BCS Written Physics key terms

A measure of how a body's mass is distributed about a chosen axis, equal to the sum of each mass element times the square of its distance from that axis. It plays the role of mass in rotational motion.

The turning effect of a force about a point or axis, found by multiplying the force by the perpendicular distance from the axis to the force's line of action. It changes a body's angular momentum.

A conserved quantity for a rotating or orbiting body, equal to the product of moment of inertia and angular velocity for a rigid body. It remains constant when no external torque acts on the system.

The minimum speed an object needs at a given point to move away from a massive body and never return, ignoring air resistance. For a spherical body it equals the square root of twice the gravitational parameter divided by the distance from the centre.

Three statements describing planetary orbits: orbits are ellipses with the Sun at one focus; a line from Sun to planet sweeps equal areas in equal times; and the square of the orbital period is proportional to the cube of the semi-major axis.

The work done per unit mass in bringing a small test mass from infinity to a specified point in a gravitational field. It is always negative for a bound point and is a scalar quantity.

A relation for steady, incompressible, non-viscous flow stating that pressure plus kinetic energy per unit volume plus gravitational potential energy per unit volume stays constant along a streamline. It links pressure, speed and height.

A statement of mass conservation for steady flow, saying the product of fluid density, cross-sectional area and flow speed is constant along a pipe. For an incompressible fluid, area times speed is constant.

A fluid's internal resistance to flow, quantified as the ratio of shear stress to velocity gradient. A high-viscosity fluid such as honey deforms slowly under a given applied shear compared with water.

The constant speed reached by a falling body when the upward drag force, including any buoyancy, exactly balances its weight. At that point its acceleration becomes zero and it falls at a steady rate.

A dimensionless ratio of inertial forces to viscous forces in a flow, equal to density times speed times length divided by viscosity. Low values indicate smooth laminar flow; high values indicate turbulent flow.

The ratio of tensile stress to tensile strain for a material stretched within its elastic limit. It measures stiffness: a larger value means the material resists stretching more strongly.

The greatest stress a material can bear and still return exactly to its original shape when the load is removed. Beyond it, permanent deformation occurs even after unloading.

The negative ratio of transverse strain to axial strain when a material is stretched. It describes how much a sample narrows sideways as it lengthens, and is typically near 0.3 for metals.

A statement of energy conservation for thermodynamic systems: the heat added to a system equals the increase in its internal energy plus the work it does on its surroundings. Sign conventions must be stated explicitly.

A state function measuring the dispersal of energy in a system, defined so that its change equals heat transferred reversibly divided by absolute temperature. It never decreases for an isolated system.

The maximum possible efficiency of a heat engine operating between two reservoirs, equal to one minus the ratio of cold to hot absolute temperatures. No real engine between those temperatures can exceed it.

The quantity of heat absorbed or released per unit mass when a substance changes phase at constant temperature, as in melting, boiling or condensation. It produces no change in temperature during the change.

The heat energy required to raise the temperature of one kilogram of a substance by one kelvin, with no phase change. Water's unusually large value explains its role in moderating climate.

The principle that heat flows spontaneously from hotter to colder bodies and that the total entropy of an isolated system cannot decrease. It sets the direction of natural processes and limits engine efficiency.

The change in observed frequency of a wave when the source, the observer, or both move relative to the medium. Approaching sources produce higher observed frequencies; receding sources produce lower ones.

The rate at which loudness rises and falls when two waves of slightly different frequency superpose. It equals the absolute difference between the two component frequencies.

A large-amplitude response that occurs when a system is driven at one of its natural frequencies. Energy transfer from the driver is then maximal, and damping controls how sharp the peak is.

A wave pattern formed by two identical waves travelling in opposite directions, producing fixed nodes and antinodes. Energy is stored rather than transported along the medium.

The spreading of waves around obstacles and through apertures, becoming pronounced when the obstacle or gap is comparable in size to the wavelength. It limits the resolution of optical instruments.

The superposition of two or more coherent waves, producing regions of reinforced amplitude and regions of cancellation. Constructive interference needs a path difference of a whole number of wavelengths.

A fixed, predictable phase relationship between two or more waves, allowing stable interference patterns to be observed. Lasers provide highly coherent light; ordinary lamps emit incoherent light.

The induced electromotive force in a circuit equals the negative rate of change of magnetic flux through it. The minus sign expresses Lenz's law, opposing the change that produced the current.

The rule that an induced current flows in a direction whose magnetic effect opposes the change in flux that caused it. It follows from energy conservation and gives the minus sign in Faraday's law.

The number of magnetic field lines passing through a surface, calculated as the field strength times the area times the cosine of the angle between field and surface normal. Its unit is the weber.

The time taken for half the nuclei in a radioactive sample to decay, a constant characteristic of each radionuclide. After n half-lives, one part in two to the power n remains.

The probability per unit time that a given radioactive nucleus will decay, related to half-life by the natural logarithm of two divided by the half-life. Activity equals decay constant times the number of nuclei.

The energy required to separate all nucleons in a nucleus completely, equivalent to the mass defect multiplied by the square of the speed of light. Higher binding energy per nucleon indicates greater nuclear stability.

The difference between the total mass of a nucleus's separate constituent nucleons and the actual measured mass of the bound nucleus. It corresponds to the energy released when the nucleus formed.

The emission of electrons from a metal surface when light of sufficiently high frequency strikes it. The effect demonstrates that light energy is delivered in discrete quanta called photons.

The minimum energy needed to remove an electron from the surface of a particular metal. In the photoelectric effect it equals the photon energy minus the maximum kinetic energy of emitted electrons.

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

The statement that certain pairs of quantities, such as position and momentum, cannot both be known with unlimited precision at the same time. The product of their uncertainties is at least Planck's constant over four pi.

The energy level at which the probability of finding an electron is one half at a given temperature in a solid. In metals at absolute zero it marks the highest occupied electron state.

The energy range between the valence band and conduction band in a solid, in which no electron states exist. Its size determines whether a material behaves as a conductor, semiconductor or insulator.

The deliberate addition of small amounts of impurity atoms to a semiconductor to increase its concentration of free electrons or holes. It creates n-type or p-type material and enables device fabrication.

A high-gain differential voltage amplifier, usually integrated, whose behaviour is set by external feedback components rather than by its own open-loop gain. It is the building block of analogue computing and instrumentation.

A mathematical operation that decomposes a function of time or space into its constituent sinusoidal frequencies, producing a spectrum. Its inverse reconstructs the original function from that spectrum.

A differential equation together with conditions specified at the boundaries of a region, whose solution must satisfy those conditions everywhere inside. Such problems typically yield discrete allowed solutions.

A consistent bias in measurements that shifts results in the same direction, arising from faulty instruments, calibration or procedure. Repeating readings does not reduce it; the method must be corrected.

Unpredictable scatter in measured values caused by uncontrolled fluctuations, which averages towards zero over many readings. Its effect is estimated from the spread, and it is reduced by averaging repeated measurements.

A measure of the spread of a set of values about their mean, equal to the square root of the average squared deviation. For repeated measurements it quantifies the uncertainty of a single reading.

The digits in a reported value that carry meaningful information about its precision, excluding leading zeros and place-holding zeros. Results should be quoted to a number consistent with the measurement's uncertainty.