the surface of a metal against attractive ionic forces, usually measured in electron volts ($1\,\text{eV} = 1.602 \times 10^{-19}\,\text{J}$).

Stopping Potential ($V_0$)

The minimum retarding potential required to stop the most energetic photoelectrons from reaching the collector plate in a photoelectric cell, satisfying $e V_0 = K_{\max}$.

Threshold Frequency ($\nu_0$)

The minimum cut-off frequency of incident electromagnetic radiation below which no photoelectric emission takes place from a given photosensitive material ($\nu_0 = \frac{\phi_0}{h}$).

de Broglie Wavelength ($\lambda$)

The wavelength associated with a material particle of momentum $p$, defined by Louis de Broglie as $\lambda = \frac{h}{p} = \frac{h}{mv}$, demonstrating matter-wave duality.

Impact Parameter ($b$)

The perpendicular distance between the initial trajectory velocity vector of an incident $\alpha$-particle and the centre of the target nucleus in Rutherford scattering.

Bohr's Quantisation Condition

Niels Bohr's postulate stating that an orbital electron in an atom moves only in stationary orbits whose orbital angular momentum $L$ is an integral multiple of $\frac{h}{2\pi}$ ($L = \frac{nh}{2\pi}$, where $n = 1, 2, 3, \ldots$).

Ionisation Energy

The minimum amount of energy needed to detach an electron completely from the ground state ($n = 1$) of an isolated atom to an infinite distance ($13.6\,\text{eV}$ for a hydrogen atom).

Unified Atomic Mass Unit ($u$)

A standard unit of atomic mass defined as exactly $\frac{1}{12}$ of the mass of one neutral carbon-12 atom ($^{12}\text{C}$), equal to $1.660539 \times 10^{-27}\,\text{kg}$ or $931.5\,\text{MeV}/c^2$.

Mass Defect ($\Delta M$)

The difference between the total mass of the individual constituent nucleons (protons and neutrons) and the actual rest mass of the bound nucleus: $\Delta M = [Z m_p + (A - Z) m_n] - M$.

Binding Energy per Nucleon ($E_{bn}$)

The ratio of the total nuclear binding energy $E_b$ of a nucleus to its mass number $A$ ($E_{bn} = \frac{E_b}{A}$), serving as a benchmark for nuclear stability.

Nuclear Fission

The neutron-induced nuclear process in which a heavy nucleus like $^{235}_{92}\text{U}$ splits into two intermediate-mass fragment nuclei with the release of secondary neutrons and about $200\,\text{MeV}$ of energy.

Thermonuclear Fusion

The nuclear reaction in which lighter atomic nuclei combine to form a heavier nucleus at extremely high temperatures ($\sim 10^7\,\text{K}$ to $10^8\,\text{K}$), providing the energy output of stars.

Energy Band Gap ($E_g$)

The forbidden energy interval separating the highest occupied valence band energy level ($E_V$) from the lowest empty or partially filled conduction band level ($E_C$) in a solid.

Intrinsic Semiconductor

A pure semiconductor crystal (such as silicon or germanium) where electrical conduction is mediated entirely by equal numbers of thermally generated free electrons and holes ($n_e = n_h = n_i$).

Depletion Region

The narrow space-charge layer formed at a $p\text{-}n$ junction comprising immobile donor and acceptor ions depleted of mobile charge carriers.

p-n Junction Diode

A two-terminal semiconductor device that allows substantial electric current to flow in the forward-biased direction while almost completely blocking current in the reverse-biased direction.

Meissner Effect

The phenomenon of total expulsion of magnetic field lines from the interior of a superconducting material cooled below its critical transition temperature ($\chi = -1$, $\mu_r = 0$).

ITER (International Thermonuclear Experimental Reactor)

A joint international fusion research and engineering project situated at Cadarache in the South of France, aimed at demonstrating the scientific feasibility of generating $500\,\text{MW}$ of fusion power.