Glossary
Every technical word in the course and what it means in plain language, with flashcards for practice.
Every technical term in the course, with a plain-language meaning. On every page these words are marked with a dotted underline: hover or tap one to see its meaning. The “Used in” links point to the sections where each term appears.
Ch 1 · Electric Charges and FieldsCh 2 · Electrostatic Potential and CapacitanceCh 3 · Current ElectricityCh 4 · Moving Charges and MagnetismCh 5 · Magnetism and MatterCh 6 · Electromagnetic InductionCh 7 · Alternating CurrentCh 8 · Electromagnetic WavesCh 9 · Ray Optics and Optical InstrumentsCh 10 · Wave OpticsCh 11 · Dual Nature of Radiation and MatterCh 12 · AtomsCh 13 · NucleiCh 14 · Semiconductor Electronics: Materials, Devices and Simple Circuits
Ch 1 · Electric Charges and Fields
| Term | Meaning | Used in |
|---|---|---|
| area vector | A vector $\Delta\mathbf A=\Delta A\,\hat{\mathbf n}$ whose size is the area of a small flat surface and whose direction is normal to it — outwards for a closed surface. | 1.3 5.1 6.1 |
| axial line | The line through the two charges of a dipole; on it, far away, the field is $2k\mathbf p/r^3$, along $\mathbf p$. | 1.2 2.1 5.1 |
| charging by induction | Giving a conductor a charge opposite to that of a nearby charged body without touching it: separate its charges, earth it, break the earth connection, then remove the body. | 1.1 |
| closed surface | A surface with no edges that encloses a volume, such as a sphere or a box; its normals are taken to point outwards. | 1.3 2.2 2.3 4.3 |
| conductor | A material through which charges (in metals, free electrons) can move easily, such as copper, the human body or the Earth. | 1.1 1.3 2.1 2.2 |
| conservation of charge | The total charge of an isolated system never changes; charge can only be moved, or created and destroyed in equal and opposite amounts. | 3.2 3.4 6.1 |
| dipole moment | The vector $\mathbf p=q(2a)$ pointing from $-q$ to $+q$ (unit C m); far from a dipole its field depends only on $\mathbf p$. | 1.2 2.1 2.3 4.4 |
| earthing | Connecting a conductor to the Earth by a conducting path, so that it can give charge to or take charge from the Earth. | 1.1 2.3 |
| electric dipole | A pair of equal and opposite charges $\pm q$ a small distance $2a$ apart; a model for polar molecules and for any neutral body whose charges are slightly separated. | 1.2 2.1 2.2 4.2 |
| electric field | The force per unit positive charge at a point, $\mathbf E=\mathbf F/q_0$, measured in N/C; it tells you the force $q\mathbf E$ that any charge placed there would feel. | 1.2 1.3 2.1 2.2 |
| electric field lines | Curves whose tangent at every point is the direction of $\mathbf E$; they start on positive and end on negative charges, never cross, and crowd together where the field is strong. | 1.2 5.1 8.1 |
| electric flux | $\Phi_E=\int\mathbf E\cdot d\mathbf A$, a measure of how much field passes through a surface (roughly, the number of field lines crossing it); unit N m²/C. | 1.3 5.1 6.1 8.1 |
| equatorial line | The perpendicular bisector of a dipole (in 3-D, the equatorial plane); on it, far away, the field is $-k\mathbf p/r^3$, opposite to $\mathbf p$. | 1.2 2.1 2.2 5.1 |
| Gauss's law | The net electric flux out of any closed surface equals the enclosed charge divided by $\varepsilon_0$: $\oint\mathbf E\cdot d\mathbf A=q_\text{enc}/\varepsilon_0$. | 1.1 1.3 2.1 2.3 |
| Gaussian surface | An imaginary closed surface chosen to apply Gauss's law, usually shaped to match the symmetry of the charges (coaxial cylinder, pillbox, concentric sphere). | 1.3 |
| insulator | A material in which charges cannot move freely, so charge stays where it is placed — glass, plastic, rubber, dry wood. | 1.1 2.3 3.2 7.3 |
| linear charge density | Charge per unit length of a line or thin wire, $\lambda=\Delta q/\Delta l$, in C/m. | 1.3 |
| quantisation of charge | The fact that every free charge is a whole-number multiple of the elementary charge $e=1.6\times10^{-19}$ C: $q=ne$. | |
| relative permittivity | The factor $\varepsilon_r\ge1$ (also called the dielectric constant) by which a medium reduces the Coulomb force between charges embedded in it. | 1.1 1.2 2.3 5.2 |
| superposition principle | The net force on a charge due to several charges is the vector sum of the forces each would exert alone. | 1.1 10.2 |
| surface charge density | Charge per unit area of a surface, $\sigma=\Delta q/\Delta A$, in C/m². | 1.3 2.3 |
| test charge | A small positive charge used (in thought) to probe a field; it must be small enough not to disturb the charges that produce the field. | 1.1 1.2 2.1 2.2 |
| uniform electric field | A field with the same magnitude and direction at every point, shown by parallel, equally spaced straight field lines (as between large parallel charged plates). | 1.2 2.2 4.1 4.4 |
| volume charge density | Charge per unit volume, $\rho=\Delta q/\Delta V$, in C/m³. | 1.3 |
Ch 2 · Electrostatic Potential and Capacitance
| Term | Meaning | Used in |
|---|---|---|
| capacitance | $C=Q/V$, the charge stored per unit potential difference between the conductors of a capacitor; it depends only on their geometry and the medium between them. | 2.3 2.4 7.1 7.2 |
| capacitor | A pair of insulated conductors carrying equal and opposite charges $\pm Q$, used to store charge and energy. | 1.3 2.2 2.3 2.4 |
| common potential | The single potential reached when charged conductors or capacitors are connected: $V=(C_1V_1+C_2V_2)/(C_1+C_2)$, by conservation of charge. | 2.3 2.4 |
| conservative force | A force whose work between two points does not depend on the path taken (and is zero round any closed path), so that a potential energy can be defined — gravity and the electrostatic force are examples. | 2.1 2.2 |
| dielectric | An insulating material placed in an electric field; it has no free charges but its molecules become polarised, which weakens the field inside it. | 1.1 2.3 2.4 5.2 |
| dielectric constant | The factor $K=\varepsilon/\varepsilon_0=1+\chi_e/\varepsilon_0$ by which a dielectric filling a region reduces the field ($E=E_0/K$) and raises the capacitance of a capacitor ($C=KC_0$). | 1.1 2.3 2.4 5.2 |
| dielectric strength | The largest field a dielectric can withstand before it breaks down and conducts (about $3\times10^6$ V/m for dry air). | |
| distance of closest approach | The smallest separation reached when a charged particle is fired straight at a like charge: all of its kinetic energy has become electrostatic potential energy, $K=kq_1q_2/d$. | 2.2 12.1 |
| electric polarisation | The dipole moment per unit volume, $\mathbf P$, of a polarised dielectric (unit C/m²); for a linear material $\mathbf P=\chi_e\mathbf E$, where $\chi_e$ is the electric susceptibility. | 2.3 |
| electron volt | The energy gained by a charge $e$ moving through a potential difference of $1$ V: $1\ \text{eV}=1.6\times10^{-19}$ J. Used for atomic and nuclear energies (keV, MeV). | 2.1 4.1 8.2 11.1 |
| electrostatic potential | The work done by an external force in bringing a unit positive test charge slowly from infinity to a point: $V=W/q$, a scalar measured in volts. For a point charge $V=kq/r$. | 2.1 2.2 2.4 3.2 |
| electrostatic potential energy | The work done by an external agent in assembling a set of charges from infinite separation: $kq_1q_2/r_{12}$ for each pair. For a charge in an external field, $U=qV$; for a dipole in a uniform field, $U=-\mathbf p\cdot\mathbf E$. | 2.1 2.2 2.4 3.2 |
| electrostatic shielding | The fact that the field is zero in an empty cavity inside a conductor, whatever charges or fields are outside; used to protect instruments and cables (a metal car in a thunderstorm). | 2.1 2.3 |
| energy density | Energy stored per unit volume of a field (J/m³). For an electric field in vacuum $u=\tfrac12\varepsilon_0E^2$; in a dielectric $u=\tfrac12K\varepsilon_0E^2$. | 2.4 6.3 8.2 |
| equipotential surface | A surface on which the potential has the same value everywhere. No work is done moving a charge along it, and the field crosses it at right angles, pointing towards lower potential. | 2.2 |
| equivalent capacitance | The capacitance of the single capacitor that would store the same charge at the same voltage as a whole network of capacitors. | 2.4 |
| farad | The SI unit of capacitance: $1\ \text{F}=1\ \text{C/V}$. Practical capacitors are measured in µF, nF and pF. | 2.3 |
| parallel combination of capacitors | Capacitors connected between the same two points: each has the same voltage, the charges add, and $C=\sum C_i$. | 2.4 |
| parallel-plate capacitor | Two flat plates of area $A$ a small distance $d$ apart; the field between them is uniform and $C=K\varepsilon_0A/d$. | 1.3 2.3 2.4 8.1 |
| polar molecule | A molecule with a permanent dipole moment (such as H₂O or HCl). In a non-polar molecule (O₂, CO₂) the centres of positive and negative charge coincide until a field separates them. | 2.3 |
| potential difference | $V_B-V_A$, the work done per unit positive charge in carrying a charge slowly from $A$ to $B$; it fixes the energy $q\,\Delta V$ gained or lost by any charge moving between the points. | 2.1 2.2 2.3 2.4 |
| potential gradient | The rate of change of potential with distance, $dV/dl$. The field is minus the potential gradient taken normal to the equipotentials: $E=-dV/dl$ (unit V/m). | 3.4 |
| series combination of capacitors | Capacitors joined end to end with no branches: each carries the same charge, the voltages add, and $1/C=\sum1/C_i$. | 2.4 |
| volt | The SI unit of potential and potential difference: $1\ \text{V}=1\ \text{J/C}$, the potential difference across which one coulomb gains or loses one joule. | 1.2 2.1 2.2 2.3 |
Ch 3 · Current Electricity
| Term | Meaning | Used in |
|---|---|---|
| cells in parallel | Cells with like terminals joined: $\varepsilon_\text{eq}/r_\text{eq}=\sum\varepsilon_i/r_i$ and $1/r_\text{eq}=\sum1/r_i$; identical cells keep the emf and divide $r$. | 3.3 |
| cells in series | Cells joined positive to negative: emfs add (with sign) and internal resistances add, $\varepsilon_\text{eq}=\sum\pm\varepsilon_i$, $r_\text{eq}=\sum r_i$. | 3.3 |
| conductivity | The reciprocal of resistivity, $\sigma=1/\rho$ (unit S/m); for a metal $\sigma=ne^2\tau/m$. | 3.1 14.1 14.2 |
| current density | Current per unit cross-sectional area, $j=I/A$ (A/m²); a vector $\mathbf j$ pointing in the direction of flow of positive charge. | 1.3 3.1 14.2 |
| drift velocity | The small average velocity of the free electrons along a conductor when a field is applied, $v_d=eE\tau/m$, superposed on their fast random motion; $I=neAv_d$. | 3.1 4.1 14.2 |
| electric current | The rate at which net charge crosses a surface, $I=dq/dt$; measured in amperes ($1\ \text{A}=1\ \text{C/s}$), in the direction positive charge flows. | 3.1 6.1 |
| emf | Electromotive force $\varepsilon$: the work done per unit charge by a source's non-electrostatic agency (chemical, magnetic…) in moving charge from its negative to its positive terminal; equal to the terminal voltage on open circuit. Measured in volts — it is not a force. | 2.3 3.3 3.4 6.1 |
| internal resistance | The resistance $r$ of the electrolyte and electrodes inside a cell, in series with its emf; it makes the terminal voltage fall when current is drawn. | 3.3 3.4 4.4 6.2 |
| Joule heating | The conversion of electrical energy into heat in a resistor, at the rate $P=I^2R$, as drifting electrons hand energy to the lattice in collisions. | 3.2 6.2 |
| kilowatt hour | The commercial unit of electrical energy (one 'unit'): $1\ \text{kWh}=1000\ \text{W}\times3600\ \text{s}=3.6\times10^6$ J. | 3.2 13.2 |
| Kirchhoff's junction rule | At any junction of a circuit carrying steady currents, the total current entering equals the total current leaving ($\sum I=0$) — conservation of charge. | 3.4 |
| Kirchhoff's loop rule | Round any closed loop of a circuit the algebraic sum of the potential changes is zero ($\sum\varepsilon=\sum IR$ with signs) — conservation of energy. | 3.4 6.2 6.3 7.1 |
| lost volts | The potential drop $Ir$ inside a cell carrying current $I$; the difference between the emf and the terminal voltage. | 3.3 |
| manganin | A copper–manganese–nickel alloy whose resistivity hardly changes with temperature; used for standard and precision resistors. | 3.2 4.4 |
| mobility | Drift speed per unit electric field, $\mu=v_d/E=e\tau/m$ (unit m² V⁻¹ s⁻¹); $\sigma=ne\mu$. | 3.1 14.2 |
| nichrome | A nickel–chromium alloy with high resistivity and a small temperature coefficient that does not oxidise when red hot; used for heating elements. | 3.1 3.2 14.1 |
| non-ohmic device | A device whose $I$–$V$ graph is not a straight line through the origin — e.g. a filament lamp, a thermistor or a diode. | 3.2 |
| null method | A measurement made by adjusting a known quantity until a detector reads zero, so the detector need not be calibrated — e.g. balancing a Wheatstone bridge. | 3.4 |
| Ohm's law | For a metal at constant temperature the current is proportional to the potential difference, $V=IR$ with $R$ constant; in point form $\mathbf j=\sigma\mathbf E$. | 3.1 3.2 14.3 |
| ohmic device | A device whose current is proportional to the potential difference across it under fixed conditions: its $I$–$V$ graph is a straight line through the origin. | 3.2 |
| ratio arms | The two arms $P$ and $Q$ of a Wheatstone bridge whose ratio $Q/P$ sets the range: at balance the unknown is $S=R\,Q/P$. | 3.4 |
| relaxation time | The average time $\tau$ between successive collisions of a free electron with the lattice (about $10^{-14}$ s in metals at room temperature). | 3.1 3.2 14.1 |
| resistivity | The material property $\rho$ in $R=\rho l/A$ (unit $\Omega$ m); it depends on the material and temperature, not on the size or shape of the sample. | 3.1 3.2 4.4 14.1 |
| short-circuit current | The current $\varepsilon/r$ a cell drives when its terminals are joined by a wire of negligible resistance. | 3.3 |
| steady current | A current that does not change with time, so that the charge passing in time $t$ is $q=It$. | 1.1 3.1 3.2 3.4 |
| strain gauge | A thin metal foil or wire glued to a surface whose resistance rises slightly when the surface is stretched; read with a Wheatstone bridge in weighing scales and load cells. | 3.4 |
| temperature coefficient of resistivity | The fractional change of resistivity per degree, $\alpha$, in $\rho_T=\rho_0[1+\alpha(T-T_0)]$; positive for metals, negative for semiconductors. | 3.2 |
| terminal voltage | The potential difference across the terminals of a cell: $V=\varepsilon-Ir$ when it delivers current, $V=\varepsilon+Ir$ when it is being charged. | 3.3 3.4 |
| thermistor | A semiconductor resistor whose resistance changes strongly with temperature; an NTC thermistor's resistance falls as it warms up. | 3.2 14.1 |
| Wheatstone bridge | Four resistors $P$, $Q$, $R$, $S$ in a ring with a cell across one diagonal and a galvanometer across the other; it is balanced (no galvanometer current) when $P/Q=R/S$. | 3.4 |
Ch 4 · Moving Charges and Magnetism
| Term | Meaning | Used in |
|---|---|---|
| ammeter | An instrument for measuring current, connected in series; a galvanometer with a small shunt in parallel, so its resistance is very small (ideally zero). | 4.4 7.1 |
| ampere | The SI unit of current. Formerly defined as the current that, in two long thin parallel wires $1$ m apart in vacuum, gives a force of $2\times10^{-7}$ N per metre; since 2019 fixed through $e=1.602176634\times10^{-19}$ C. | 1.1 3.1 3.2 3.3 |
| Amperian loop | An imaginary closed path chosen for applying Ampère's law, usually one along which $B$ is constant and parallel or perpendicular to the path. | 4.3 |
| Ampère's circuital law | For steady currents, the circulation of $\mathbf B$ round any closed loop equals $\mu_0$ times the net current threading the loop: $\oint\mathbf B\cdot d\mathbf l=\mu_0I_\text{enc}$. | 4.2 4.3 8.1 |
| Biot–Savart law | The field of a current element: $d\mathbf B=\dfrac{\mu_0}{4\pi}\dfrac{I\,d\mathbf l\times\hat{\mathbf r}}{r^2}$ — inverse-square, perpendicular to both $d\mathbf l$ and $\mathbf r$, zero along the line of the element. | 4.2 4.3 6.3 |
| circulation of B | The line integral $\oint\mathbf B\cdot d\mathbf l$ round a closed curve — the sum of (component of $\mathbf B$ along the path) × (step length). | 4.3 |
| current element | A very short length $d\mathbf l$ of a wire carrying current $I$, written $I\,d\mathbf l$ and pointing along the current; the magnetic analogue of a point charge. | 4.2 |
| current sensitivity | The deflection of a galvanometer per unit current, $\phi/I=NAB/k$. | 4.4 |
| cyclotron | A particle accelerator in which ions circle in a uniform magnetic field and are kicked by an alternating voltage at the cyclotron frequency $qB/(2\pi m)$ (beyond the NCERT syllabus; JEE/NEET). | 4.1 |
| Fleming's left-hand rule | First finger along the Field, second finger along the Current, thumb gives the force (Motion) on a current-carrying conductor — an equivalent of $\mathbf F=I\mathbf l\times\mathbf B$. | 4.1 6.2 |
| helical path | The spiral path of a charge whose velocity makes an angle $\theta$ with a uniform $\mathbf B$: circling with radius $mv\sin\theta/(|q|B)$ while drifting along $\mathbf B$ by one pitch $2\pi mv\cos\theta/(|q|B)$ per turn. | 4.1 4.3 |
| Lorentz force | The total force on a point charge in electric and magnetic fields, $\mathbf F=q(\mathbf E+\mathbf v\times\mathbf B)$; its magnetic part is perpendicular to the velocity and does no work. | 4.1 |
| magnetic dipole | A source whose far field has the dipole pattern $B\propto m/r^3$ (axial field twice the equatorial) — a small current loop, or a bar magnet; unlike an electric dipole it cannot be split into separate poles. | 4.2 4.4 5.1 |
| magnetic dipole moment | For a plane current loop, $\mathbf m=NIA\,\hat{\mathbf n}$ (normal by the right-hand thumb rule), unit A m²; it sets the torque $\mathbf m\times\mathbf B$ on the loop and its far field. | 4.4 5.1 5.2 |
| magnetic field | The vector field $\mathbf B$ that exerts a force $q\mathbf v\times\mathbf B$ on a charge moving through it; produced by moving charges (currents) and magnets, measured in tesla. | 1.2 4.1 4.2 4.3 |
| moving-coil galvanometer | A meter in which a coil in a radial magnetic field turns against a spring; its deflection $\phi=(NAB/k)I$ is proportional to the current. | 3.4 4.4 6.1 |
| permeability of free space | The constant $\mu_0=4\pi\times10^{-7}$ T m/A that fixes the strength of magnetic fields produced by currents in vacuum; $1/\sqrt{\mu_0\varepsilon_0}$ is the speed of light. | 4.2 |
| radial magnetic field | A field directed along the radii of a cylindrical core, made by concave pole pieces and a soft-iron cylinder, so that the plane of a galvanometer coil is always parallel to $\mathbf B$. | 4.4 |
| right-hand grip rule | Grip a straight current with the right hand, thumb along the current: the fingers curl in the direction of the magnetic field lines round it. | 4.2 4.3 8.1 |
| right-hand thumb rule | Curl the fingers of the right hand round a loop or coil in the direction of the current: the thumb points along the magnetic field on its axis (and inside a solenoid). | 4.2 4.3 4.4 |
| shunt | A small resistance connected in parallel with a galvanometer so that most of the current bypasses the coil; it turns the galvanometer into an ammeter, $S=I_gG/(I-I_g)$. | 4.4 |
| solenoid | A long tightly wound helical coil; inside a long solenoid the field is uniform and axial, $B=\mu_0nI$ ($n$ turns per metre), and outside it is nearly zero. | 4.3 5.1 5.2 6.1 |
| tesla | The SI unit of magnetic field: $1\ \text{T}=1\ \text{N A}^{-1}\text{m}^{-1}$, the field that pushes with $1$ N on $1$ C moving at $1$ m/s at right angles to it ($1$ gauss $=10^{-4}$ T). | 4.1 5.2 6.1 |
| toroid | A solenoid bent into a closed ring; its field $\mu_0NI/(2\pi r)$ is confined to the core (beyond the NCERT syllabus; JEE/NEET). | 4.3 |
| velocity selector | Crossed uniform $\mathbf E$ and $\mathbf B$ fields that let only particles of speed $v=E/B$ pass undeflected (beyond the NCERT syllabus; JEE/NEET). | 4.1 13.1 |
| voltage sensitivity | The deflection of a galvanometer per unit potential difference across it, $\phi/V=NAB/(kG)$. | 4.4 |
| voltmeter | An instrument for measuring potential difference, connected in parallel; a galvanometer with a large resistance $R=V/I_g-G$ in series, so it draws very little current. | 3.3 3.4 4.4 6.2 |
Ch 5 · Magnetism and Matter
| Term | Meaning | Used in |
|---|---|---|
| angle of dip | The angle $I$ between the Earth's total magnetic field and the horizontal, measured in the magnetic meridian; $\tan I=B_V/B_H$ (beyond the current syllabus). | 5.1 |
| bar magnet | A straight permanent magnet with a north pole at one end and a south pole at the other; outside itself its field is that of a magnetic dipole of moment $\mathbf m$ pointing from S to N. | 4.3 4.4 5.1 5.2 |
| coercivity | The reverse magnetic intensity $H_c$ needed to reduce $B$ in a magnetised ferromagnet to zero; small for soft iron, large for permanent-magnet materials (beyond syllabus). | 5.2 |
| Curie temperature | The temperature $T_C$ above which a ferromagnet loses its spontaneous alignment and becomes paramagnetic (iron $1043$ K, nickel $631$ K). | 5.2 |
| diamagnetic | Having a small negative susceptibility: magnetised against the field and pushed towards weaker field (bismuth, copper, water); superconductors are perfect diamagnets, $\chi=-1$. | 5.2 |
| ferromagnetic | Strongly magnetised along the field ($\chi\gg1$) because atomic moments align spontaneously in domains (iron, cobalt, nickel, gadolinium); becomes paramagnetic above its Curie temperature. | 5.2 |
| Gauss's law for magnetism | The net magnetic flux through any closed surface is zero, $\oint\mathbf B\cdot d\mathbf A=0$: magnetic field lines have no beginning or end. | 5.1 8.1 |
| hysteresis | The lagging of $B$ behind $H$ in a ferromagnet: $B$ depends on the sample's past, so a cycle of $H$ traces a closed B–H loop (beyond syllabus). | 5.2 7.3 |
| magnetic declination | The angle between geographic north and the direction of a horizontal compass needle at a place (beyond the current syllabus). | 5.1 |
| magnetic field lines | Curves whose tangent gives the direction of $\mathbf B$; they form closed loops — N to S outside a magnet and S to N inside it — and never cross. | 4.3 5.1 6.1 8.1 |
| magnetic intensity | The vector $\mathbf H=\mathbf B/\mu_0-\mathbf M$ (unit A/m), fixed by the free currents; inside a long solenoid $H=nI$ whatever the core. | 5.2 |
| magnetic meridian | The vertical plane at a place that contains the direction in which a freely suspended horizontal compass needle points (beyond the current syllabus). | 5.1 |
| magnetic monopole | A hypothetical isolated N or S pole. None has ever been found; Gauss's law for magnetism expresses their absence. | 5.1 8.1 |
| magnetic poles | The regions near the ends of a magnet where its field is strongest; like poles repel, unlike poles attract, and N and S poles always occur together. | 4.1 5.1 6.1 6.3 |
| magnetic susceptibility | The dimensionless ratio $\chi=M/H$: negative for diamagnets, small and positive for paramagnets, very large for ferromagnets. | 2.3 5.2 |
| magnetisation | The net magnetic moment per unit volume of a material, $\mathbf M=\mathbf m_\text{net}/V$, measured in A/m. | 5.1 5.2 |
| paramagnetic | Having a small positive susceptibility: permanent atomic moments are lined up a little by the field, so the material is weakly attracted (aluminium, oxygen); $\chi\propto1/T$. | 5.2 |
| pole strength | The quantity $q_m$ (unit A m) assigned to each pole in the pole model of a magnet, so that its magnetic moment is $m=q_m(2l)$, with $2l$ the magnetic length. | 5.1 |
| relative permeability | The ratio $\mu_r=1+\chi=\mu/\mu_0$; filling a long solenoid with a material multiplies the field inside by $\mu_r$. | 5.2 6.3 |
| retentivity | The field $B_r$ that remains in a ferromagnet when $H$ is reduced to zero after saturation (beyond syllabus). | 5.2 |
Ch 6 · Electromagnetic Induction
| Term | Meaning | Used in |
|---|---|---|
| AC generator | A coil of $N$ turns rotating in a magnetic field (or a magnet rotating inside coils); its emf is $\varepsilon=NBA\omega\sin\omega t$. | 3.4 6.3 7.1 |
| back emf | The emf induced in a coil (or motor) by the change of its own current or motion; by Lenz's law it opposes the change that causes it. | 6.3 7.1 7.2 |
| eddy currents | Induced currents circulating inside a bulk piece of metal that moves in a magnetic field or sits in a changing one; they oppose the motion and heat the metal (beyond the board syllabus). | 6.1 6.2 7.3 |
| electromagnetic induction | The production of an emf in a circuit by a changing magnetic flux through it — discovered by Faraday and Henry in 1831. | 6.1 6.3 |
| Faraday disc | A metal disc spinning about an axis parallel to a magnetic field; a steady emf $\tfrac12B\omega r^2$ appears between its centre and its rim. | 6.2 |
| Faraday's law of induction | The induced emf in a circuit equals the negative rate of change of the flux linkage: $\varepsilon=-N\,d\Phi_B/dt$. | 6.1 6.2 6.3 8.1 |
| Fleming's right-hand rule | Thumb along the motion of a conductor, first finger along the field, second finger along the induced current (all at right angles). Used for generators. | 6.2 |
| flux linkage | The total flux through all the turns of a coil, $N\Phi_B$ for $N$ turns each threaded by flux $\Phi_B$. | 6.1 6.3 |
| henry | The SI unit of inductance: $1\ \text{H}=1\ \text{Wb/A}=1\ \text{V s/A}$ — a current changing at $1$ A/s induces $1$ V. | 6.3 |
| induced current | The current driven round a closed circuit by an induced emf, $I=\varepsilon/R$. Its direction is given by Lenz's law. | 5.2 6.1 6.2 |
| induced emf | An emf produced in a circuit by a changing magnetic flux (or by a conductor moving through a field), rather than by a cell. | 6.1 6.2 6.3 7.3 |
| inductor | A coil used for its self-inductance. It opposes changes of current (not steady current) and stores energy $\tfrac12LI^2$ in its magnetic field. | 6.3 7.1 7.2 7.3 |
| Lenz's law | The induced current flows in the direction whose magnetic field opposes the change of flux that produces it — a consequence of energy conservation. | 5.2 6.1 6.2 6.3 |
| magnetic damping | The slowing of a moving conductor (or the settling of a meter's needle) by the opposing force on its induced currents, the kinetic energy becoming heat. | 6.2 |
| magnetic flux | The amount of magnetic field threading a surface: $\Phi_B=\mathbf B\cdot\mathbf A=BA\cos\theta$ for a plane surface in a uniform field ($\theta$ measured from the normal). A scalar, measured in webers. | 5.1 6.1 7.3 8.1 |
| motional emf | The emf across a conductor moving through a magnetic field, caused by the magnetic force $q\mathbf v\times\mathbf B$ on its free charges: $\varepsilon=Blv$ for a rod perpendicular to $\mathbf B$ and $\mathbf v$. | 6.2 |
| mutual inductance | The flux linkage of one coil per unit current in another, $N_2\Phi_2=MI_1$, so that $\varepsilon_2=-M\,dI_1/dt$. For two long coaxial solenoids $M=\mu_0n_1n_2\pi r_1^2l$. | 6.3 7.3 |
| search coil | A small coil of known area and turns joined to a charge-measuring meter; flipping it or removing it from a field gives $B$ from $Q=N\,\Delta\Phi_B/R$. | 6.1 8.1 |
| self-inductance | The flux linkage of a coil per unit current in itself, $N\Phi_B=LI$; the coil's back emf is $-L\,dI/dt$. For a long solenoid $L=\mu_0N^2A/l$. | 6.3 |
| slip rings | Metal rings on the axle of an AC generator, joined to the ends of the coil; stationary carbon brushes pressing on them carry the current out. | 6.3 |
| terminal velocity | The constant speed reached when a speed-dependent opposing force has grown to balance the driving force. For a rod on rails driven by a force $F$: $v_t=FR/(B^2l^2)$. | 6.2 |
| weber | The SI unit of magnetic flux: $1\ \text{Wb}=1\ \text{T m}^2=1\ \text{V s}$. | 5.1 6.1 |
Ch 7 · Alternating Current
| Term | Meaning | Used in |
|---|---|---|
| alternating current | A current that reverses direction periodically, usually sinusoidally: $i=i_m\sin(\omega t+\phi)$. The mains in India is $50$ Hz AC. | 4.3 6.3 7.1 7.3 |
| alternating voltage | A voltage that varies sinusoidally with time, $v=v_m\sin\omega t$, with peak value $v_m$ and angular frequency $\omega=2\pi\nu$. | 4.1 7.1 7.3 8.1 |
| apparent power | The product $VI$ of rms voltage and current in an AC circuit, measured in volt-amperes (VA); the average power is the apparent power times the power factor. | 7.3 |
| bandwidth | For a resonant circuit, the width $\omega_2-\omega_1$ between the half-power frequencies; for a series LCR circuit it equals $R/L$ (rad/s). | 7.2 |
| capacitive reactance | The opposition of a capacitor to AC, $X_C=1/\omega C=1/(2\pi\nu C)$, in ohms; $I=V/X_C$. It decreases with frequency and is infinite for a steady current. | 7.1 |
| half-power frequencies | The two frequencies $\omega_1$, $\omega_2$ either side of resonance at which the current is $1/\sqrt2$ of its peak value, so the power in $R$ is half its maximum; there $|X_L-X_C|=R$. | 7.2 7.3 |
| impedance | The total opposition of an AC circuit, $Z=V/I$; for a series LCR circuit $Z=\sqrt{R^2+(X_C-X_L)^2}$, in ohms. | 7.2 7.3 |
| impedance triangle | The right-angled triangle with sides $R$ and $|X_C-X_L|$ and hypotenuse $Z$; its angle at $R$ is the phase angle $\phi$ between current and voltage. | 7.2 |
| inductive reactance | The opposition of an inductor to AC, $X_L=\omega L=2\pi\nu L$, in ohms; $I=V/X_L$. It increases with frequency and is zero for a steady current. | 7.1 |
| laminated core | A transformer core built from thin, insulated sheets of soft iron, which cut down the heating caused by currents induced in the iron itself. | 6.2 7.3 |
| LC oscillations | The oscillation of charge and current in a circuit of an inductor and a charged capacitor, at $\omega_0=1/\sqrt{LC}$, with energy passing between the electric and magnetic fields (beyond the NCERT syllabus). | 7.2 |
| peak value | The amplitude $v_m$ or $i_m$ of an alternating voltage or current — the largest value it reaches in a cycle. For a sinusoid it is $\sqrt2$ times the rms value. | 6.3 7.1 7.2 7.3 |
| phasor | An arrow of length equal to the peak value of a sinusoidal quantity, rotating anticlockwise at angular speed $\omega$; its projection on the vertical axis is the instantaneous value. The angle between two phasors is their phase difference. | 7.1 7.2 7.3 10.2 |
| power factor | $\cos\phi$, where $\phi$ is the phase difference between current and voltage; the ratio of the average power to $VI$. For a series LCR circuit $\cos\phi=R/Z$; it is $1$ for a resistor and $0$ for a pure inductor or capacitor. | 7.3 |
| quality factor | $Q=\omega_0L/R=\omega_0/(\omega_2-\omega_1)$, a measure of the sharpness of resonance; at resonance the voltage across $L$ or $C$ is $Q$ times the supply voltage. | 7.2 |
| resonance in a series LCR circuit | The condition $X_L=X_C$, reached at $\omega_0=1/\sqrt{LC}$, at which the impedance is least ($Z=R$), the current is largest and in phase with the voltage. | 7.2 |
| resonant frequency | The frequency at which a series LCR circuit resonates: $\omega_0=1/\sqrt{LC}$, or $\nu_0=1/(2\pi\sqrt{LC})$. It does not depend on $R$. | 7.1 7.2 |
| rms value | The square root of the mean of the square of a quantity over one cycle; for AC, the steady value that would give the same heating in a resistor. For a sinusoid $I=i_m/\sqrt2$, $V=v_m/\sqrt2$. Meters and ratings give rms values. | 7.1 7.3 |
| series LCR circuit | A resistor $R$, an inductor $L$ and a capacitor $C$ joined in series to an AC source, so that the same current flows through all three while their voltages differ in phase. | 7.2 7.3 |
| step-down transformer | A transformer with fewer turns on the secondary than on the primary ($N_s\lt N_p$), giving an output voltage smaller than the input (and a larger current). | 7.3 |
| step-up transformer | A transformer with more turns on the secondary than on the primary ($N_s\gt N_p$), giving an output voltage larger than the input (and a smaller current). | 7.3 |
| transformer | A device of two coils on a common iron core that changes an alternating voltage by mutual induction; ideally $V_s/V_p=N_s/N_p$ and $I_s/I_p=N_p/N_s$. It does not work on d.c. | 3.2 4.3 5.2 6.2 |
| turns ratio | The ratio $N_s/N_p$ of the numbers of turns on the secondary and primary of a transformer; for an ideal transformer it equals $V_s/V_p$. | 7.3 14.3 |
| wattless current | The component $I\sin\phi$ of an alternating current that is $90^\circ$ out of phase with the voltage; it carries no average power but still heats the supply wires. | 7.3 |
Ch 8 · Electromagnetic Waves
| Term | Meaning | Used in |
|---|---|---|
| Ampère–Maxwell law | Ampère's circuital law completed by Maxwell: $\oint\mathbf B\cdot d\mathbf l=\mu_0(i_c+i_d)=\mu_0i_c+\mu_0\varepsilon_0\,d\Phi_E/dt$. Valid for changing as well as steady currents. | 8.1 8.2 |
| conduction current | The ordinary current carried by moving charges, $i_c=dq/dt$ through a surface — as distinct from the displacement current. | 8.1 |
| displacement current | $\varepsilon_0$ times the rate of change of electric flux through a surface, $i_d=\varepsilon_0\,d\Phi_E/dt$. Not a flow of charge, but a source of magnetic field just like a current; between the plates of a charging capacitor it equals the current in the leads. | 4.3 8.1 |
| electromagnetic spectrum | The whole range of electromagnetic waves arranged by wavelength or frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. | 8.2 12.2 |
| electromagnetic wave | Oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel, that sustain each other and travel through vacuum at $c=1/\sqrt{\mu_0\varepsilon_0}\approx3.0\times10^8$ m/s, with $E_0=cB_0$. Produced by accelerated charges. | 2.4 8.1 8.2 10.3 |
| gamma rays | The most energetic electromagnetic radiation (wavelengths below about $1$ pm), emitted by radioactive nuclei and in nuclear reactions; used in cancer therapy and sterilisation. | 8.2 13.1 13.2 13.3 |
| infrared | Electromagnetic radiation of wavelength about $700$ nm to $1$ mm, emitted strongly by warm bodies; used in remote controls, night vision and fibre-optic links. | 8.2 9.2 11.1 11.2 |
| intensity of an electromagnetic wave | Average energy carried per unit time through unit area perpendicular to the direction of travel: $I=\tfrac12c\varepsilon_0E_0^2$ in vacuum (W/m²). | 8.2 |
| Maxwell's equations | The four laws of electromagnetism: Gauss's law for $\mathbf E$, Gauss's law for $\mathbf B$, Faraday's law and the Ampère–Maxwell law. With the Lorentz force they describe all classical electric and magnetic phenomena, and they predict electromagnetic waves. | 1.3 8.1 8.2 |
| microwaves | Electromagnetic waves of wavelength about $1$ mm to $0.1$ m; used in radar, satellite links, Wi-Fi and microwave ovens (2.45 GHz). | 2.3 8.2 10.2 10.3 |
| radiation pressure | The pressure exerted by an electromagnetic wave on a surface: $I/c$ for complete absorption and $2I/c$ for perfect reflection at normal incidence (beyond the NCERT syllabus; JEE/NEET). | 8.2 11.2 |
| radio waves | Electromagnetic waves longer than about $0.1$ m, made by charges oscillating in antennas; used for broadcasting, mobile networks and radio astronomy. | 8.2 |
| time-varying electric field | An electric field whose value at a point changes with time. By the Ampère–Maxwell law it produces a magnetic field, just as a current does. | 8.1 8.2 |
| ultraviolet | Electromagnetic radiation of wavelength about $1$ nm to $400$ nm; its photons can break chemical bonds. Most of the Sun's ultraviolet is absorbed by the ozone layer. | 8.2 11.1 11.2 12.1 |
| X-rays | Electromagnetic radiation of wavelength about $1$ pm to $1$ nm, produced when fast electrons are stopped in a metal target; used for medical imaging and studying crystals. | 8.2 11.1 11.2 11.3 |
Ch 9 · Ray Optics and Optical Instruments
| Term | Meaning | Used in |
|---|---|---|
| acceptance angle | The largest angle to the axis at which light entering the end of an optical fibre is still guided; in air $\sin\theta_{a}=\sqrt{n_1^2-n_2^2}$. | 9.2 |
| angle of deviation | The angle $\delta$ between the direction of the ray entering a prism and the direction of the emergent ray; $\delta=i+e-A$. | 9.4 |
| angle of minimum deviation | The smallest deviation $\delta_m$ a prism can produce, reached when the ray passes symmetrically ($i=e$, $r=A/2$); then $n=\sin\frac{A+\delta_m}{2}\big/\sin\frac A2$. | 9.4 |
| angle of the prism | The angle $A$ between the two refracting faces of a prism. | 9.4 |
| apparent depth | The depth at which an object under a transparent medium seems to lie when viewed from above; for near-normal viewing it is the real depth divided by $n$. | 9.2 |
| astronomical telescope | A telescope with a long-focus objective lens and a short-focus eyepiece; in normal adjustment $m=f_o/f_e$ and its length is $f_o+f_e$. | 9.4 |
| Cartesian sign convention | Distances are measured from the pole (or optical centre); those along the incident light are positive, those against it negative; heights above the axis are positive. | 9.1 9.3 |
| centre of curvature | The centre $C$ of the sphere of which a spherical mirror (or a lens surface) is a part; the normal at any point of the surface passes through it. | 9.1 9.3 |
| compound microscope | A microscope with a short-focus objective that forms a real enlarged image and an eyepiece that magnifies it: $m=m_om_e\approx\frac{L}{f_o}\frac{D}{f_e}$. | 9.4 |
| converging lens | A lens (usually thicker in the middle, convex) that bends parallel rays towards a real focus; $f\gt0$ and $P\gt0$. | 9.2 9.3 9.4 10.1 |
| critical angle | The angle of incidence in the denser medium for which the angle of refraction is $90^\circ$; $\sin C=n_\text{rarer}/n_\text{denser}$ ($=1/n$ against air). | 9.2 9.4 10.1 |
| dioptre | The unit of the power of a lens: $1\ \text{D}=1\ \text{m}^{-1}$, the power of a lens of focal length $1$ m. | 9.3 |
| dispersion | The splitting of white light into colours because the refractive index depends on wavelength; for a thin prism the angular dispersion is $(n_v-n_r)A$ (beyond the board syllabus). | 9.4 |
| diverging lens | A lens (usually thinner in the middle, concave) that spreads parallel rays as if from a virtual focus; $f\lt0$ and $P\lt0$. | 9.3 9.4 |
| lateral shift | The perpendicular distance between the incident ray and the parallel emergent ray after passing through a parallel-sided slab: $d=t\sin(i-r)/\cos r$. | 9.2 |
| least distance of distinct vision | The closest distance at which a normal eye sees clearly without strain, $D=25$ cm; the eye's near point. | 9.4 |
| lens maker's formula | $\dfrac1f=(n_{21}-1)\left(\dfrac1{R_1}-\dfrac1{R_2}\right)$: the focal length of a thin lens from its refractive index relative to the surrounding medium and the radii of its faces (Cartesian signs). | |
| linear magnification | The ratio $m=h'/h$ of image height to object height; for a mirror $m=-v/u$, for a lens $m=v/u$. Negative $m$ means an inverted image. | 9.1 9.4 |
| magnifying power | The ratio $\beta/\alpha$ of the angle subtended at the eye by the final image to the angle subtended by the object seen directly (at $D$ for a microscope, at its actual distance for a telescope). | 9.4 |
| mirage | An image of the sky (or of distant objects) seen on or near the ground, produced by the bending and finally total internal reflection of light in air layers of varying temperature. | 9.2 10.1 |
| mirror equation | $\dfrac1v+\dfrac1u=\dfrac1f$, relating object distance, image distance and focal length of a spherical mirror for paraxial rays (Cartesian signs). | 9.1 |
| optical centre | The point on the principal axis of a lens through which rays pass without deviation; lens distances are measured from it. | 9.1 9.3 |
| optical fibre | A thin strand of glass with a core of index $n_1$ inside a cladding of lower index $n_2$; light is guided along the core by repeated total internal reflection. | 9.2 10.1 10.3 |
| optically denser | Of two media, the optically denser one has the larger refractive index (light travels more slowly in it); the other is optically rarer. | 9.2 10.1 |
| paraxial rays | Rays that travel close to the principal axis and make small angles with it; the mirror and lens formulas hold exactly only for them. | 9.1 9.3 |
| pole of a mirror | The centre $P$ of the reflecting surface of a spherical mirror; distances for the mirror equation are measured from it. | |
| power of a lens | $P=1/f$ with $f$ in metres; it measures how strongly a lens bends light and is positive for converging lenses. | 9.3 |
| principal axis | The line through the pole and the centre of curvature of a mirror (or through the optical centre of a lens and the centres of its surfaces); the axis of symmetry. | 9.1 9.3 |
| principal focus | The point $F$ on the principal axis through which paraxial rays parallel to the axis pass after reflection or refraction (or from which they appear to diverge). Its distance from the pole or optical centre is the focal length $f$, signed by the Cartesian convention. | 9.1 9.3 |
| radius of curvature | The radius $R$ of the sphere of which a mirror or lens surface is a part; for a mirror, $R=2f$. | 9.1 9.3 |
| reflecting telescope | A telescope whose objective is a concave mirror (often with a small secondary mirror, as in the Cassegrain design); free of chromatic aberration and buildable in very large sizes. | 9.4 |
| refractive index | For a medium, $n=c/v$, the speed of light in vacuum divided by its speed in the medium. The relative index $n_{21}=n_2/n_1=\sin i/\sin r$ for light passing from medium 1 into medium 2. | 8.2 9.2 9.3 9.4 |
| simple microscope | A single converging lens of short focal length used close to the eye: $m=1+D/f$ (image at the near point) or $D/f$ (image at infinity). | 9.3 9.4 |
| Snell's law | The law of refraction: $n_1\sin i=n_2\sin r$, with both angles measured from the normal, and the incident ray, refracted ray and normal in one plane. | 9.2 9.3 9.4 10.1 |
| spherical aberration | The blurring of the focus of a spherical mirror or lens because rays far from the axis are brought to a focus at a slightly different point from paraxial rays. | 9.1 9.4 |
| thin lens | A lens whose thickness is negligible compared with the radii of curvature of its faces and with the object and image distances, so that both refractions can be treated as happening at one plane. | 9.3 |
| thin lens formula | $\dfrac1v-\dfrac1u=\dfrac1f$ for a thin lens and paraxial rays, with distances from the optical centre in the Cartesian convention; the magnification is $m=v/u$. | 9.1 9.3 9.4 |
| total internal reflection | Complete reflection of light back into a denser medium when it meets the boundary with a rarer medium at an angle of incidence greater than the critical angle. | 9.2 10.1 |
Ch 10 · Wave Optics
| Term | Meaning | Used in |
|---|---|---|
| Brewster's law | (Beyond NCERT.) Reflected light is completely plane-polarised when $\tan i_B=n$; the reflected and refracted rays are then perpendicular. | 10.3 |
| central maximum | The bright band straight ahead of a slit, between the first minima on either side. Its width is $2\lambda D/a$ (angular width $2\lambda/a$), twice that of each secondary maximum. | 10.2 10.3 |
| coherent sources | Sources (or waves) of the same frequency whose phase difference stays constant in time. Only coherent waves give a steady interference pattern. | 10.2 10.3 |
| constructive interference | Superposition of waves arriving in phase (path difference $n\lambda$ for in-phase sources), giving maximum intensity $\left(\sqrt{I_1}+\sqrt{I_2}\right)^2$. | 10.2 10.3 |
| destructive interference | Superposition of waves arriving in antiphase (path difference $(n+\tfrac12)\lambda$ for in-phase sources), giving minimum intensity $\left(\sqrt{I_1}-\sqrt{I_2}\right)^2$ — zero for equal waves. | 10.2 |
| diffraction envelope | In a double slit with slits of finite width, the single-slit pattern that sets the brightness of the interference fringes; fringes at its minima are missing. | 10.3 |
| diffraction of light | The spreading of light into the geometrical shadow at edges and openings, with bright and dark bands, caused by interference of the secondary wavelets from the unobstructed wavefront. Large when the opening is comparable with $\lambda$. | 10.3 11.3 |
| fringe width | The distance $\beta$ between successive bright (or dark) fringes. In Young's experiment $\beta=\lambda D/d$; the angular fringe width is $\lambda/d$. | 10.2 10.3 |
| Huygens' principle | Every point on a wavefront is a source of secondary wavelets spreading forward at the wave speed; a time $\Delta t$ later the new wavefront is the forward envelope (common tangent) of the wavelets. | 10.1 10.3 |
| incoherent sources | Sources whose phase difference changes randomly with time, such as two separate lamps. Their intensities simply add, $I=I_1+I_2$, and no fringes are seen. | 10.2 |
| interference fringes | The alternate bright and dark bands produced where coherent light waves overlap. | 10.1 10.2 10.3 |
| interference of light | The redistribution of light intensity where waves from coherent sources overlap: bright where they arrive in phase, dark where they arrive in antiphase. | 10.2 |
| Malus's law | Plane-polarised light of intensity $I_1$ passing through a polaroid whose axis is at $\theta$ to its plane of vibration emerges with intensity $I=I_1\cos^2\theta$. | 10.3 |
| optical path length | The product $nL$ of refractive index and geometrical length: the distance light would travel in vacuum in the time it takes to cover $L$ in the medium. A sheet of thickness $t$ adds an extra optical path $(n-1)t$. | 10.1 10.2 |
| path difference | The difference $\Delta$ between the distances travelled by two waves from their sources to a point. It gives a phase difference $\phi=2\pi\Delta/\lambda$. | 10.2 10.3 |
| plane wavefront | A flat wavefront with parallel rays — what a spherical wavefront becomes far from its source (sunlight, starlight, or a point source at the focus of a lens). | 10.1 |
| plane-polarised light | Light whose electric field oscillates along one fixed direction perpendicular to the direction of travel. | 10.3 |
| polarisation of light | The restriction of the electric-field vibrations of light to particular directions, showing that light is a transverse wave. | |
| polaroid | A sheet that transmits only the component of $\mathbf E$ along its transmission axis; it halves the intensity of unpolarised light and gives plane-polarised light. | 10.3 |
| secondary maxima | The faint bright bands of a single-slit pattern, roughly midway between the minima, at $a\sin\theta\approx(n+\tfrac12)\lambda$; about $4.7\%$, $1.6\%$… of the central intensity. | 10.3 |
| secondary wavelets | The small spherical waves that, in Huygens' principle, spread forward from every point of a wavefront with the speed of the wave in that medium. | 10.1 10.3 |
| single-slit diffraction | The pattern produced by one slit of width $a$: a bright central maximum with minima at $a\sin\theta=n\lambda$ ($n=\pm1,\pm2,\dots$) and faint secondary maxima between them. | 10.3 |
| spherical wavefront | The wavefront of a point source in a uniform medium: a sphere centred on the source, with radial rays. Its intensity falls as $1/r^2$. | 10.1 |
| wavefront | A surface on which every point of a wave oscillates in the same phase (e.g. all at a crest). It moves out from the source at the wave speed; rays are perpendicular to it. | 10.1 10.2 10.3 11.1 |
| Young's double-slit experiment | Light from one source passes through two narrow slits a distance $d$ apart, which act as coherent sources; equally spaced bright and dark fringes appear on a screen at distance $D$. | 10.2 10.3 |
Ch 11 · Dual Nature of Radiation and Matter
| Term | Meaning | Used in |
|---|---|---|
| Davisson–Germer experiment | The 1927 experiment in which $54$ V electrons scattered from a nickel crystal showed a diffraction peak at $50^\circ$, matching the de Broglie wavelength $0.167$ nm — the first direct evidence of matter waves (beyond the board syllabus). | 11.3 |
| de Broglie wavelength | The wavelength $\lambda=h/p=h/(mv)$ of the matter wave associated with a particle of momentum $p$; for an electron accelerated from rest through $V$ volts, $\lambda\approx1.227/\sqrt V$ nm. | 11.3 12.3 |
| Einstein's photoelectric equation | $K_{\max}=h\nu-\phi_0$ (equivalently $eV_0=h\nu-\phi_0$): one photon of energy $h\nu$ is absorbed by one electron, which spends at least $\phi_0$ to escape and keeps the rest as kinetic energy. | 11.1 11.2 |
| electron diffraction | The diffraction of a beam of electrons by the regularly spaced atoms of a crystal or thin foil, showing that electrons behave as waves; strongest when the de Broglie wavelength is comparable to the atomic spacing ($\sim0.1$–$0.3$ nm). | 11.3 |
| electron emission | The release of electrons from the surface of a material when they are given at least the work function in energy — by heating (thermionic), a strong field (field emission) or light (photoelectric). | 11.1 |
| electron microscope | A microscope that images with a beam of electrons focused by magnetic lenses; because their de Broglie wavelength is about $10^5$ times shorter than that of light, it can resolve details down to about $0.1$ nm. | 2.1 11.1 11.3 |
| field emission | Emission of electrons pulled out of a metal by a very strong electric field (about $10^8$–$10^9$ V/m), usually at a sharp tip. | 11.1 |
| matter waves | Waves associated with moving material particles (electrons, neutrons, atoms), with wavelength $h/p$; they show diffraction and interference when they meet structures comparable in size to their wavelength. Their intensity gives the probability of finding the particle. | 11.3 12.2 12.3 |
| photoelectric effect | The emission of electrons from a surface when electromagnetic radiation of high enough frequency (above the threshold frequency) falls on it. | 11.1 11.2 11.3 |
| photoelectrons | Electrons emitted from a surface by the photoelectric effect; their kinetic energies range from zero up to $K_{\max}=eV_0$. | 11.1 11.2 |
| photon | A quantum of electromagnetic radiation, behaving as a particle with energy $E=h\nu=hc/\lambda$ and momentum $p=h/\lambda$, travelling at $c$ with zero rest mass. In eV, $E=1240/\lambda(\text{nm})$. | 1.1 8.2 11.2 11.3 |
| photon flux | The number of photons crossing unit area per second, $\Phi=I/(h\nu)$; a beam of power $P$ carries $N=P\lambda/(hc)$ photons per second. | 11.2 |
| Planck constant | The fundamental constant $h=6.63\times10^{-34}$ J s linking the energy of a quantum of radiation to its frequency, $E=h\nu$; it can be measured from the slope $h/e$ of a stopping-potential–frequency graph. | 11.1 11.2 12.3 |
| quantum of radiation | The smallest amount, $h\nu$, in which radiation of frequency $\nu$ exchanges energy with matter; light of a given frequency is a stream of such packets. | 11.2 12.2 |
| saturation current | The largest photocurrent for given light, reached when every photoelectron emitted is collected: $I_\text{sat}=ne$, proportional to the intensity of the light. | 11.1 11.2 |
| stopping potential | The smallest negative collector potential (relative to the emitter) $V_0$ that makes the photocurrent zero; $eV_0=K_{\max}$. It depends on the frequency of the light and the metal, not on the intensity. | 11.1 11.2 |
| thermionic emission | Emission of electrons from a heated metal, whose fastest free electrons gain enough thermal energy to overcome the work function; used in the filament of an X-ray tube. | 11.1 |
| threshold frequency | The minimum frequency $\nu_0$ of radiation that can cause photoelectric emission from a given surface, whatever the intensity; the threshold wavelength is $\lambda_0=c/\nu_0$, and emission needs $\lambda\lt\lambda_0$. | 11.1 11.2 |
| wave–particle duality | The fact that radiation and matter both show wave properties (interference, diffraction) in some experiments and particle properties (localised impacts with definite energy and momentum) in others, linked by $E=h\nu$ and $p=h/\lambda$. | 11.3 |
| work function | The minimum energy $\phi_0$ that must be given to an electron inside a surface to remove it from the surface; a few eV for metals (caesium $2.14$ eV, platinum $5.65$ eV) and sensitive to the state of the surface. | 11.1 11.2 |
Ch 12 · Atoms
| Term | Meaning | Used in |
|---|---|---|
| absorption line spectrum | Dark lines in an otherwise continuous spectrum, produced when white light passes through a gas whose atoms absorb certain wavelengths — the wavelengths of some of their emission lines. | 12.2 12.3 |
| alpha particle | The nucleus of a helium-4 atom (two protons and two neutrons): charge $+2e$, mass about $6.64\times10^{-27}$ kg, roughly $7300$ electron masses. Emitted by some radioactive substances with energies of a few MeV. | 1.1 2.1 2.2 4.1 |
| Balmer series | The lines of hydrogen with $n_f=2$: $\frac1\lambda=R\left(\frac14-\frac1{n_i^2}\right)$, $n_i=3,4,\dots$; from $656.3$ nm (red) to the limit $364.6$ nm — the visible lines of hydrogen. | 12.2 12.3 |
| Bohr radius | The radius of the smallest ($n=1$) Bohr orbit of hydrogen, $a_0=\dfrac{\varepsilon_0h^2}{\pi me^2}\approx0.53$ Å $=5.3\times10^{-11}$ m. | 12.2 12.3 |
| emission line spectrum | The set of sharp bright lines, on a dark background, emitted by an excited gas of an element (in a discharge tube or a flame). Each element has its own pattern. | 12.2 |
| energy level | One of the allowed energies of an atom. For hydrogen $E_n=-13.6/n^2$ eV, $n=1,2,3,\dots$; an atom emits or absorbs light only by jumping between energy levels. | 8.2 12.3 14.1 14.2 |
| energy-level diagram | A ladder of horizontal lines showing the allowed energies of an atom, with $E=0$ (electron free and at rest) at the top; vertical arrows between lines show transitions and the photons emitted or absorbed. | 12.3 |
| excitation energy | The energy needed to raise an atom from its ground state to a given excited level, $E_n-E_1$. For hydrogen the first excitation energy is $10.2$ eV (first excitation potential $10.2$ V). | 12.3 |
| excited state | Any energy level above the ground state ($n\ge2$ for hydrogen). An atom stays in an excited state only briefly before dropping to a lower level and emitting a photon. | 12.3 13.3 |
| Geiger–Marsden experiment | The alpha-particle scattering experiment (1909–1913): a beam of alpha particles was fired at a thin gold foil and the number scattered through each angle was counted. A very few bounced back, revealing the nucleus. | 12.1 |
| ground state | The lowest energy level of an atom ($n=1$ for hydrogen, $E_1=-13.6$ eV), in which an undisturbed atom is normally found. | 12.3 |
| impact parameter | The perpendicular distance $b$ from the centre of a nucleus to the initial line of motion of a projectile such as an alpha particle. The smaller $b$, the larger the scattering angle; $b=0$ is a head-on collision. | 12.1 |
| ionisation energy | The least energy needed to remove an electron from an atom in its ground state: $13.6$ eV for hydrogen. From a level $n$ it is $|E_n|$. | 12.2 12.3 14.2 |
| ionisation potential | The ionisation energy expressed in volts — the potential difference through which an electron must be accelerated to gain that energy: $13.6$ V for hydrogen. | 12.3 |
| Lyman series | The lines of hydrogen with $n_f=1$: $\frac1\lambda=R\left(1-\frac1{n_i^2}\right)$, $n_i=2,3,\dots$; from $121.5$ nm down to the limit $91.2$ nm, in the ultraviolet. | 12.2 12.3 |
| Paschen series | The lines of hydrogen with $n_f=3$: $\frac1\lambda=R\left(\frac19-\frac1{n_i^2}\right)$, $n_i=4,5,\dots$; from $1875$ nm to $820$ nm, in the infrared. (Brackett, $n_f=4$, and Pfund, $n_f=5$, lie further into the infrared.) | 12.2 12.3 |
| principal quantum number | The whole number $n=1,2,3,\dots$ that labels a Bohr orbit, through $mvr=nh/2\pi$. For hydrogen $r_n=0.53n^2$ Å and $E_n=-13.6/n^2$ eV. | 12.2 |
| Rutherford's nuclear model | The model in which all the positive charge and more than $99.9\%$ of the mass of an atom are concentrated in a tiny central nucleus (radius about $10^{-15}$–$10^{-14}$ m), with the electrons moving around it at distances of about $10^{-10}$ m. | 12.1 12.2 |
| Rydberg constant | The constant $R=1.097\times10^7\ \text{m}^{-1}$ in the formula $\frac1\lambda=R\left(\frac1{n_f^2}-\frac1{n_i^2}\right)$ for the spectral lines of hydrogen. | 12.2 12.3 |
| scattering angle | The angle $\theta$ between the initial and final directions of motion of a scattered particle. $\theta=180^\circ$ means the particle comes straight back. | 12.1 |
| series limit | The shortest wavelength of a spectral series, $\lambda_\infty=n_f^2/R$ for hydrogen, approached as $n_i\to\infty$; the lines crowd together towards it. | 12.2 12.3 |
| stationary orbit | In Bohr's model, an allowed orbit in which the electron does not radiate, so that its energy stays constant. The allowed orbits have angular momentum $mvr=nh/2\pi$. | 12.2 12.3 |
| Thomson's model | The early model in which an atom is a sphere of radius about $10^{-10}$ m with its positive charge spread uniformly through it and the electrons embedded in it. Ruled out by alpha-particle scattering. | 12.1 |
Ch 13 · Nuclei
| Term | Meaning | Used in |
|---|---|---|
| atomic mass unit | One-twelfth of the mass of a neutral carbon-12 atom: $1\ \text{u}=1.66\times10^{-27}$ kg, roughly the mass of one nucleon. | 13.1 13.2 |
| binding energy | The energy needed to separate a bound system completely into its parts. For a nucleus it is the energy to free all its protons and neutrons, $E_b=\Delta M\,c^2$ — equally, the energy released when it forms from them. | 12.1 13.2 13.3 |
| binding energy per nucleon | The binding energy divided by the mass number, $E_b/A$ — a measure of how tightly bound a nucleus is; its maximum, about $8.8$ MeV, is near iron. | 13.2 |
| chain reaction | A self-sustaining sequence of fissions in which neutrons from one generation of fissions cause the next. | 13.3 |
| control rods | Rods of a strongly neutron-absorbing material (cadmium, boron) moved in and out of a reactor core to adjust the multiplication factor. | 13.3 |
| critical mass | The smallest mass (or size) of fissile material, for a given shape, in which a chain reaction can sustain itself ($k=1$); smaller lumps lose too many neutrons through the surface. | 13.3 |
| isobars | Nuclides with the same mass number $A$ but different atomic numbers $Z$. | 13.1 |
| isotones | Nuclides with the same neutron number $N$ but different atomic numbers $Z$. | 13.1 |
| isotopes | Nuclides with the same atomic number $Z$ but different neutron numbers — the same element with different masses. | 4.1 13.1 |
| mass defect | The amount by which the mass of a nucleus is less than the total mass of its separated protons and neutrons, $\Delta M=Zm_p+Nm_n-M$. | 13.2 |
| mass number | The total number of nucleons in a nucleus, $A=Z+N$. | 13.1 13.2 13.3 |
| mass spectrometer | An instrument that ionises atoms and sorts the ions by mass, using the radius $r=mv/qB$ of their circular paths in a magnetic field. | 4.1 13.1 |
| mass–energy equivalence | Einstein's result that mass is a form of energy: a mass $m$ corresponds to an energy $E=mc^2$, and $1\ \text{u}$ corresponds to $931.5$ MeV. | 13.2 |
| moderator | A material of light nuclei (water, heavy water, graphite) that slows fast fission neutrons by elastic collisions, so that they cause fission in uranium-235 more readily. | 13.3 |
| multiplication factor | The ratio $k$ of the number of fissions in one generation to the number in the previous one: $k\lt 1$ subcritical, $k=1$ critical, $k\gt 1$ supercritical. | 13.3 |
| neutron number | The number of neutrons in a nucleus, $N=A-Z$. | 13.1 |
| nuclear density | The density of matter inside a nucleus, about $2.3\times10^{17}$ kg/m³ — the same for all nuclei because volume is proportional to $A$. | 13.1 13.2 |
| nuclear fission | The splitting of a heavy nucleus (such as uranium-235 after absorbing a slow neutron) into two medium nuclei, with 2–3 neutrons and about $200$ MeV released. | 13.2 13.3 |
| nuclear force | The very strong, short-range (about $2$–$3$ fm), charge-independent attraction between nucleons that holds a nucleus together; repulsive below about $0.8$ fm. | 12.1 13.1 13.2 13.3 |
| nuclear fusion | The joining of two light nuclei into a heavier one, with the release of energy; it needs very high temperatures (thermonuclear fusion) to overcome the Coulomb repulsion. | 4.3 13.2 13.3 |
| nuclear reactor | A device in which a controlled fission chain reaction ($k=1$) produces heat, using fuel, a moderator, control rods, a coolant and shielding. | 6.3 11.3 13.2 13.3 |
| nucleon | A proton or a neutron — the particles that make up a nucleus. | 13.1 13.2 13.3 |
| nuclide | A particular kind of nucleus with a definite number of protons and neutrons, written ${}^{A}_{Z}\text{X}$. | 13.1 13.2 |
| proton–proton chain | The sequence of fusion reactions by which the Sun turns four protons into a helium-4 nucleus, releasing $26.7$ MeV. | 13.3 |
| Q-value | The energy released in a nuclear reaction, $Q=(m_\text{initial}-m_\text{final})c^2$; positive for an exothermic reaction, negative for an endothermic one. | |
| rest-mass energy | The energy $mc^2$ that a body has because of its mass alone, when at rest — $0.511$ MeV for an electron, $938.3$ MeV for a proton. | 13.2 13.3 |
| saturation of the nuclear force | The property that each nucleon is bound only to its nearest neighbours, so the binding energy grows in proportion to $A$ and $E_b/A$ is nearly constant. | 13.2 |
Ch 14 · Semiconductor Electronics: Materials, Devices and Simple Circuits
| Term | Meaning | Used in |
|---|---|---|
| acceptor | A trivalent dopant atom that accepts an electron from a neighbouring bond, creating a hole and becoming a fixed negative ion; its level $E_A$ lies just above $E_V$. | 14.2 14.3 |
| band gap | The energy range $E_g=E_C-E_V$ between the top of the valence band and the bottom of the conduction band, in which a perfect crystal has no allowed electron levels. | 12.3 14.1 14.2 |
| barrier potential | The potential difference $V_0$ across the depletion region of an unbiased junction (n side higher), about $0.7$ V for Si and $0.3$ V for Ge; it opposes diffusion of majority carriers. | 14.3 |
| compound semiconductor | A semiconductor made of two or more elements, such as gallium arsenide (GaAs), cadmium sulphide (CdS) or indium phosphide (InP). | 14.1 |
| conduction band | The band just above the valence band; electrons in it are free to move through the crystal and carry current. Its bottom edge is $E_C$. | 14.1 14.2 |
| depletion region | The thin layer at a p–n junction (about $0.1$–$1\ \mu$m wide) emptied of mobile carriers, containing only fixed negative acceptor ions (p side) and positive donor ions (n side). | 14.3 |
| donor | A pentavalent dopant atom whose fifth valence electron is easily freed into the conduction band, leaving a fixed positive ion; its level $E_D$ lies just below $E_C$. | 14.2 14.3 |
| doping | Adding a small, controlled amount of a suitable impurity (the dopant), typically one atom in $10^5$ to $10^9$, to a pure semiconductor to change its conductivity. | 3.2 14.2 14.3 |
| electron–hole pair | A free electron in the conduction band together with the hole (vacancy in a covalent bond) it leaves behind; created when thermal energy or a photon breaks a bond. A hole behaves as a mobile carrier of charge $+e$. | 14.1 14.2 |
| energy band | A range of very closely spaced allowed electron energies in a crystal, formed when one atomic level splits into $N$ levels as $N$ atoms come together. | 14.1 |
| extrinsic semiconductor | A semiconductor whose carrier densities are set by deliberately added impurity (dopant) atoms; it is either n-type or p-type. | 14.2 |
| forward bias | Connecting the p side of a junction to the positive terminal: the barrier falls to $V_0-V$, the depletion region narrows and a large current flows above the knee voltage. | 14.1 14.3 |
| full-wave rectifier | A rectifier (two diodes with a centre-tap transformer, or a four-diode bridge) that passes both halves of each cycle in the same direction; $V_\text{dc}=2V_m/\pi$, output pulses at $2f$. | 7.1 14.3 |
| half-wave rectifier | A single diode in series with the load, passing only one half of each AC cycle; $V_\text{dc}=V_m/\pi$, output pulses at the supply frequency $f$. | 14.3 |
| intrinsic semiconductor | A pure semiconductor, whose carriers come only from thermally broken bonds, so that $n_e=n_h=n_i$. | 14.2 |
| knee voltage | The forward voltage (about $0.7$ V for Si, $0.2$–$0.3$ V for Ge) above which a diode's current rises steeply. | 14.3 |
| majority carriers | The more numerous kind of mobile carrier in an extrinsic semiconductor: electrons in n-type, holes in p-type. | 14.2 14.3 |
| minority carriers | The less numerous kind of mobile carrier in an extrinsic semiconductor (holes in n-type, electrons in p-type), produced thermally; their density is $n_i^2$ divided by the majority density. | 14.2 14.3 |
| n-type semiconductor | A semiconductor doped with pentavalent donor atoms (P, As, Sb); electrons are the majority carriers. | 14.2 14.3 |
| p-type semiconductor | A semiconductor doped with trivalent acceptor atoms (B, Al, Ga, In); holes are the majority carriers. | 14.2 14.3 |
| p–n junction | The boundary between a p-type and an n-type region of one semiconductor crystal; the basis of diodes, LEDs and solar cells. | 14.1 14.3 |
| reverse bias | Connecting the p side of a junction to the negative terminal: the barrier rises to $V_0+V$, the depletion region widens and only a tiny minority-carrier current flows (until breakdown). | 14.1 14.3 |
| ripple | The remaining periodic variation in the output of a rectifier; a filter capacitor across the load reduces it, more so for larger $C$ and load resistance. | 10.1 14.3 |
| semiconductor | A solid with resistivity between metals and insulators (roughly $10^{-5}$ to $10^{6}\ \Omega$ m) that falls as the temperature rises and can be changed greatly by impurities or light; e.g. Si, Ge, GaAs. | 1.1 3.1 3.2 12.1 |
| semiconductor diode | A p–n junction with metal contacts; it conducts easily when forward biased and passes only a tiny current when reverse biased. | 3.2 14.3 |
| valence band | The band containing the valence electrons — the highest band that is filled (or partly filled) at absolute zero; its top edge is $E_V$. | 14.1 14.2 |