LC Physics and Chemistry key terms

The straight-line vector distance from a starting point to a final point, including direction. It differs from total distance travelled, which ignores direction and only adds path lengths.

The rate at which velocity changes with time, measured in metres per second squared. It occurs whenever speed changes, direction changes, or both change together.

The resultant force on a body equals its mass multiplied by its acceleration (F = ma). The acceleration points in the same direction as the resultant force.

The product of a body's mass and velocity, a vector quantity measured in kilogram metres per second. In a closed system with no external resultant force, total momentum is conserved in collisions.

Energy cannot be created or destroyed, only transferred between stores or transformed from one form to another. In mechanical problems, the total of kinetic, potential and internal energy stays constant.

The energy needed to raise the temperature of one kilogram of a substance by one kelvin, measured in joules per kilogram per kelvin. Water's high value explains its use as a coolant.

The thermal energy absorbed or released when a substance changes state at constant temperature, without any change in temperature. It is quoted per kilogram for a named change such as melting or vaporisation.

The total kinetic and potential energy of all the particles in a system. Heating raises it by increasing particle kinetic energy; a change of state alters its potential energy component.

Temperature measured on the kelvin scale, starting at absolute zero where particle kinetic energy is minimal. A kelvin equals a Celsius degree, and kelvin values are used in gas-law calculations.

The relationship pV = nRT linking pressure, volume, amount in moles, and absolute temperature for an ideal gas. It combines Boyle's, Charles's and Avogadro's laws into one equation.

The number of complete oscillations or wave cycles passing a point each second, measured in hertz. It is set by the source and stays constant when a wave crosses into a new medium.

The distance between two consecutive corresponding points on a wave, such as crest to crest, measured in metres. It changes with wave speed when a wave enters a different medium.

The change in direction of a wave as it passes between media of different optical or acoustic density, caused by a change in wave speed. Frequency is unchanged while wavelength and speed alter.

The complete reflection of light back into a denser medium when the angle of incidence exceeds the critical angle. It requires travel from a higher to a lower refractive index medium, as in optical fibres.

A large amplitude response that occurs when a driving frequency matches an object's natural frequency. It explains tuning circuits, musical instruments, and the collapse of structures under matching vibration.

A stationary vibration pattern formed when two identical waves travelling in opposite directions superpose, producing fixed nodes and antinodes. It occurs on stretched strings and in air columns.

For a metallic conductor at constant temperature, current is directly proportional to potential difference, giving V = IR. Components such as filament lamps do not obey it because their resistance changes with temperature.

The opposition to current flow in a component, equal to potential difference divided by current and measured in ohms. It increases with length and temperature but decreases with greater cross-sectional area.

The energy transferred per unit charge between two points in a circuit, measured in volts. In a closed loop, the supply potential difference equals the sum of the potential differences across components.

The generation of a potential difference in a conductor when it experiences a changing magnetic flux. The induced current opposes the change that produced it, as stated by Lenz's law.

The emission of electrons from a metal surface when light above a threshold frequency strikes it. It provides evidence that light energy is delivered in discrete quanta called photons.

A discrete packet of electromagnetic energy whose energy equals Planck's constant multiplied by frequency. Higher-frequency light carries more energetic photons, explaining threshold behaviour.

The minimum energy required to remove an electron from the surface of a particular metal. It determines the threshold frequency below which no photoelectric emission occurs.

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

The strong electrostatic attraction between oppositely charged ions formed by electron transfer from a metal to a non-metal. It produces giant lattices with high melting points that conduct when molten.

A bond formed when two atoms share one or more pairs of electrons. The shared pair is attracted to both nuclei, and the bond strength increases with the number of shared pairs.

A measure of how strongly an atom attracts the shared electrons in a covalent bond. Large differences between bonded atoms create polar bonds and can lead to ionic character.

The amount of substance containing 6.02 x 10^23 elementary entities, equal to the Avogadro constant. It links mass, molar mass, gas volume and solution concentration in calculations.

The mass of one mole of a substance in grams per mole, numerically equal to its relative formula mass. It converts between mass in grams and amount in moles.

The quantitative relationship between amounts of reactants and products shown by a balanced equation. Coefficients give the mole ratio used to calculate expected masses or volumes.

The reactant that is used up first in a reaction and therefore determines the maximum amount of product formed. Other reactants are described as in excess.

The concentration of a solution expressed as moles of solute per cubic decimetre of solution, written mol dm^-3. It is used to calculate amounts in titration and dilution problems.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm^-3. Values below seven indicate acidity, and each unit change represents a tenfold change in hydrogen ion concentration.

A solution that resists pH change when small amounts of acid or alkali are added, usually containing a weak acid with its conjugate base. It is important in biological and analytical systems.

A value expressing the ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient. It depends on temperature and indicates the reaction's position.

When a system at equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to oppose that change. It predicts effects of temperature, pressure and concentration changes.

A specific group of atoms within an organic molecule that determines its characteristic reactions and properties. Examples include hydroxyl, carboxyl, carbonyl, amine and ester groups.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving different properties. Structural and stereoisomerism are the two main classes.

A reaction in which an electron-rich nucleophile replaces a leaving group on a carbon atom. Haloalkanes undergo this with hydroxide ions to form alcohols.

The heat energy transferred at constant pressure during a reaction, given the symbol delta H. Negative values indicate exothermic processes and positive values indicate endothermic ones.

The minimum energy colliding particles need to react successfully, shown as the height of the barrier on an energy profile. Catalysts lower it without changing the overall enthalpy change.

The change in concentration of a reactant or product per unit time. It increases with concentration, temperature, surface area and catalysts, as explained by collision theory.

The loss of electrons by a species, accompanied by an increase in oxidation number. Oxidation and reduction always occur together in redox reactions.

The gain of electrons by a species, accompanied by a decrease in oxidation number. In electrolysis it occurs at the cathode, where positive ions gain electrons.

The potential difference measured for a half-cell under standard conditions relative to the hydrogen electrode, quoted in volts. More positive values indicate a greater tendency to be reduced.

A volumetric procedure in which a solution of known concentration is added from a burette until the reaction with a measured volume of analyte is just complete. A suitable indicator signals the end point.

A quantity deliberately held constant throughout an experiment so that it cannot influence the measured dependent variable. Identifying controls is essential when evaluating experimental validity.

The estimated range within which a measured value is likely to lie, often quoted as plus or minus half the smallest scale division. Uncertainties are combined when calculated results depend on several measurements.