BSEH SS Physics key terms

The measure of the number of electric field lines passing through a given surface area, calculated as the surface integral of the electric field over that area. Its SI unit is the volt-metre.

A fundamental law stating that the total electric flux through any closed surface equals the net charge enclosed divided by the permittivity of free space. It simplifies field calculations for symmetric charge distributions.

A vector quantity for a pair of equal and opposite charges, equal to charge times the separation vector pointing from negative to positive charge. It determines torque and field behaviour in external fields.

The ability of a conductor or capacitor to store electric charge per unit potential difference, measured in farads. For a parallel-plate capacitor it equals permittivity times area divided by plate separation.

The ratio of a material's permittivity to the permittivity of free space, also called relative permittivity. It quantifies how much a dielectric reduces the electric field and increases capacitance.

The average velocity with which free electrons move through a conductor under an applied electric field, superimposed on their random thermal motion. It is typically very small, on the order of millimetres per second.

The drift velocity acquired by a charge carrier per unit applied electric field, measured in square metres per volt-second. It links conductivity to carrier concentration and charge.

The electric current per unit cross-sectional area, a vector pointing along conventional current flow, measured in amperes per square metre. It equals conductivity times the electric field.

The resistance within a source of electromotive force that causes its terminal voltage to fall below the emf when current flows. It dissipates energy as heat inside the source.

The energy supplied per unit charge by a source such as a cell or generator, measured in volts. It equals the terminal potential difference only when no current is drawn.

Two circuit laws: the junction rule states that charge entering a junction equals charge leaving, and the loop rule states that potential changes around any closed loop sum to zero. They solve complex networks.

A four-resistance network used to measure an unknown resistance. It is balanced, giving zero galvanometer deflection, when the ratio of resistances in one branch equals that in the other.

An instrument that compares or measures emf and potential difference by balancing an unknown voltage against a known potential drop along a uniform wire, drawing no current at balance.

A law giving the magnetic field produced at a point by a small current element, proportional to current, element length, sine of the angle, and inversely proportional to distance squared.

A law stating that the line integral of magnetic field around any closed loop equals permeability times the net current enclosed by the loop. It is useful for highly symmetric current distributions.

The total force on a charge moving in electric and magnetic fields, equal to the electric force plus the magnetic force qv cross B. The magnetic part is always perpendicular to velocity.

A long coil of wire carrying current, producing a nearly uniform magnetic field inside it equal to permeability times turns per unit length times current. It behaves like a bar magnet externally.

A dimensionless measure of how strongly a material becomes magnetised in response to an applied magnetic field. It is small and positive for paramagnets, negative for diamagnets, and large for ferromagnets.

The lag of magnetisation behind the magnetising field in ferromagnetic materials, producing a loop when magnetisation is plotted against field. Loop area represents energy lost per cycle.

A particle accelerator in which a magnetic field bends charged particles into spiral paths while an alternating electric field repeatedly accelerates them across a gap. It cannot accelerate uncharged particles.

A law stating that the induced emf in a circuit equals the negative rate of change of magnetic flux through it. The negative sign expresses Lenz's law.

A principle stating that an induced current flows in a direction that opposes the change in magnetic flux producing it. It is a consequence of energy conservation.

The property of a coil by which a changing current in it induces an emf opposing that change, equal to flux linkage per unit current, measured in henries.

Circulating currents induced within the body of a conductor when it experiences a changing magnetic flux. They cause energy loss as heat and can be reduced by lamination.

The total opposition a circuit presents to alternating current, combining resistance and reactance, measured in ohms. It equals the phasor ratio of voltage to current.

The condition in a series LCR circuit where inductive and capacitive reactances cancel, making impedance minimum and current maximum at a particular frequency. It is used in tuning circuits.

The cosine of the phase angle between voltage and current in an AC circuit, equal to the ratio of true power to apparent power. It equals one for a purely resistive circuit.

A term introduced by Maxwell equal to permittivity times the rate of change of electric flux, which acts like a current in producing a magnetic field and completes Ampere's law.

A self-propagating oscillation of coupled electric and magnetic fields perpendicular to each other and to the direction of propagation, travelling in vacuum at the speed of light.

A vector equal to the cross product of electric and magnetic field vectors divided by permeability, giving the rate of energy flow per unit area in an electromagnetic wave.

The complete reflection of light back into a denser medium when it strikes a boundary at an angle greater than the critical angle. It requires travel from denser to rarer medium.

The angle of incidence in the denser medium for which the refracted ray grazes the boundary at 90 degrees. Beyond it, total internal reflection occurs.

The law relating angles of incidence and refraction at an interface, stating that the ratio of their sines equals the ratio of refractive indices of the two media.

The superposition of two or more coherent waves producing regions of reinforced and cancelled intensity. Constructive interference requires a path difference of a whole number of wavelengths.

The spreading of waves around obstacles and through apertures, producing fringes of varying intensity. It becomes pronounced when the obstacle or slit size is comparable to the wavelength.

The property of two waves having a constant phase difference over time, which is necessary for stable interference patterns. Laser light is highly coherent.

The emission of electrons from a metal surface when light of frequency above a threshold falls on it. It demonstrates the particle nature of light and is explained by photon energy.

The minimum energy required to remove an electron from the surface of a metal, measured in electronvolts. It determines the threshold frequency for the photoelectric effect.

The negative potential applied to stop the fastest photoelectrons from reaching the collector, producing zero photocurrent. It measures the maximum kinetic energy of emitted electrons.

The wavelength associated with a moving particle, equal to Planck's constant divided by its momentum. It explains electron diffraction and quantised atomic energy levels.

The radius of the electron's lowest-energy orbit in the hydrogen atom, approximately 0.053 nanometres. It sets the scale for atomic sizes.

The time required for half the nuclei in a radioactive sample to decay. It is constant for a given isotope and relates to the decay constant by a natural logarithm factor.

The difference between the sum of masses of separate nucleons and the mass of the bound nucleus. It appears as binding energy according to mass-energy equivalence.

The zone around a p-n junction depleted of mobile charge carriers, leaving fixed ionised dopant atoms. It creates the built-in potential barrier across the junction.

A heavily doped diode designed to operate safely in reverse breakdown, maintaining a nearly constant voltage. It is used for voltage regulation.

A circuit that converts alternating current into direct current using diodes, either half-wave or full-wave. It exploits the one-way conduction of a p-n junction.

The smallest measurement an instrument can read reliably, such as the value of one division on a vernier scale. It determines the precision of a reading.

A systematic error in an instrument when it does not read zero for a true zero quantity. It is corrected by adding or subtracting the error from readings.

The digits in a measured value that carry meaningful information about its precision, including the last estimated digit. Results of calculations are rounded to match the least precise input.

A technique using the base dimensions of physical quantities to check equations, derive relations, and convert units. It cannot determine dimensionless numerical constants.

The process of determining how uncertainties in measured quantities combine to give the uncertainty in a calculated result. Absolute errors add for sums, fractional errors add for products.

Two ways of multiplying vectors: the scalar product gives a scalar equal to product of magnitudes times cosine of angle, while the vector product gives a perpendicular vector involving sine.