GCE A/L Physics key terms

A consistent deviation of measured values from the true value, always in the same direction, caused by faulty apparatus, calibration or method. It shifts the mean and is not reduced by repeating readings.

Unpredictable variation in readings about a mean value, caused by fluctuations in measurement conditions or observer judgement. It scatters results in both directions and is reduced by taking many readings and averaging.

The degree of agreement among repeated measurements of the same quantity, reflected in the spread of values. A precise set has small random uncertainty but may still be inaccurate if a systematic error is present.

How close a measured value is to the true value of the quantity. Accuracy is limited by systematic errors, whereas precision is limited by the spread of random errors.

The interval within which the true value of a measurement is expected to lie, quoted with a stated confidence. It combines the resolution of the instrument with the scatter of repeated readings.

The smallest change in a quantity that an instrument can detect and display. It sets a limit on the uncertainty of a single reading, often taken as half the smallest scale division.

A physical quantity fully described by magnitude alone, with no direction. Mass, speed, energy, work and temperature are scalars, and they add by simple arithmetic.

A physical quantity having both magnitude and direction, which adds by the parallelogram or triangle rule. Displacement, velocity, acceleration, force and momentum are vectors.

The single force that produces the same effect as all the forces acting on a body combined. A body in equilibrium has zero resultant force, so its velocity does not change.

The turning effect of a force about a point, equal to the force multiplied by the perpendicular distance from the point to the line of action of the force. Measured in newton metres.

The point through which the entire weight of a body appears to act, whatever the body's orientation. For a uniform body it coincides with the geometric centre.

The product of a force and the time for which it acts, equal to the change in momentum produced. Its unit is the newton second, equivalent to the kilogram metre per second.

The principle that the total momentum of a system remains constant provided no external resultant force acts on it. It applies to collisions and explosions in any direction.

The force acting per unit cross-sectional area of a material, measured in pascals. Tensile stress stretches a sample, compressive stress shortens it, and shear stress acts parallel to a surface.

The fractional change in a dimension of a body produced by stress, such as extension divided by original length. It is dimensionless and has no units.

The ratio of tensile stress to tensile strain in the linear region of a stress-strain graph. It is a property of the material, measured in pascals, and is independent of sample dimensions.

The greatest stress a material can withstand and still return to its original shape when the load is removed. Beyond it, permanent plastic deformation occurs.

The principle that when two or more waves meet at a point, the resultant displacement equals the vector sum of the individual displacements. It underlies interference and stationary waves.

The superposition of waves from two or more sources, giving regions of reinforcement where displacements add and cancellation where they oppose. Coherence is needed for a stable pattern.

The property of two wave sources having a constant phase difference and the same frequency, so their interference pattern is stable and observable. Laser light is highly coherent.

A wave pattern formed by two identical waves travelling in opposite directions, with nodes of zero amplitude and antinodes of maximum amplitude. Energy is stored rather than transferred along the wave.

The spreading of waves when they pass through an aperture or around an obstacle, becoming more pronounced when the gap is comparable in size to the wavelength. It explains fringe patterns from narrow slits.

The resistance of a conductor of unit length and unit cross-sectional area, a property of the material. It depends on temperature and is measured in ohm metres.

The opposition of a component to the flow of electric current, defined as potential difference across it divided by the current through it. Measured in ohms.

The charge stored on one plate of a capacitor per unit potential difference across it, measured in farads. It depends on plate area, separation and the dielectric between the plates.

The energy transferred from other forms to electrical energy per unit charge driven round a circuit by a source. It equals the potential difference across a source when no current flows.

The force per unit positive charge experienced at a point in an electric field, a vector measured in newtons per coulomb. It equals the negative gradient of potential.

The work done per unit positive charge in bringing a small positive test charge from infinity to a point in an electric field. It is a scalar measured in volts.

The force per unit current per unit length on a conductor placed at right angles to a magnetic field, a vector measured in teslas. It describes the strength of the field.

The product of magnetic flux density, area and the number of turns of a coil, measured in webers or weber turns. A changing flux linkage induces an electromotive force.

The sum of the randomly distributed kinetic and potential energies of the particles making up a system. For an ideal gas it depends only on temperature.

The statement that the increase in internal energy of a system equals the heat added to it minus the work done by it. It is a statement of conservation of energy.

The lowest possible temperature, zero kelvin, at which particles have minimum kinetic energy. It is approximately minus 273 degrees Celsius and is unattainable in practice.

The time taken for half the nuclei in a radioactive sample to decay, or for the activity to fall to half its initial value. It is constant for a given isotope.

The energy required to separate a nucleus completely into its constituent nucleons, equal to the mass defect multiplied by the speed of light squared. Binding energy per nucleon peaks near iron-56.

The minimum energy needed to remove an electron from the surface of a metal. It is a property of the metal and appears in the photoelectric equation.

The lowest frequency of incident light that can eject photoelectrons from a metal surface. Below it, no emission occurs however intense the light.

Oscillation in which acceleration is proportional to displacement from equilibrium and directed towards it. The period is independent of amplitude for such motion.

The large-amplitude oscillation that occurs when a system is driven at its natural frequency. Damping reduces the peak amplitude and broadens the response curve.

The increase in wavelength of light from a receding source, observed as spectral lines shifted towards the red end. Cosmological redshift indicates the expansion of the universe.

The constant of proportionality relating the recession speed of a distant galaxy to its distance, giving the current rate of expansion of the universe. Its reciprocal estimates the age of the universe.