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HSC Board 2026 Physics Important Definitions Class 12

HSC Board 2024 Physics Important Definitions

HSC Board 2026

Imp Definitions for Board Exam 2026

1. ROTATIONAL DYNAMICS
(1) Uniform circular motion.
Ans. A particle is said to perform uniform circular motion if it moves in a circle or a circular arc at constant linear speed or constant angular velocity.
(2) Centripetal force.
Ans. In the uniform circular motion of a particle, the centripetal force is the force on the particle which at every instant points radially inward and produces the centripetal acceleration necessary to make the particle move in its circular path.
(3) Centrifugal force.
Ans. In the reference frame of a particle performing circular motion, centrifugal force is defined as a fictitious, radially outward force on the particle and is equal in magnitude to the particle's mass times the centripetal acceleration of the reference frame, as measured from an inertial frame of reference.
(4) Angle of banking.
Ans. Angle of banking is the angle of inclination of a banked road with the horizontal.
(5) Conical pendulum.
Ans. A conical pendulum is a simple pendulum whose bob revolves in a horizontal circle with constant speed such that the string describes the surface of a right circular cone.
(6) Moment of inertia.
Ans. The moment of inertia of a body about a given axis of rotation is defined as the sum of the products of the masses of the particles of the body and the squares of their respective distances from the axis of rotation.
(7) Radius of gyration.
Ans. The radius of gyration of a body rotating about an axis is defined as the distance between the axis of rotation and the point at which the entire mass of the body can be supposed to be concentrated so as to give the same moment of inertia as that of the body about the given axis.
(8) Angular momentum of a particle.
Ans. The angular momentum of a particle is defined as the moment of the linear momentum of the particle.

HSC Physics Board Papers with Solution

2. MECHANICAL PROPERTIES OF FLUIDS
(1) Coefficient of viscosity.
Ans. The coefficient of viscosity of a fluid is defined as the viscous drag per unit area acting on a fluid layer per unit velocity gradient established in a steady flow.
(2) Angle of contact.
Ans. The angle of contact for a liquid-solid pair (a liquid in contact with a solid) is defined as the angle between the surface of the solid and the tangent drawn to the free surface of the liquid at the extreme edge of the liquid, as measured through the liquid.
(3) Pressure.
Ans. The pressure at a point in a fluid in hydrostatic equilibrium is defined as the normal force per unit area exerted by the fluid on a surface of infinitesimal area containing the point.
(4) Absolute pressure.
Ans. The absolute pressure, or total pressure, is measured relative to absolute zero on the pressure scale—which is a perfect vacuum—and is the sum of gauge pressure and atmospheric pressure. It is the same as the thermodynamic pressure.
(5) Gauge pressure.
Ans. Gauge pressure is the pressure exerted by a fluid relative to the local atmospheric pressure. Gauge pressure, \( p_g = p - p_0 \) where \( p \) is the absolute pressure and \( p_0 \) is the local atmospheric pressure.
(6) Range of molecular attraction or molecular range.
Ans. Range of molecular attraction is defined as the maximum distance between two molecules up to which the intermolecular force of attraction is appreciable.
(7) Sphere of influence.
Ans. The sphere of influence of a molecule is defined as an imaginary sphere drawn with the molecule as the centre and radius equal to the range of molecular attraction.
(8) Surface tension.
Ans. Surface tension of a liquid is defined as the tangential force per unit length, acting at right angles on either side of an imaginary line drawn on the free surface of the liquid.
(9) Surface energy.
Ans. Surface energy is defined as the extra (i.e., increased) potential energy of a liquid surface with an isothermal increase in the surface area.
(10) Velocity gradient in a steady flow.
Ans. In a steady flow of a fluid past a solid surface, the rate at which the velocity changes with distance within a limiting distance from the surface is called the velocity gradient.
(11) Volume flux.
Ans. The volume of fluid passing by a given point per unit time through an area is called the volume flux or volume flow rate.
(12) Mass flux.
Ans. The mass of fluid passing by a given point per unit time through an area is called the mass flux or mass flow rate.
3. KINETIC THEORY OF GASES AND RADIATION
(1) Coefficient of emission (emissivity) of a body.
Ans. Coefficient of emission (emissivity) of a body is defined as the ratio of the emissive power of the body to the emissive power of a blackbody at the same temperature as that of the body.
(2) Emissive power of a body.
Ans. Emissive power of a body at a given temperature is defined as the quantity of radiant energy emitted by the body per unit time per unit surface area of the body at that temperature.
(3) Mean free path.
Ans. The average distance travelled by a gas molecule between successive collisions, the average being taken over a large number of free paths (or collisions), is called the mean free path.
(4) Root mean square speed of gas molecules.
Ans. Root mean square speed of gas molecules is defined as the square root of the arithmetic mean of the squares of the speeds of all molecules of the gas at a given temperature.
(5) Molar heat capacity of a gas at constant volume.
Ans. Molar heat capacity of a gas at constant volume is defined as the quantity of heat required to raise the temperature of one mole of the gas through one degree (\(1^{\circ}\text{C}\) or 1 K), when its volume is kept constant.
(6) Molar heat capacity of a gas at constant pressure.
Ans. Molar heat capacity of a gas at constant pressure is defined as the quantity of heat required to raise the temperature of one mole of the gas through one degree (\(1^{\circ}\text{C}\) or 1 K), when its pressure is kept constant.
(7) Coefficient of absorption (Absorptance).
Ans. The coefficient of absorption (absorptance or absorptive power) of a body is defined as the ratio of the quantity of radiant energy absorbed by the body to the quantity of radiant energy incident on the body in the same time.
(8) Coefficient of reflection (Reflectance).
Ans. The coefficient of reflection (reflectance) of the surface of a body is defined as the ratio of the quantity of radiant energy reflected by the surface to the quantity of radiant energy incident on the surface in the same time.
(9) Coefficient of transmission (Transmittance).
Ans. The coefficient of transmission (transmittance) of a body is defined as the ratio of the quantity of radiant energy transmitted by the body to the quantity of radiant energy incident on the body in the same time.
4. THERMODYNAMICS
1. Internal energy.
Ans. Internal energy of a system is defined as the sum of the kinetic energies of the atoms and molecules belonging to the system, and the potential energies associated with the interactions between these constituents (atoms and molecules).
2. Zeroth law of thermodynamics:
If two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.
3. First law of thermodynamics :
According to the first law of thermodynamics, "the total energy of a system and surroundings remains constant when the system changes from an initial state to final state."
4. Second law of thermodynamics :
i. Kelvin-Planck statement: Heat \( Q_H \) cannot be taken out of a hot reservoir and used in its whole for labour W. It is necessary for \( Q_C \) to exhaust (give away) some of its heat to a cold reservoir. This rules out the development of an ideal heat engine.

ii. Clausius statement: Heat cannot transfer from a colder body to a warmer body unless some effort is made to do this. This rules out the creation of the ideal refrigerator.
5. Mechanical equilibrium:
When there are no unbalanced forces within the system and between the system and its surrounding, the system is in mechanical equilibrium.
6. Chemical equilibrium:
If there is no net chemical reaction between two thermodynamic systems in contact with each other then it is said to be in chemical equilibrium.
7. Thermal equilibrium:
Two systems are said to be in thermal equilibrium with each other if they are at the same temperature, which will not change with time.
8. Quasi-static process:
A quasi-static process is an infinitely slow process in which the system changes its variables (\( P, V, T \)) so slowly such that it remains in thermal, mechanical, and chemical equilibrium with its surroundings throughout.
9. Reversible processes:
A thermodynamic process can be considered reversible only if it possible to retrace the path in the opposite direction in such a way that the system and surroundings pass through the same states as in the initial, direct process.
10. Irreversible processes:
All natural processes are irreversible. Irreversible processes cannot be plotted in a PV diagram, because these processes cannot have unique values of pressure, the temperature at every stage of the process.
11. Isothermal process:
It is a process in which the temperature remains constant but the pressure and volume of a thermodynamic system will change.
12. Isobaric process:
A thermodynamic process that is carried out at constant pressure i.e., \( \Delta p = 0 \) is called the isobaric process.
13. Isochoric process:
A thermodynamic process in which the volume of the system is kept constant is called the isochoric process.
14. Adiabatic process:
It is defined as one in which there is no exchange of heat (q) between the system and surrounding during operations.
5. OSCILLATIONS
(1) Periodic motion.
Ans. A motion that repeats itself at definite intervals of time is said to be a periodic motion.
(2) Oscillatory motion.
Ans. A periodic motion in which a body moves back and forth over the same path, straight or curved, between alternate extremes is said to be an oscillatory motion.
(3) Linear simple harmonic motion.
Ans. Linear simple harmonic motion is defined as the linear periodic motion of a body in which the force on the body (or its acceleration) is always directed towards the mean position of the body and its magnitude is proportional to the displacement of the body from the mean position.
(4) Period or periodic time of SHM.
Ans. The time taken by a particle performing simple harmonic motion to complete one oscillation is called the period or periodic time of SHM.
(5) Frequency of SHM.
Ans. The number of oscillations performed per unit time by a particle executing SHM is called the frequency of SHM.
(6) Amplitude of SHM.
Ans. The magnitude of the maximum displacement of a particle performing SHM from its mean position is called the amplitude of SHM.
(7) Phase of SHM.
Ans. Phase of simple harmonic motion represents the state of oscillation of the particle performing simple harmonic motion (SHM), i.e., it gives the displacement of the particle and its direction of motion from the equilibrium position.
(8) Ideal simple pendulum.
Ans. An ideal simple pendulum is a heavy point mass suspended from a rigid support by a weightless, inextensible and twistless string, and set oscillating under gravity through a small angle in a vertical plane.
(9) Seconds pendulum.
Ans. A simple pendulum of period two seconds is called a seconds pendulum.
(10) Angular SHM.
Ans. Angular SHM is defined as the oscillatory motion of a body in which the restoring torque responsible for angular acceleration is directly proportional to the angular displacement and its direction is opposite to that of angular displacement.
11. Damped oscillations.
Ans. Oscillations of gradually decreasing amplitude in the presence of dissipative frictional forces are called damped oscillations.
6. SUPERPOSITION OF WAVES
(1) Progressive wave OR Travelling wave.
Ans. A progressive wave or a wave motion is a periodic or oscillatory disturbance in a medium or in vacuum which is propagated without any damping and obstruction from one place to another at a finite speed.
(2) Transverse progressive wave.
Ans. A progressive wave in which the vibration of the individual particles of the medium is perpendicular to the direction of propagation of the wave is called a transverse progressive wave.
(3) Longitudinal progressive wave.
Ans. A progressive wave in which the vibration of the individual particles of the medium is along the line of propagation of the wave is called a longitudinal progressive wave.
(4) Stationary wave OR Standing wave.
Ans. When two identical progressive waves, i.e., waves having the same amplitude, wavelength and speed, propagate in opposite directions through the same region of a medium, their superposition under certain conditions creates a stationary interference pattern called a stationary wave or a standing wave.
(5) Transverse stationary wave.
Ans. When two identical transverse progressive waves travelling in opposite directions along the same line superimpose, the resultant wave produced is called a transverse stationary wave.
(6) Longitudinal stationary wave.
Ans. When two identical longitudinal progressive waves travelling in opposite directions along the same line superimpose, the resultant wave produced is called a longitudinal stationary wave.
(7) Free vibrations.
Ans. Vibrations of a body, free to vibrate, when it is disturbed from its equilibrium position and left to itself are called free vibrations.
(8) Forced vibrations.
Ans. The vibrations of a body in response to an external periodic force are called forced vibrations.
(9) Resonance.
Ans. If a body is made to vibrate by an external periodic force, whose frequency is equal to the natural frequency (or nearly so) of the body, the body vibrates with maximum amplitude. This phenomenon is called resonance.
(10) Beats.
Ans. A periodic variation in loudness (or intensity) when two sound notes of slightly different frequencies are sounded at the same time is called beats.
(11) Period of beats.
Ans. The time interval between successive maxima or minima of sound at a given place is called the period of beats.
(12) Beat frequency.
Ans. The number of beats produced per unit time is called the beat frequency.
7. WAVE OPTICS
(1) Wavefront.
Ans. A wavefront is defined as a surface of all neighbouring points which receive light waves from a source at the same instant and are in the same phase.
(2) Wave normal.
Ans. A wave normal at a point on a wavefront is defined as a line drawn perpendicular to the wavefront in the direction of propagation of the wavefront.
(3) Plane of vibration.
Ans. The plane of vibration of an electromagnetic wave is the plane of vibration of the electric field vector containing the direction of propagation of the wave. Experiment shows that it is the electric field vector \( E \) which produces the optical polarization effects.
(4) Plane of polarization.
Ans. The plane of polarization of an electromagnetic wave is defined as the plane perpendicular to the plane of vibration. It is the plane containing the magnetic field vector and the direction of propagation of the wave.
(5) The Brewster angle OR the polarizing angle.
Ans. The Brewster angle or the polarizing angle for an interface is the angle of incidence for a ray of unpolarized light at which the reflected ray is completely plane polarized.
(6) Interference of light.
Ans. Interference of light is the phenomenon in which the superposition of two or more light waves produces a resultant disturbance of redistributed light intensity or energy.
(7) Diffraction of light.
Ans. Diffraction of light is the phenomenon of bending of light waves at an edge into the region of the geometrical shadow.
(8) Resolving power of an optical instrument.
Ans. The resolving power of an optical instrument is defined as the reciprocal of its limit of resolution which is the smallest linear or angular separation between two point objects which appear just resolved when viewed through the instrument.
(9) Resolving power of a microscope.
Ans. The resolving power of a microscope is defined as the reciprocal of the least separation between two closely-spaced points on an object which are just resolved when viewed through the microscope.
(10) Resolving power of a telescope.
Ans. The resolving power of a telescope is defined as the reciprocal of the angular limit of resolution between two closely-spaced distant objecl so that they are just resolved when seen through the telescope.
8. ELECTROSTATICS
(1) Electric potential difference.
Ans. The electric potential difference between two points in an electric field is defined as the work done per unit charge by an external agent against the electric force in moving an infinitesimal positive charge from one point to the other without acceleration.
(2) Electric potential.
Ans. The electric potential at a point in an electric field is defined as the work per unit charge that must be done by an external agent against the electric force to move without acceleration a sufficiently small positive test charge from infinity to the point of interest.
(3) The electronvolt.
Ans. The electronvolt (symbol, eV) is the increase in the kinetic energy of a particle with a charge equal in magnitude to the elementary charge \( e \) when the particle is accelerated through a potential difference of one volt.
(4) Electric potential gradient.
Ans. The rate of change of electric potential with distance in a specified direction is called the electric potential gradient in that direction.
(5) Electric polarization.
Ans. The electric polarization at every point within a dielectric is defined as the electric dipole moment per unit volume. It has the direction of the external electric field.
(6) Capacitance of a capacitor.
Ans. The capacitance of a capacitor is defined as the ratio of the charge on either conductor to the potential difference between the two conductors forming the capacitor.
9. CURRENT ELECTRICITY
1. Junction:
Any point in an electric circuit where two or more conductors are joined together is a junction
2. Loop:
Any closed conducting path in an electric network is called a loop or mesh.
3. Branch:
A branch is any part of the network that lies between two junctions.
4. Kirchhoff’s First Law: (Current law/ Junction law) :
The algebraic sum of the currents at a junction in an electrical network, is zero
5. Kirchhoff’s Second Law: (Voltage law )
The algebraic sum of the potential differences (products of current and resistance) and the electromotive forces (emfs) in a closed loop is zero.
6. Voltmeter :
A voltmeter is a device which is used for measuring potential difference between two points in a circuit.
7. Potentiometer:
Potentiometer is one such device which does not draw any current from the circuit.
8. Shunt:
Moving coil galvanometer is converted into an ammeter by connecting a low resistance in parallel with the galvanometer, which effectively reduces the resistance of the galvanometer. This low resistance connected in parallel is called as shunt (S).
9. Potential gradient:
With a potential difference applied across a uniform resistance wire, potential gradient along the wire is the potential difference (the fall of potential from the high potential end) per unit length of the wire.
10. MAGNETIC FIELDS DUE TO ELECTRIC CURRENT
(1) The ampere.
Ans. The ampere is that constant current which if maintained in two infinitely long straight parallel wires, placed one metre apart in vacuum, would cause each wire to experience a force per unit length of \( 2 \times 10^{-7} \) newton per metre.
(2) The SI unit of magnetic field/induction OR The tesla.
Ans. The SI unit of magnetic field/induction is the tesla. The magnitude of magnetic induction is said to be one tesla if a charge of one coulomb experiences a force of one newton when it moves at one metre per second in a magnetic field in a direction perpendicular to the direction of the field.
11. MAGNETIC MATERIALS
(1) Magnetization.
Ans. The magnetization of the material is defined as the net magnetic moment per unit volume of a material.
(2) Magnetic intensity.
Ans. The magnetic intensity is defined as the magnetic induction in an isotropic medium divided by the permeability of the medium.
12. ELECTROMAGNETIC INDUCTION
(1) Magnetic flux.
Ans. The magnetic flux through a given area in a magnetic field is defined as the total number of magnetic lines of force passing normally through that area.
(2) Electromagnetic induction.
Ans. Electromagnetic induction is the phenomenon of production of emf in a conductor or circuit due to the motion of the conductor in a magnetic field or by a changing magnetic flux through the circuit.
(3) Self induction.
Ans. The phenomenon of production of induced emf in a coil, due to the change of current in the same coil, is called self induction.
(4) Self inductance OR Coefficient of self induction.
Ans. The self inductance or the coefficient of self induction of a coil is defined as the emf induced in the coil per unit time rate of change of current in the same coil.
(5) The henry.
Ans. The self inductance of a coil is 1 henry if an emf of 1 volt is induced in the coil when the current through the same coil changes at the rate of 1 ampere per second.
(6) Mutual induction.
Ans. The phenomenon of production of induced emf in one coil due to changing current in a magnetically linked neighbouring coil is called mutual induction.
(7) Mutual inductance OR Coefficient of mutual induction.
Ans. The mutual inductance or the coefficient of mutual induction of two magnetically linked coils is equal to the flux linkage of one coil per unit current in the neighbouring coil.
13. AC CIRCUITS
(1) Inductive reactance.
Ans. The resistance offered by an inductor to the alternating current through it is called the inductive reactance.
(2) Capacitive reactance.
Ans. The resistance offered by a capacitor to the alternating current ilirough it is called the capacitive reactance.
(3) Impedance.
Ans. In an AC circuit containing resistance and inductance and/or capacitance, the effective resistance offered by the circuit is called impedance.
(4) Average or mean value of A.C.
Average or mean value of A.C. is the average of all values of the voltage (or current) over the one-half cycle.
14. DUAL NATURE OF RADIATION AND MATTER
(1) Threshold frequency.
Ans. The threshold frequency for a given metal surface is defined as the characteristic minimum frequency of the incident radiation below which no photoelectrons are emitted from that metal surface.
(2) Threshold wavelength.
Ans. The threshold wavelength for a given metal surface is defined as the characteristic maximum wavelength of the incident radiation above which no photoelectrons are emitted from that metal surface.
(3) Stopping potential.
Ans. The stopping potential is defined as the value of the retarding potential difference that is just sufficient to stop the most energetic photoelectrons from reaching the collector so that the photoelectric current in a photocell reduces to zero.
(4) Photoelectric work function.
Ans. The photoelectric work function of a metal is defined as the minimum photon energy that will eject an electron from the metal.
15. STRUCTURE OF ATOMS AND NUCLEI
(1) Stationary (stable) orbit.
Ans. In the Bohr model of a hydrogen atom, a stationary or stable orbit is defined as any of the discrete allowed orbits such that the electron does not radiate energy while it is in such orbits.
(2) Ground state of an atom.
Ans. Ground state of an atom is defined as the lowest stable energy state of the atom.
3. Excitation energy of an atomic electron.
Ans. The energy required to transfer an electron from the ground state to an excited state (a state of higher energy) is called the excitation energy of the electron in that state.
4. Binding energy of an atomic electron.
Ans. Binding energy of an electron in an atom is defined as the minimum energy that should be provided to an orbital electron to remove it from the atom such that its total energy is zero.
(5) Ionization energy of an atomic electron OR Ionization energy of an atom.
Ans. Ionization energy of an electron in an atom is defined as the minimum energy required to remove the least strongly bound electron from a neutral atom such that its total energy is zero.
(6) Radioactivity.
Ans. Radioactivity is the phenomenon in which unstable nuclei of an element spontaneously distintegrate into nuclei of another element by emitting \( \alpha \) particles, or \( \beta \) particles, accompanied by \( \gamma \)-rays.
(7) Half-life of a radioactive element.
Ans. The half-life of a radioactive element is defined as the average time interval during which half of the initial number of nuclei of the element disintegrate.
(8) Decay constant or disintegration constant.
Ans. The decay constant or disintegration constant of a radioactive element is defined as the ratio of the disintegration rate at an instant to the number of undecayed nuclei of the element present at that instant.
(9) Mean-life of a radioactive element.
Ans. The mean-life of a radioactive element is the average time for which the undecayed nuclei of the element exist before decaying. It is equal in the reciprocal of the decay constant of that element.
(10) Nuclear fission.
Ans. Nuclear fission is a nuclear reaction in which a heavy nucleus of an atom splits into two or more fragments of comparable size either spontaneously or when bombarded by a neutron, with the release of enormous amount of energy.
(11) Nuclear fusion.
Ans. Nuclear fusion is a type of nuclear reaction in which lighter atomic nuclei (of low atomic numbers) fuse to form a heavier nucleus (of higher atomic number) with the release of enormous amount of energy.
(12) Chain reaction.
Ans. A chain reaction is a self-multiplying nuclear fission process in which neutrons ejected in one nuclear fission strike neighbouring nuclei of fissile material and cause more fissions.
16. SEMICONDUCTOR DEVICES
(1) Depletion layer (region).
Ans. The depletion layer or depletion region is the region of the junction between a \( p \)-type layer and an \( n \)-type layer within a single semiconducting crystal which is depleted of free charge carriers.
(2) Barrier potential.
Ans. The barrier potential is defined as the electric potential difference across the depletion region of a \( p-n \)-junction.
(3) Rectification.
Ans. The process of converting an alternating voltage (or current) to a direct voltage (or current) is called rectification.
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