G2 Science Physics Definitions
Definitions for G2 Science Physics, with precise meanings and common distinctions for the K223 / K224 Physics component.
On this page
Key terms for G2 Science Physics (K223 / K224, 2027), grouped to help you compare related ideas. Learn the meaning and the condition together, then use the full lessons for examples and applications.
Recall with understanding
Cover the meaning column and explain a term from memory. For a ratio, name what is measured per unit of what. For a vector, include direction. Then check the last column for the distinction that makes the definition useful.
Measurement
| Term | Meaning | Remember |
|---|---|---|
| Physical quantity | A measurable property expressed by a numerical value and a unit. | A number alone does not specify a physical measurement. |
| Base quantity | A quantity chosen as independent of other quantities in the system of units. | Base does not mean more important. |
| Derived quantity | A quantity expressed in terms of base quantities. | Density combines mass and volume. |
| Scalar | A quantity with magnitude only. | Speed, mass and energy are scalars. |
| Vector | A quantity with magnitude and direction. | Velocity, acceleration and force are vectors. |
| Accuracy | Closeness of a measured value to the true or accepted value. | A small scatter does not remove a systematic bias. |
| Precision | Closeness of repeated measurements to one another. | Repeats may be precise but inaccurate. |
| Random error | An error that varies unpredictably between readings, producing scatter. | Repeating and averaging can reduce its effect. |
| Systematic error | An error that biases readings consistently. | Repeating alone does not remove it. |
| Zero error | A non-zero instrument reading when the measured quantity is zero. | It is a signed systematic error. |
| Parallax error | A reading error caused by viewing a scale from an inappropriate angle. | View perpendicular to the scale. |
Motion and forces
| Term | Meaning | Remember |
|---|---|---|
| Distance | Total length of the path travelled. | Scalar; a return journey still adds distance. |
| Displacement | Change in position from the initial to the final position, with direction. | A complete return to the start has zero displacement. |
| Speed | Distance travelled per unit time. | Average speed uses total distance and total time. |
| Velocity | Rate of change of displacement. | Includes direction. |
| Acceleration | Rate of change of velocity. | Changing direction can cause acceleration even at constant speed. |
| Uniform acceleration | Equal changes in velocity in equal time intervals. | Constant speed in a straight line means zero acceleration. |
| Mass | A measure of the amount of matter in a body and of its inertia. | Unit kg; mass is not weight. |
| Weight | Gravitational force acting on a body. | Unit N; depends on gravitational field strength. |
| Inertia | Tendency of a body to resist a change in velocity. | Greater mass means greater inertia. |
| Resultant force | A single force with the same effect as the combined forces on a body. | Add forces with their directions. |
| Gravitational field | A region in which a mass experiences a gravitational force. | A field exists even without a test mass. |
| Gravitational field strength | Gravitational force per unit mass at a point. | Unit N kg⁻¹. |
| Terminal velocity | Constant velocity reached when the resultant force on a moving body is zero. | For a falling body, drag balances weight (with buoyancy included if significant). |
Pressure and energy
| Term | Meaning | Remember |
|---|---|---|
| Density | Mass per unit volume. | Unit kg m⁻³. |
| Pressure | Normal force per unit area. | Unit Pa; not simply “force”. |
| Work done | Energy transferred by a force through displacement in the direction of that force. | No displacement means no mechanical work by that force. |
| Kinetic energy | Energy a body has because of its motion. | Depends on speed squared. |
| Gravitational potential energy | Energy associated with a body’s position in a gravitational field. | Choose a reference height when calculating near Earth. |
| Conservation of energy | Energy cannot be created or destroyed; the total energy of an isolated system remains constant. | Energy transferred to the surroundings must still be counted. |
| Power | Rate of energy transfer or rate of doing work. | Unit W = J/s; energy and power are different quantities. |
Thermal physics
| Term | Meaning | Remember |
|---|---|---|
| Temperature | Quantity that indicates how hot or cold a body is and determines the direction of net thermal energy transfer. | Temperature is not a measure of total stored energy. |
| Thermal equilibrium | State in which bodies have the same temperature and no net energy transfer by heating occurs between them. | Microscopic exchanges need not stop. |
| Conduction | Transfer of thermal energy through a material without bulk movement of the material. | Metals also transfer energy through mobile electrons. |
| Convection | Transfer of thermal energy by bulk movement of a fluid. | Density differences can drive the circulation. |
| Thermal radiation | Transfer of energy by electromagnetic waves. | Requires no material medium. |
| Boiling | Change from liquid to gas throughout the liquid at its boiling point. | Boiling point depends on pressure. |
| Evaporation | Escape of particles from the surface of a liquid into the gas phase. | Can occur below the boiling point; cools the remaining liquid. |
| Condensation | Change from gas to liquid. | Energy is released to the surroundings. |
| Brownian motion | Irregular motion of visible suspended particles caused by unequal molecular collisions. | Observe the suspended particles, not the individual molecules. |
Waves
| Term | Meaning | Remember |
|---|---|---|
| Wave | A disturbance that transfers energy without net transfer of matter with the wave. | Particles of a medium oscillate about equilibrium. |
| Transverse wave | Wave with oscillations perpendicular to its direction of energy transfer. | All electromagnetic waves are transverse. |
| Longitudinal wave | Wave with oscillations parallel to its direction of energy transfer. | Sound in air has compressions and rarefactions. |
| Amplitude | Maximum displacement from the equilibrium position. | Crest-to-trough distance is twice the amplitude. |
| Wavelength | Shortest distance between two points in phase on a wave. | Successive crests or successive compressions are examples. |
| Frequency | Number of complete oscillations per unit time. | Unit Hz; count full cycles, not half-cycles. |
| Period | Time for one complete oscillation. | Unit s; T = 1/f. |
| Wave speed | Rate at which a wave disturbance travels through a medium. | Different from a particle’s oscillation speed. |
Electricity and circuits
| Term | Meaning | Remember |
|---|---|---|
| Electric current | Rate of flow of electric charge. | Unit A; conventional current is opposite to electron drift in a metal. |
| Potential difference | Energy transferred per unit charge between two points. | Unit V = J/C; specify the two points or component. |
| Electromotive force | Energy supplied by a source per unit charge passing through it. | Despite its name, e.m.f. is not a force. |
| Resistance | Ratio of p.d. across a component to current through it. | Unit Ω; may change with operating conditions. |
| Series arrangement | Components connected along one path, carrying the same current. | Their p.d.s add. |
| Parallel arrangement | Components connected between the same two circuit points. | Same p.d. across each branch; currents divide. |
| Fuse | Safety device containing a wire that melts and opens the circuit when excessive current heats it sufficiently. | It protects against excessive current; not every electric shock causes it to blow. |
| Circuit breaker | Device that opens a circuit when an unsafe electrical condition is detected. | Unlike a fuse, it can usually be reset after the fault is cleared. |
| Double insulation | Protection using insulating barriers so accessible parts cannot become live through a single insulation fault. | Such appliances do not need an earth connection to their casing. |
Radioactivity and the nuclear atom
| Term | Meaning | Remember |
|---|---|---|
| Proton number | Number of protons in a nucleus. | Determines the element. |
| Nucleon number | Total number of protons and neutrons in a nucleus. | Neutron number = nucleon number − proton number. |
| Nuclide | Type of nucleus specified by its proton and nucleon numbers. | Written ^A_ZX. |
| Isotopes | Atoms of the same element with different neutron numbers. | Same proton number; different nucleon number. |
| Radioactive decay | Random, spontaneous disintegration of an unstable nucleus with emission of radiation. | Random: cannot predict an individual decay; spontaneous: no external trigger is needed. |
| Alpha particle | Helium-4 nucleus with two protons and two neutrons. | Charge + 2e; strongly ionising and weakly penetrating. |
| Beta-minus particle | Fast electron emitted when a neutron converts to a proton in a nucleus. | It is not an orbital electron expelled from an atom. |
| Gamma radiation | High-energy electromagnetic radiation emitted by a nucleus. | No charge or rest mass; does not change proton or nucleon number. |
| Half-life | Time for the number of undecayed nuclei, or the activity of a sample, to fall to half its initial value. | Use background-corrected count rate as a proxy in detector measurements. |
| Background radiation | Ionising radiation present from natural and artificial sources in the surroundings. | Subtract its contribution when measuring a source. |
Practical language
| Term | Meaning | Remember |
|---|---|---|
| Independent variable | Quantity deliberately changed in an investigation. | Usually plotted on the horizontal axis. |
| Dependent variable | Quantity measured in response to changes in the independent variable. | Usually plotted on the vertical axis. |
| Control variable | Quantity kept constant to make a comparison fair. | State how it is controlled. |
| Repeatability | Closeness of results obtained using the same method, observer and equipment under the same conditions. | Repeat trials test this. |
| Reproducibility | Closeness of results obtained when relevant conditions such as observer or equipment change. | Another investigator can check the result. |
| Best-fit line | Line or curve representing the overall trend of the data. | Do not join readings point to point unless the task calls for it. |
Use a distinction
A lamp has energy transferred through it per unit charge: this is its potential difference. A cell supplies energy per unit charge: this is its e.m.f. Both are measured in volts, but their roles differ.
G2 Science requires you to identify action–reaction force pairs; memorising the formal statements of Newton’s laws is not required. The pair acts on two different bodies, so it cannot cancel within the free-body diagram of just one body.
Choose a topic from the course hub, revise the explanation, then use its topic check.