Physics terms: D

D

Dampingalso damped, damping force

The loss of energy from an oscillating system to its surroundings because of resistive forces, which makes the amplitude decrease.

Introduced in C.1 Simple harmonic motion

Data bookletalso physics data booklet

The booklet of equations, constants, prefixes, unit conversions and circuit symbols provided for every physics paper.

Databasealso databases, database investigation

An organised, published collection of measurements from which an investigator selects data to answer a research question.

Daughter nucleusalso daughter, daughter nuclide, daughter product

The nucleus left after a parent nucleus has decayed.

Introduced in E.3 Radioactive decay

de Broglie wavelengthHL only · also de Broglie wavelengths, de Broglie relation, de Broglie hypothesis

The wavelength of a particle of momentum $p$: $\lambda = h/p$.

Introduced in E.2 Quantum physics

Decay constantHL only · also λ

The constant $\lambda$ in $A = \lambda N$: the fraction of undecayed nuclei that decay per unit time, related to the half-life by $T_{1/2} = \ln 2/\lambda$

Decay heat

The energy released by the radioactive decay of fission products, which continues after a reactor's chain reaction has stopped.

Introduced in E.4 Fission

Decay seriesalso decay chain, radioactive series

A sequence of radioactive decays in which each daughter is itself unstable, ending at a stable nuclide.

Introduced in E.3 Radioactive decay

Deep geological repositoryalso geological repository

A store for long-lived radioactive waste, hundreds of metres underground in stable rock, with several barriers between the waste and groundwater.

Introduced in E.4 Fission

Densityalso densities

Mass per unit volume, $\rho = m/V$; measured in $\text{kg m}^{-3}$.

Introduced in B.1 Thermal energy transfers

Dependent variablealso dependent variables, DV

The variable that is measured to find how it responds to changes in the independent variable.

Derived unitalso derived units

A unit formed by multiplying and dividing base units according to the defining equation of a quantity, such as the newton, $\text{kg m s}^{-2}$.

Destructive interference

Superposition of waves that meet in antiphase, so that their displacements cancel; for in-phase sources it occurs where the path difference is $(n + \tfrac12)\lambda$.

Introduced in C.3 Wave phenomena

Diffractionalso diffract, diffracted, diffracts

The spreading of a wave as it passes through an aperture or past the edge of an obstacle; greatest when the aperture is about the same size as the wavelength.

Introduced in C.2 Wave model

Diffraction gratingHL only · also grating, gratings

A large number of identical, equally spaced parallel slits, which produces very sharp maxima at angles given by $n\lambda = d\sin\theta$.

Introduced in C.3 Wave phenomena

Direct currentalso dc, d.c.

A current that flows in one direction only, such as the current from a cell or battery.

Introduced in B.5 Current and circuits

Directly proportionalalso direct proportion, proportional, proportionality

Describes two quantities whose ratio is constant: a graph of one against the other is a straight line through the origin.

Discharge tubealso discharge tubes, gas discharge tube

A tube of gas at low pressure that glows when a high potential difference is applied across it, giving the emission spectrum of the gas.

Introduced in E.1 Structure of the atom

Displacementalso displacements

The change in position of a body, from its initial to its final position; a vector quantity measured in metres.

Introduced in A.1 Kinematics

Displacement–time graphalso s–t graph, position–time graph, distance–time graph

A graph of displacement (or position) against time; its gradient at any time is the velocity.

Introduced in A.1 Kinematics

Dissipationalso dissipated, dissipate, dissipated energy

The spreading of energy into the surroundings, mostly as internal energy, in a form that can no longer do useful work.

Introduced in A.3 Work, energy and power

Distancealso distance travelled

The total length of the path travelled by a body; a scalar quantity measured in metres.

Introduced in A.1 Kinematics

Distance of closest approachHL only · also closest approach

The minimum separation of a charged particle from a nucleus in a head-on collision, where all its initial kinetic energy has become electric potential energy: $E_\text{k} = kQq/d$.

Introduced in E.1 Structure of the atom

Distractoralso distractors

A wrong option in a multiple-choice question, designed so that a specific, common mistake leads to it.

Doppler effectalso Doppler shift, Doppler shifts

The change in the observed frequency and wavelength of a wave when the source and the observer move relative to each other.

Introduced in C.5 Doppler effect

Doppler ultrasoundalso Doppler ultrasound scan

A medical technique that measures the speed and direction of blood flow from the frequency shift of ultrasound reflected by moving blood cells.

Introduced in C.5 Doppler effect

Driving forcealso driving forces, tractive force

The forward force that propels a vehicle, exerted by the road on the driven tyres. At constant velocity it equals the total resistive force.

Introduced in A.3 Work, energy and power

Driving frequencyalso forcing frequency

The frequency of the periodic external force acting on an oscillating system.

Introduced in C.4 Standing waves and resonance

Dynamic frictionalso kinetic friction, sliding friction

Friction between surfaces that are sliding over each other, $F_\mathrm{f} = \mu_\mathrm{d}F_\mathrm{N}$; usually less than the maximum static friction.

Introduced in A.2 Forces and momentum