EJU Science key terms

The net force on an object equals its mass multiplied by its acceleration (F = ma). It predicts how motion changes when forces act, and reduces to constant velocity when net force is zero.

In an isolated system, total momentum stays constant. For collisions and explosions, the vector sum of momenta before equals that after, regardless of internal forces.

Energy an object possesses due to motion, calculated as one-half mass times speed squared. It is a scalar, always non-negative, and changes when net work is done.

Oscillation where restoring force is proportional to displacement and directed opposite it. Motion is sinusoidal with constant period independent of amplitude, exemplified by an ideal spring-mass system.

Bending of a wave when it crosses a boundary between media with different wave speeds. Described by Snell's law; frequency stays constant while wavelength and direction change.

For a metallic conductor at constant temperature, current is proportional to potential difference. The constant ratio is resistance (V = IR), valid only for ohmic devices.

A changing magnetic flux through a circuit induces an electromotive force. Faraday's law quantifies it, and Lenz's law gives the induced current direction opposing the flux change.

Light above a threshold frequency ejects electrons from a metal surface instantly. Electron energy depends on frequency, not intensity, supporting the photon model of light.

Relates pressure, volume, amount, and absolute temperature of an ideal gas: PV = nRT. It combines Boyle's, Charles's, and Avogadro's laws and ignores intermolecular forces.

Energy conservation for thermodynamic systems: change in internal energy equals heat added minus work done by the system. It defines how heat and work transfer energy.

Acceleration directed toward the center of circular motion, magnitude v²/r. It changes velocity direction but not speed, and requires a net inward force.

Superposition of waves meeting at a point. Constructive interference gives larger amplitude when waves are in phase; destructive gives smaller amplitude when out of phase.

Amount of substance containing Avogadro's number of particles. It connects mass to particle count via molar mass and lets chemists count atoms and molecules by weighing.

Number of particles in one mole, approximately 6.02 × 10²³ per mole. It links microscopic particle counts to macroscopic amounts used in stoichiometry.

Concentration expressed as moles of solute per liter of solution. It is temperature-dependent because solution volume changes with temperature; used in dilution and titration calculations.

Reactant completely consumed first in a chemical reaction, determining the maximum product amount. Excess reactants remain, and theoretical yield is calculated from this reactant.

When a system at equilibrium is disturbed by concentration, pressure, or temperature change, it shifts in the direction that partially offsets the disturbance. It predicts qualitative equilibrium responses.

Ratio of product concentrations to reactant concentrations at equilibrium, each raised to its stoichiometric coefficient. A large value favors products; it depends on temperature only.

Equilibrium constant for the ionization of a weak acid in water. A larger value means a stronger acid; it relates to pH and buffer behavior.

Reaction involving electron transfer, with oxidation (loss of electrons) and reduction (gain of electrons) occurring together. Oxidation numbers change, and the reaction can be split into half-equations.

Driving a nonspontaneous chemical reaction with an external electrical current. Oxidation occurs at the anode and reduction at the cathode; used for plating and metal extraction.

Heat exchanged at constant pressure during a reaction. Exothermic reactions release heat with negative enthalpy change; endothermic absorb heat with positive change.

The overall enthalpy change of a reaction is the sum of enthalpy changes of its steps, regardless of pathway. It allows calculation from formation or combustion data.

Molecules with the same molecular formula but different atom arrangements or spatial orientations. Connectivity differences define structural types; three-dimensional differences define stereochemical types.

Reaction of a carboxylic acid with an alcohol, producing an ester and water, usually acid-catalyzed. Esters often have fruity odors and are used as flavorings and solvents.

Net movement of water across a selectively permeable membrane from lower to higher solute concentration. It drives cell volume changes in hypotonic and hypertonic solutions.

Protein catalyst that speeds up biochemical reactions by lowering activation energy. It binds substrates at its active site and is sensitive to temperature and pH.

Adenosine triphosphate, the main energy currency of cells. Hydrolysis of its terminal phosphate bond releases energy used for muscle contraction, transport, and biosynthesis.

Process where plants, algae, and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. It occurs in chloroplasts and underpins most food chains.

Series of reactions breaking down glucose to release energy, producing ATP, carbon dioxide, and water. Aerobic respiration uses oxygen; anaerobic pathways include glycolysis and fermentation.

Semi-conservative process where a double-stranded DNA molecule unwinds and each strand templates a new complementary strand. It ensures genetic information is copied before cell division.

Synthesis of an RNA molecule from a DNA template, catalyzed by RNA polymerase. In eukaryotes it occurs in the nucleus and is followed by RNA processing.

Process where ribosomes read mRNA codons and assemble amino acids into a polypeptide chain, using tRNA and the genetic code.

Cell division producing four haploid gametes with half the chromosome number. Crossing over and independent assortment create genetic variation among offspring.

Mechanism of evolution where individuals with inherited traits better suited to their environment survive and reproduce more, increasing those traits' frequency in later generations.

Maintenance of stable internal conditions, such as temperature and blood glucose, despite external changes. It relies on negative feedback loops involving sensors, control centers, and effectors.

Rapid electrical signal traveling along a neuron's membrane, caused by ion channel opening and depolarization followed by repolarization. It transmits information to other neurons or muscles.

Community of organisms interacting with each other and their physical environment, through energy flow and nutrient cycling. Producers, consumers, and decomposers are linked in food webs.

Signaling molecule secreted by glands into the bloodstream, acting on target cells with specific receptors. Hormones regulate growth, metabolism, reproduction, and homeostasis.

Random change in allele frequencies in a population, especially strong in small populations. Unlike natural selection, it is not driven by fitness differences.