OCR • J259 • 780 key terms
10-dot strips
A length of ticker tape containing 10 gaps between dots, representing a time interval of 0.2 seconds at 50 Hz.
From: PAG 3: Motion
Absolute temperature
Temperature measured using the Kelvin scale, starting from absolute zero.
From: Particle Motion and Gas Pressure
Absolute uncertainty
The expected interval within which the true value lies, calculated as half the range of repeat readings.
From: Graphing and Statistical Analysis
Absolute zero
The lowest possible temperature (-273°C or 0 K) where particles theoretically have zero kinetic energy and exert no pressure.
From: Particle Motion and Gas Pressure
Absorber
A material or object that takes in electromagnetic radiation, converting its energy into other forms.
From: Properties of the Electromagnetic Spectrum
Absorption
The process where a wave's energy is transferred to the thermal energy store of the material it is travelling through, causing its amplitude to decrease.
From: PAG 8: Interactions of waves
Absorption spectrum
A continuous spectrum of light interrupted by dark lines or bands, caused by specific wavelengths being absorbed by elements in a gas.
From: Red-shift and Evidence for the Big Bang
Acceleration
The rate of change of speed (or velocity) over time, measured in m/s².
From: PAG 3: Motion
Acceleration of free fall (g)
The constant acceleration of an object falling solely under the influence of Earth's gravity (in the absence of air resistance), valued at 10 m/s² for OCR B. The symbol for this value is g.
From: Acceleration and Equations of Motion
Accretion
The process where gravity pulls together small particles and clumps to form larger astronomical bodies like planets.
From: Formation and Stability of Stars
Accuracy
A measure of how close a calculated or measured value is to the true, actual value.
From: PAG 4: Measuring Waves
AC power supply
The type of electricity supply required by a ticker timer to ensure the pin vibrates at the correct frequency (50 Hz).
From: PAG 3: Motion
Activity
The rate at which a source of unstable nuclei decays, measured in Becquerels (Bq).
From: Biological Effects and Radiation Hazards
Activity-time graph
A plot showing how the activity of a radioactive source (y-axis) changes over time (x-axis).
From: Half-Life and Radioactive Decay Calculations
Actual risk
An objective measurement of danger calculated using data, statistics, and probabilities.
From: Technological Applications and Risks
Airbags
Safety devices that inflate during a crash to provide a soft surface and increase the time it takes for a passenger's head or chest to stop.
From: Momentum and Force Relations
Air density
The mass of air molecules per unit volume of space, which decreases as altitude increases.
From: Atmospheric and Liquid Pressure
Air resistance
A type of friction (drag) caused by an object moving through air particles.
From: Forces
ALARA
An acronym for 'As Low As Reasonably Achievable', a safety principle used to minimise exposure to hazards like radiation without requiring impractical measures.
From: Risk Analysis and Decision Making
Albedo
A measure of the reflectivity of a surface, determining how much incoming radiation is reflected rather than absorbed.
From: Climate change and evidence
Alpha particle
A highly ionising type of radiation consisting of two protons and two neutrons (a helium nucleus).
From: Contamination and Irradiation
Alpha particles
A particle consisting of two protons and two neutrons emitted from an unstable nucleus.
From: Isotopes and Radioactive Emissions
Alternating current
An electric current that repeatedly reverses its direction, required for a transformer to function.
From: Transformers and the Transformer Equations
Alternating current (a.c.)
An electric current that repeatedly reverses its direction of flow, typically produced by an alternator.
From: Alternators and Dynamos
Alternating current (AC)
An electric current that continuously and rapidly changes its direction.
From: Solenoids and their Applications
Alternator
A type of generator that converts mechanical energy into electrical energy in the form of alternating current (a.c.).
From: Alternators and Dynamos
Altitude
The vertical height of an object or point above sea level or ground level.
From: Atmospheric and Liquid Pressure
Ammeter
A device with near-zero resistance placed in series to measure the flow of electric current in Amperes.
From: PAG 7: Series and Parallel Circuits
Amperes (A)
The standard unit of electric current, equivalent to a flow of one Coulomb of charge per second.
From: Electric Charge, Current and Circuit Symbols
Amplitude
The maximum displacement of a wave from its undisturbed (rest) position, which determines the volume of a sound.
From: Principles of Electromagnetic Induction and Microphones
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