PSEB SS Physics key terms

The rule giving the force between two point charges at rest: it is proportional to the product of the charges and inversely proportional to the square of the distance between them, acting along the line joining them.

The electric force per unit positive charge at a point; a vector pointing the way a positive test charge would be pushed. Its SI unit is the newton per coulomb or volt per metre.

A vector for a pair of equal and opposite charges: its magnitude is charge times separation, its direction runs from the negative to the positive charge. Unit is coulomb metre.

The total electric flux through any closed surface equals the net charge enclosed divided by the permittivity of free space. It is a convenient tool for finding fields of symmetric charge distributions.

The charge stored on a conductor per unit potential difference applied, measured in farads. For a parallel-plate capacitor it equals permittivity times plate area divided by plate separation.

The small average velocity with which free electrons move along a conductor under an applied electric field, superimposed on their random thermal motion. It is usually a fraction of a millimetre per second.

A property of a material rather than of a particular wire, equal to resistance times cross-sectional area divided by length. It measures how strongly the material opposes current flow and varies with temperature.

Electromotive force: the energy per unit charge supplied by a source such as a cell or generator to drive charge around a circuit. It is measured in volts, like potential difference.

Two circuit laws: the junction rule says charge entering a junction equals charge leaving; the loop rule says the sum of potential changes around any closed loop is zero. Both follow from conservation laws.

A uniform resistance wire used to compare emfs or measure potential differences while drawing almost no current from the source. The balance point is where the galvanometer shows zero deflection.

The force on a charge moving in a magnetic field, equal to charge times velocity crossed with the field. It acts perpendicular to both velocity and field, so it does no work on the charge.

Gives the magnetic field produced by a small current element at a point: proportional to current times element length and to the sine of the angle between them, and inversely proportional to the square of the distance.

The line integral of magnetic field around any closed path equals the permeability of free space times the net current passing through the surface bounded by that path. It suits highly symmetric current arrangements.

A vector for a current loop: magnitude equals current times loop area, direction is normal to the loop by the right-hand rule. It determines the torque the loop feels in a magnetic field.

The temperature above which a ferromagnetic material loses its spontaneous magnetization and becomes paramagnetic, because thermal agitation destroys the alignment of atomic magnetic moments. Each material has its own value.

A measure of the magnetic field lines passing through a surface, equal to field strength times area times the cosine of the angle between the field and the surface normal. It is measured in webers.

The induced emf in a circuit equals the negative rate of change of magnetic flux linked with it. Rapid flux changes or a coil with many turns give larger induced emfs.

The direction of an induced current is such that it opposes the change in magnetic flux that produced it. This follows from conservation of energy and explains the minus sign in Faraday's law.

The property of a coil by which a changing current in it induces an emf in the same coil, opposing the change. It equals flux linkage per unit current and is measured in henries.

The total opposition a circuit offers to alternating current, combining resistance and reactance. It is the ratio of rms voltage to rms current and is measured in ohms.

The condition in a series circuit containing inductance and capacitance where the two reactances are equal and cancel, so the current is maximum and the impedance is purely resistive. It occurs at one particular frequency.

A term introduced by Maxwell: a changing electric field acts like a current and produces a magnetic field. It equals the permittivity times the rate of change of electric flux.

A self-sustaining wave of oscillating electric and magnetic fields at right angles to each other and to the direction of travel, produced by accelerating charges. It needs no medium and travels at the speed of light in vacuum.

The ordered range of electromagnetic waves from long radio waves through microwaves, infrared, visible light and ultraviolet to X-rays and gamma rays. The members differ in frequency and wavelength but share the same vacuum speed.

When light travelling in a denser medium strikes a boundary beyond the critical angle, it is completely reflected back with no refracted ray. It requires travel from a denser to a rarer medium.

The reciprocal of the focal length measured in metres, expressed in dioptres. A converging lens has positive power, a diverging lens negative power, and powers of thin lenses in contact add.

The splitting of white light into its component colours because the refractive index of a material depends on wavelength, so different colours bend by different amounts through a prism.

A property of two waves that keep a constant phase difference over time, so they produce a stable interference pattern. Lasers give coherent light, while ordinary sources do not.

The spreading of waves around obstacles and through narrow gaps, becoming noticeable when the obstacle or gap is comparable in size to the wavelength. It explains the bending of light at sharp edges.

The restriction of light's electric field oscillations to one plane, which shows that light is a transverse wave. It occurs on reflection at a suitable angle, and through polaroid filters or scattering.

The minimum energy needed to remove an electron from the surface of a metal. It is a property of the material and sets the threshold frequency below which no photoelectrons are emitted.

The reverse potential difference just able to stop the fastest photoelectrons from reaching the collector, making the photocurrent zero. It measures the maximum kinetic energy of the emitted electrons.

The wavelength associated with a moving particle, equal to Planck's constant divided by its momentum. It is larger for slow, light particles and is confirmed by electron diffraction experiments.

The difference between the total mass of the separate nucleons and the actual mass of the nucleus they form. It is equivalent, through Einstein's relation, to the binding energy holding the nucleus together.

The time in which half the nuclei in a radioactive sample decay. It is constant for a given radionuclide and equals 0.693 divided by the decay constant.

The splitting of a heavy nucleus into two lighter nuclei of roughly equal mass, releasing large energy and usually a few neutrons. These neutrons can trigger further fissions, giving a chain reaction.

The narrow zone around a p-n junction where mobile charge carriers have diffused away and recombined, leaving fixed ionized atoms. It holds an internal electric field that opposes further diffusion.

A circuit that converts alternating current into direct current, typically using one or two diodes that conduct only during part of each cycle. A full-wave rectifier uses both halves of the cycle.

A heavily doped diode designed to work safely in reverse breakdown at a precise voltage. It serves as a voltage regulator because the voltage across it stays nearly constant while the current varies.

A basic digital circuit that produces one output from one or more inputs according to a fixed rule, such as AND, OR or NOT. Gates are built from transistors and form the basis of digital electronics.