Page 559 - Physics Coursebook 2015 (A level)

Glossary
closed system A system of interacting objects in which there are no external forces.
coaxial cable An electrical cable with an inner conductor surrounded by a tubular insulating layer and an outside conducting layer.
coherent Two sources are coherent when they emit waves with a constant phase difference.
collimated beam A parallel-sided beam of radiation. components (of a vector) The magnitudes of a vector
quantity in two perpendicular directions.
compression A region in a sound wave where the air pressure is greater than its mean value.
compressive Describes a force that squeezes an object.
computerised axial tomography A technique in which X-rays are used to image the human body in order to produce a computerised 3-D image.
conduction band A range of electron energies in a solid; electrons in the conduction band are free to move throughout the material.
conservation of momentum In a closed system, when bodies interact, the total momentum in any specified direction remains constant.
constructive interference When two waves reinforce to give increased amplitude.
contact force The force an object exerts on another with which it is in contact.
contrast In a high-contrast image, there is a big difference in brightness between bright and dark areas.
contrast media Materials such as barium that easily absorb X-rays. A contrast medium is used to reveal the outlines or edges of soft tissues in an X-ray image.
coulomb The SI unit of electrical charge (abbreviated C). A charge of 1 C passes a point when a current of 1 A flows for 1s. 1C=1As.
Coulomb’s law Any two point charges exert an electrical force on each other that is proportional to the product of their charges and inversely proportional to the square of the distance between them.
count rate The number of particles (beta or alpha) or gamma-ray photons detected per unit time by a Geiger– Müller tube. Count rate is always a fraction of the activity of a sample.
couple A pair of equal and antiparallel forces having a turning effect but no resultant force.
damped Describes an oscillatory motion in which the amplitude decreases with time due to energy losses.
de Broglie wavelength The wavelength associated with a moving particle, given by the equation:
λ = h mv
decay constant The constant λ for an isotope that appears in the equation A = −λN. It is the probability of an individual nucleus decaying per unit time interval.
decibel A logarithmic unit of measurement that expresses the relative sizes of two powers using the formula 10lg(P1). density The mass per unit volume of a material: P2
ρ = mV
Unit: kg m−3.
dependent variable The variable in an experiment with a value that changes as the independent variable is altered by the experimenter.
derived units Units which are combinations of the base units of the SI system.
destructive interference When two waves cancel to give reduced amplitude.
diffraction The spreading of a wave when it passes through a gap or past the edge of an object.
digital signal A signal that has only a few possible values, often only two.
digital-to-analogueconversion(DAC) Conversionof a series of digital numbers into a continuous analogue signal.
dispersion The splitting of light into its different wavelengths.
displacement The distance moved by an object in a particular direction (measured from a fixed starting point).
Doppler effect The change in frequency or wavelength of a wave observed when the source of the wave is moving towards or away from the observer (or the observer is moving relative to the source).
drag A force that resists the movement of a body through a fluid.
drift velocity, mean The average speed of a collection of charged particles when a current flows.
dynamics The study of motion using quantities such as force and mass.
e.m.f. The total work done when unit charge is moved round a complete circuit. Unit: J C−1 or volt (V).
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