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STEM Glossary

What does a word mean in science, maths or Python? Each page gives the meaning in plain words, a worked example, and where to learn more. 151 terms so far.

PhysicsChemistryBiologyMathematicsPython and data

Physics

Motion, forces, energy, waves, light, electricity and heat.

  • AccelerationAcceleration is the rate at which velocity changes with time, a = (Δ v)/(Δ t). Its unit is the metre per second squared (m/s²).
  • Acceleration Due to Gravity (g)The acceleration due to gravity, written g, is the acceleration of a freely falling object. Near the Earth's surface it is about 9.8 m/s² downwards.
  • Adiabatic ProcessAn adiabatic process is a change in which no heat enters or leaves the system, so Q = 0. The system changes its temperature only because work is done on it or by it.
  • AmplitudeThe amplitude of a wave or a vibration is the largest displacement of a particle from its rest position. It tells you how much energy the wave carries.
  • Angle of IncidenceThe angle of incidence is the angle between an incoming ray of light and the normal to the surface at the point where the ray arrives.
  • Angle of ReflectionThe angle of reflection is the angle between a reflected ray of light and the normal, the line drawn at right angles to the surface at the point where the ray hits it.
  • Archimedes' PrincipleArchimedes' principle says that an object placed in a fluid feels an upward force, called the buoyant force, equal to the weight of the fluid it pushes aside.
  • Conservation of MomentumThe law of conservation of momentum says that the total momentum of a system stays the same if no outside force acts on it.
  • DensityDensity is the mass of a material in each unit of its volume, ρ = (m)/(V). It is measured in kilograms per cubic metre (kg/m³).
  • Dispersion of LightDispersion of light is the splitting of white light into its colours, because each colour travels at a slightly different speed in glass and so is bent by a different amount.
  • Electric CurrentElectric current is the rate of flow of electric charge, I = (Q)/(t). It is measured in amperes (A), where one ampere is one coulomb of charge each second.
  • Electrical ResistanceElectrical resistance is the opposition a conductor offers to the flow of electric current through it. It is measured in ohms (Ω).
  • Equations of Uniform AccelerationThe equations of uniform acceleration link initial velocity u, final velocity v, acceleration a, time t and displacement s for an object whose acceleration is constant.
  • Errors in MeasurementAn error in a measurement is the difference between the measured value and the true value. Systematic errors push every reading the same way, and random errors scatter the readings.
  • Escape SpeedEscape speed is the minimum speed with which an object must be thrown from the surface of a planet so that it never falls back, ignoring air resistance. For the Earth it is about 11.2 km/s.
  • First Law of ThermodynamicsThe first law of thermodynamics says that the change in the internal energy of a system equals the heat added to it minus the work the system does: Δ U = Q - W.
  • FrequencyFrequency is the number of complete waves, or complete vibrations, that pass a point or happen every second. Its unit is the hertz (Hz).
  • FrictionFriction is the force that opposes the motion, or the attempt at motion, of two surfaces in contact. It acts along the surfaces, in the direction opposite to the sliding.
  • Heat and TemperatureTemperature measures how hot something is, and is related to the average kinetic energy of its particles. Heat is the energy that flows from a hotter object to a colder one.
  • Hooke's LawHooke's law says that the extension of a spring or a wire is proportional to the force stretching it, as long as the elastic limit is not passed: F = kx.
  • Isothermal ProcessAn isothermal process is a change in which the temperature of the system stays constant. For an ideal gas, the internal energy then does not change.
  • Kinetic Energy and Potential EnergyKinetic energy is the energy an object has because it is moving, 1/2mv². Potential energy is the energy stored because of position or condition, for example height, mgh.
  • Least CountThe least count of a measuring instrument is the smallest value it can measure reliably. It is the difference between two neighbouring marks on its scale.
  • Lens FormulaThe thin lens formula connects the object distance u, the image distance v and the focal length f of a lens: (1)/(v) - (1)/(u) = (1)/(f).
  • Magnetic FieldA magnetic field is the region around a magnet or a moving charge in which a magnetic force can be felt. It is drawn with field lines that run from the north pole to the south pole outside a magnet.
  • Mirror FormulaThe mirror formula connects the object distance u, the image distance v and the focal length f of a spherical mirror: (1)/(v) + (1)/(u) = (1)/(f).
  • Newton's Second Law (F = ma)Newton's second law says that the net force on an object equals its mass multiplied by its acceleration, F = ma. The acceleration is in the direction of the net force.
  • Newton's Third LawNewton's third law says that when one object pushes on a second object, the second object pushes back on the first with a force of equal size in the opposite direction.
  • Ohm's LawOhm's law says that the current through a conductor is proportional to the potential difference across it, if the temperature stays the same. It is written V = IR.
  • Position-Time GraphA position-time graph shows where an object is at each moment. Time is on the horizontal axis, position is on the vertical axis, and the slope of the line is the velocity.
  • PressurePressure is the force acting on each unit of area, P = (F)/(A). Its unit is the pascal (Pa), which is one newton per square metre.
  • Projectile MotionProjectile motion is the motion of an object thrown into the air that is acted on only by gravity. Its path is a parabola.
  • Refractive IndexThe refractive index of a material is the ratio of the speed of light in vacuum to the speed of light in the material. It tells you how much the material slows light and bends it.
  • ResistivityResistivity is a property of a material that tells you how strongly it resists current, whatever the shape of the wire. It is measured in ohm metres (Ω m).
  • Snell's LawSnell's law says that when light passes from one medium into another, the sine of the angle of incidence divided by the sine of the angle of refraction is a constant for the two media.
  • SoundSound is a wave produced by a vibrating object. It travels through a material as alternating compressions and rarefactions, and it needs a medium to travel in.
  • Speed and VelocitySpeed is how fast an object moves, the distance covered per unit time. Velocity is speed in a stated direction, so it can be positive or negative.
  • Spherical MirrorA spherical mirror is a mirror whose reflecting surface is part of a sphere. A concave mirror curves inwards and converges light, and a convex mirror curves outwards and spreads light out.
  • Stress and StrainStress is the force acting on each unit area of a material, and strain is the fractional change in its size caused by the stress. The ratio of the two is a constant called the modulus.
  • Surface TensionSurface tension is the force acting along the surface of a liquid, per unit length, that makes the surface behave like a stretched elastic skin. It is measured in newtons per metre.
  • Total Internal ReflectionTotal internal reflection happens when light tries to leave a denser medium for a rarer one at an angle larger than the critical angle, and all of it is reflected back inside.
  • Tyndall EffectThe Tyndall effect is the scattering of light by tiny particles spread through a gas or a liquid, which makes the path of a light beam visible.
  • Wave Speed Equation (v = fλ)The wave speed equation v = fλ says that the speed of a wave equals its frequency multiplied by its wavelength.
  • WavelengthWavelength is the distance between two neighbouring points of a wave that are in the same state, for example from one crest to the next crest. It is measured in metres.
  • Weight and MassMass is the amount of matter in an object, in kilograms. Weight is the force of gravity on it, W = mg, in newtons.
  • Work and PowerWork is done when a force moves an object, W = Fdcosθ, and it is measured in joules. Power is the rate of doing work, P = (W)/(t), measured in watts.

Chemistry

Atoms, the periodic table, bonding, reactions and carbon compounds.

  • Acids, Bases and pHAn acid gives hydrogen ions in water and has a pH below 7. A base gives hydroxide ions and has a pH above 7. A neutral solution has a pH of 7.
  • Alkane (Saturated Hydrocarbon)An alkane is a hydrocarbon in which all the carbon-carbon bonds are single bonds. Alkanes are called saturated because they hold the greatest number of hydrogen atoms possible.
  • AllotropeAllotropes are different forms of the same element in the same physical state, with the atoms joined in different ways. Diamond and graphite are allotropes of carbon.
  • AtomAn atom is the smallest unit of an element that keeps the properties of that element. It has a tiny central nucleus of protons and neutrons, with electrons around it.
  • Bohr Model of the AtomThe Bohr model pictures a hydrogen atom as an electron moving in a circular orbit of fixed energy around the nucleus, and it explains the lines in the hydrogen spectrum.
  • CatalystA catalyst is a substance that makes a chemical reaction faster without being used up. It works by giving the reaction an easier route with a lower activation energy.
  • Chemical Formulae and EquationsA chemical formula shows the elements in a compound and how many atoms of each there are. A chemical equation shows a reaction, with the reactants on the left and the products on the right.
  • Chemical ReactionA chemical reaction is a process in which one or more substances, the reactants, change into one or more new substances, the products, by breaking and forming chemical bonds.
  • Chlor-Alkali ProcessThe chlor-alkali process is the electrolysis of concentrated sodium chloride solution (brine), which makes chlorine gas, hydrogen gas and sodium hydroxide.
  • Covalent BondA covalent bond is formed when two atoms share a pair of electrons. It usually forms between non-metal atoms.
  • ElectrolysisElectrolysis is the breaking down of an ionic compound, molten or dissolved, by passing an electric current through it.
  • ElectronegativityElectronegativity is a measure of how strongly an atom attracts the shared electrons in a covalent bond. Fluorine has the highest value.
  • Elements, Compounds and MixturesAn element has only one kind of atom. A compound has two or more elements chemically joined in a fixed ratio. A mixture has substances that are not chemically joined and can be in any ratio.
  • HybridisationHybridisation is the mixing of atomic orbitals of one atom to form new, equivalent hybrid orbitals that are used for bonding. It explains the shapes of molecules such as methane.
  • Ionic BondAn ionic bond is the strong electrostatic attraction between positive and negative ions, formed when one atom gives electrons to another. It usually forms between a metal and a non-metal.
  • Ionic RadiusThe ionic radius is the size of an ion in an ionic crystal. A positive ion is smaller than its atom, and a negative ion is larger than its atom.
  • Ionisation EnthalpyIonisation enthalpy is the energy needed to remove the outermost electron from a gaseous atom, measured in kilojoules per mole (kJ mol⁻¹).
  • IsomerismIsomers are compounds with the same molecular formula but different structures, so they have different properties.
  • IsotopeIsotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers.
  • Law of Conservation of MassThe law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products.
  • Law of Constant ProportionsThe law of constant proportions says that a pure chemical compound always contains the same elements in the same proportion by mass, however it was made or wherever it came from.
  • Mass SpectrometryMass spectrometry is a method that separates ions by their mass-to-charge ratio and measures how much of each there is. It finds the isotopes in an element and the mass of a molecule.
  • MetalloidA metalloid is an element with properties between those of a metal and a non-metal. Boron, silicon, germanium, arsenic, antimony and tellurium are the usual examples.
  • Metals and Non-metalsMetals are shiny, malleable, ductile and good conductors of heat and electricity. Non-metals are usually dull, brittle and poor conductors.
  • Modern Periodic TableThe modern periodic table arranges the elements in order of increasing atomic number, in 18 groups (columns) and 7 periods (rows), so that elements with similar properties fall in the same group.
  • MoleA mole is the amount of a substance that contains 6.022 × 10²³ particles, which may be atoms, molecules or ions. This number is the Avogadro constant.
  • Molecular MassThe molecular mass of a substance is the sum of the atomic masses of all the atoms in one molecule. It is measured in atomic mass units (u).
  • Oxidation and Reduction (Redox)Oxidation is the loss of electrons and reduction is the gain of electrons. They always happen together in a redox reaction.
  • Pauli Exclusion PrincipleThe Pauli exclusion principle says that no two electrons in an atom can have the same set of four quantum numbers. An orbital can therefore hold at most two electrons, with opposite spins.
  • Periodic TrendsPeriodic trends are regular changes in properties, such as atomic size, ionisation enthalpy and electronegativity, as you move across a period or down a group of the periodic table.
  • Quantum NumbersQuantum numbers are four numbers that describe an electron in an atom: its shell, its subshell, its orbital and its spin.
  • RancidityRancidity is the spoiling of fats and oils when they are oxidised by oxygen in the air, which gives food an unpleasant smell and taste.
  • Reactivity SeriesThe reactivity series is a list of metals in order of how readily they react, with the most reactive at the top. A more reactive metal can displace a less reactive one from its compounds.
  • Signs of a Chemical ChangeThe signs of a chemical change are a new substance forming, a change of colour, a gas given off, a solid (precipitate) forming, or heat, light or a smell given out or taken in.
  • Solution, Solute and SolventA solution is a uniform mixture of a solute dissolved in a solvent. The solvent is the substance present in the larger amount, usually the liquid.
  • State FunctionA state function is a property whose value depends only on the present state of a system, not on the path by which it arrived there. Internal energy, enthalpy, entropy and pressure are state functions.
  • Valence ElectronsValence electrons are the electrons in the outermost shell of an atom. They decide how the atom bonds and how it reacts.
  • ValencyValency is the combining power of an element: the number of electrons an atom gains, loses or shares to form a compound. It also tells you how many bonds an atom can form.
  • VSEPR TheoryVSEPR theory predicts the shape of a molecule from the idea that electron pairs around the central atom repel one another and so move as far apart as possible.

Biology

Cells, tissues, classification, life processes and inheritance.

  • AlgaeAlgae are simple plant-like organisms that make food by photosynthesis and live mostly in water. They have no true roots, stems or leaves.
  • BacteriaBacteria are single-celled organisms with no nucleus. They are found almost everywhere, and some cause disease while many are useful.
  • Binomial NomenclatureBinomial nomenclature is the scientific system of giving each species a two-part Latin name: the genus name first, then the species name. It was introduced by Carl Linnaeus.
  • CellA cell is the basic structural and functional unit of life. Every living thing is made of one or more cells, and every cell comes from an existing cell.
  • Cellular RespirationCellular respiration is the process in which cells break down glucose to release energy, stored as ATP. It is not the same as breathing.
  • Diffusion and OsmosisDiffusion is the movement of particles from a region of higher concentration to a region of lower concentration. Osmosis is the diffusion of water through a partially permeable membrane.
  • DNADNA (deoxyribonucleic acid) is the molecule that carries the genetic information of a living thing. It is a double helix made of two strands of nucleotides.
  • EcosystemAn ecosystem is a community of living things together with the non-living environment they interact with, such as soil, water, air and sunlight.
  • EnzymeAn enzyme is a protein that speeds up a chemical reaction in a living thing without being used up. Each enzyme works on one kind of molecule, its substrate.
  • Epithelial TissueEpithelial tissue is the animal tissue that covers the outside of the body and lines the inside of organs and cavities. Its cells are packed tightly with very little material between them.
  • Five Kingdom ClassificationThe five kingdom classification, proposed by R. H. Whittaker, divides living things into Monera, Protista, Fungi, Plantae and Animalia.
  • Food Chains and Food WebsA food chain shows who eats whom, as a line of arrows from a producer to the animals that eat it. A food web joins many food chains together.
  • Genes and ChromosomesA gene is a section of DNA that carries the instruction for one protein, and so for one inherited feature. Genes are arranged in a line along thread-like chromosomes.
  • HomeostasisHomeostasis is the keeping of the conditions inside the body, such as temperature, water and sugar levels, steady and within narrow limits even when the outside conditions change.
  • HormoneA hormone is a chemical messenger made by a gland and carried by the blood to a target organ, where it changes how that organ works.
  • KaryogamyKaryogamy is the fusion of two nuclei into one. In fungi it follows plasmogamy, the fusion of the cytoplasm of two cells.
  • Mitosis and MeiosisMitosis is cell division that makes two identical daughter cells with the same number of chromosomes. Meiosis makes four cells with half the number of chromosomes, and is used to make gametes.
  • Natural SelectionNatural selection is the process by which organisms with features better suited to their environment survive and reproduce more, so those features become more common over generations.
  • Parts of a Cell (Organelles)The parts of a cell are the cell membrane, the cytoplasm and the organelles. In a plant cell there is also a cell wall, a large vacuole and chloroplasts.
  • PhotosynthesisPhotosynthesis is the process in which green plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water, and release oxygen.
  • Reflex ActionA reflex action is a quick, automatic response to a stimulus that does not need thinking. It follows a short path called the reflex arc, usually through the spinal cord.
  • Taxonomic HierarchyThe taxonomic hierarchy is the ranked list of groups used to classify living things, from the largest group down to a single species: kingdom, phylum, class, order, family, genus, species.
  • TissueA tissue is a group of similar cells that work together to do one job. Tissues join to form organs, and organs join to form organ systems.
  • VirusA virus is a tiny infectious particle made of genetic material in a protein coat. It can multiply only inside the living cell of a host.

Mathematics

Number, algebra, geometry, trigonometry and probability.

  • Algebraic IdentityAn algebraic identity is an equation that is true for every value of the variables, such as (a + b)² = a² + 2ab + b².
  • Arithmetic ProgressionAn arithmetic progression (AP) is a list of numbers in which each term is found by adding the same number, the common difference, to the one before.
  • Complex NumberA complex number is a number of the form a + bi, where a and b are real and i is the imaginary unit with i² = -1.
  • DerivativeThe derivative of a function is its rate of change: the slope of the tangent to its graph at a point. It is written (dy)/(dx) or f'(x).
  • FunctionA function is a rule that gives exactly one output for each input. The set of allowed inputs is the domain and the set of outputs is the range.
  • Isosceles TriangleAn isosceles triangle has two equal sides, and the angles opposite those sides are also equal.
  • Linear EquationA linear equation is an equation in which each variable has power 1, such as 2x + 3y = 12. Its graph is a straight line.
  • LogarithmA logarithm answers the question: what power must the base be raised to in order to give a number? We write log_b x = y when b^y = x.
  • MatrixA matrix is a rectangular arrangement of numbers in rows and columns, written inside brackets. A matrix with m rows and n columns is an m × n matrix.
  • Ordered PairAn ordered pair (a, b) is two objects written in a fixed order. The pair (1, 2) is different from (2, 1).
  • PercentageA percentage is a number written as a fraction of 100. The symbol % means "per hundred", so 25% = (25)/(100) = 0.25.
  • Permutations and CombinationsA permutation is an arrangement of objects in which the order matters. A combination is a selection of objects in which the order does not matter.
  • PolynomialA polynomial is an expression made of terms that each have a number multiplied by a variable raised to a whole-number power, such as 3x² - 5x + 2.
  • Prime FactorisationPrime factorisation is writing a number as a product of prime numbers. By the fundamental theorem of arithmetic, this can be done in only one way, apart from the order.
  • ProbabilityProbability is a number from 0 to 1 that measures how likely an event is. It equals the number of favourable outcomes divided by the number of all possible, equally likely outcomes.
  • Pythagoras TheoremPythagoras' theorem says that in a right-angled triangle the square of the hypotenuse equals the sum of the squares of the other two sides: c² = a² + b².
  • Quadratic EquationA quadratic equation is an equation of the form ax² + bx + c = 0, where a ≠ 0. It has at most two real solutions, called roots.
  • RadianA radian is the angle at the centre of a circle made by an arc whose length equals the radius. A full turn is 2π radians, which is 360°.
  • Real NumbersReal numbers are all the numbers on the number line: the rational numbers, which can be written as a fraction, and the irrational numbers, which cannot.
  • SetA set is a collection of distinct objects, called elements, written inside curly brackets, for example A = {1, 2, 3}.
  • Slope of a LineThe slope of a line is its steepness: the change in y divided by the change in x between any two points, m = (y_2 - y_1)/(x_2 - x_1).
  • Trigonometric RatiosThe trigonometric ratios sine, cosine and tangent compare the sides of a right-angled triangle: sinθ = dfracoppositehypotenuse, cosθ = dfracadjacenthypotenuse and tanθ = dfracoppositeadjacent.
  • VectorA vector is a quantity with both size (magnitude) and direction, drawn as an arrow. Displacement, velocity, force and acceleration are vectors.

Python and data

Python ideas used in science, and the statistics that go with them.

  • Cell (in a Python notebook)A cell is one block of a notebook. A code cell holds Python code that you run on its own, and a text cell holds writing or notes.
  • Class and Special Methods (in Python)A class is a blueprint for making objects that hold data and the functions that work on it. Special methods, with names like __init__ and __str__, let an object behave like a built-in type.
  • Dictionary (in Python)A dictionary stores pairs of a key and a value in curly brackets, such as "H": 1.008, "C": 12.011. You look up a value by its key.
  • For LoopA for loop repeats a block of code once for each item in a sequence, such as a list or a range of numbers.
  • Function (in Python)A function is a named block of code that does one job. You give it inputs, called arguments, and it can give back a result with return. It is sometimes compared to a recipe that you can use again and again.
  • If Statement (in Python)An if statement runs a block of code only when a condition is true. With elif and else, it chooses between several blocks.
  • Import (Python modules and libraries)A module is a file of ready-made Python code, and a library is a collection of modules. The import command lets your program use them.
  • List (in Python)A list is an ordered collection of values written in square brackets, for example [3, 7, 2]. Its items can be changed, added and removed.
  • Mean, Median and ModeThe mean is the sum of the values divided by how many there are. The median is the middle value when they are in order. The mode is the value that occurs most often.
  • NumPy ArrayA NumPy array is a grid of numbers of the same type that lets you do arithmetic on all of its elements at once. You can make one from a list with np.array.
  • Program (in Python)A program is a list of instructions written in a programming language that the computer follows one after another. A Python program is saved in a file that ends in .py.
  • RecursionRecursion is when a function calls itself to solve a smaller version of the same problem. Every recursive function needs a base case that stops the calls.
  • SamplingSampling is choosing a small group from a larger population to study, so that what you find can fairly describe the whole population.
  • Split and Join (Python strings)split breaks a string into a list of smaller strings at a separator, and join puts a list of strings together into one string with a separator between them.
  • Standard DeviationThe standard deviation is a measure of how spread out the values in a data set are around their mean. A small value means the data are close together.
  • Standard ErrorThe standard error of the mean tells you how far the mean of a sample is likely to be from the true mean of the population. It equals dfracs√(n).
  • String (in Python)A string is a piece of text in quotation marks, such as "hello". A string is a sequence of characters that you cannot change in place.
  • Variable (in Python)A variable is a name that refers to a value stored in the computer's memory. You create one by writing the name, an equals sign and the value, for example speed = 12.
  • While Loop (Repeat Until)A while loop repeats a block of code as long as a condition is true. Python has no "repeat until" command, so you write while not condition to repeat until something happens.

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