LC Physics key terms

The straight-line distance and direction from a starting point to a finishing point. It is a vector quantity, so reversing direction can reduce displacement even while distance travelled keeps increasing.

The rate at which displacement changes with time, calculated as displacement divided by the time taken. Because displacement is a vector, velocity has both magnitude and direction and can be negative.

The rate at which velocity changes with time. An object accelerates when its speed, its direction, or both change; the acceleration is in the direction of the resultant force.

The resultant force on an object equals its mass multiplied by its acceleration, expressed as F = ma. The acceleration is in the same direction as the resultant force.

The product of an object's mass and its velocity, giving a vector quantity measured in kilogram metres per second. Momentum depends on both how heavy and how fast an object is.

In an isolated system with no external resultant force, the total momentum before an interaction equals the total momentum after it. This principle applies to collisions and explosions.

The energy transferred when a force moves an object through a distance. Work equals force multiplied by displacement in the direction of the force, measured in joules.

The energy an object possesses because of its motion, calculated as half its mass multiplied by the square of its speed. Doubling speed quadruples kinetic energy.

The rate at which work is done or energy is transferred, measured in watts. One watt equals one joule of energy transferred each second.

Oscillation in which the acceleration is directly proportional to the displacement from equilibrium and always directed back towards that equilibrium point. Examples include a mass on a spring and a simple pendulum.

The distance between two successive points in phase on a wave, such as from one crest to the next. It is measured in metres and determines colour for light.

The number of complete oscillations or wave cycles passing a point each second, measured in hertz. Frequency is the reciprocal of the periodic time of one cycle.

The maximum displacement of a wave or oscillation from its equilibrium position. For a wave, larger amplitude generally means more energy is being carried.

The spreading of waves when they pass through a gap or around an obstacle. Diffraction is noticeable when the gap width is similar to the wavelength.

The superposition of two or more waves that meet at the same place, producing regions of reinforcement and cancellation. Constructive interference gives larger amplitude; destructive interference gives smaller amplitude.

A stable pattern formed when two waves of equal frequency travel in opposite directions and superpose. It contains nodes of zero displacement and antinodes of maximum displacement.

A large increase in amplitude that occurs when a system is driven at its natural frequency. The driving force transfers energy efficiently, and damping can reduce the size of the response.

The change in observed frequency of a wave when the source, the observer, or both move relative to the medium. Approach raises observed frequency; recession lowers it.

The change in direction of a wave when it crosses a boundary between media and its speed changes. Light bending as it enters glass or water is a common example.

The ratio of the speed of light in a vacuum to the speed of light in a given medium. A higher refractive index means light travels more slowly in that material.

The complete reflection of light inside a medium when it strikes a boundary with a less optically dense medium at an angle greater than the critical angle. No light refracts out.

The restriction of transverse wave oscillations to one plane. Light can be polarised by filters or by reflection, showing that light is a transverse wave.

A region in space where a charged object experiences an electric force. Field strength at a point equals force per unit positive charge, measured in newtons per coulomb.

The work done per unit charge when charge moves between two points in a circuit. It is measured in volts, where one volt equals one joule per coulomb.

The rate of flow of electric charge past a point in a circuit, measured in amperes. One ampere equals one coulomb of charge passing each second.

The opposition a component offers to the flow of electric current. It is defined by voltage divided by current, measured in ohms, and depends on material, length and thickness.

The charge stored by a capacitor per unit potential difference across it. It is measured in farads, where one farad stores one coulomb per volt.

The production of an electromotive force in a conductor when the magnetic flux through it changes. Changing the flux can be done by moving a magnet or changing current.

The induced current always flows in a direction that opposes the change in magnetic flux that produced it. This law follows from conservation of energy.

The emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency shines on it. The effect provides evidence that light behaves as particles.

A discrete packet or quantum of electromagnetic radiation. Its energy equals Planck's constant multiplied by frequency, so higher-frequency radiation carries more energetic photons.

The minimum energy needed to remove an electron from the surface of a particular metal. It depends on the metal and is measured in electronvolts or joules.

The time required for half of the unstable nuclei in a radioactive sample to decay. It is a constant characteristic of each radionuclide and does not depend on sample size.

Atoms of the same element with the same number of protons but different numbers of neutrons. Isotopes have identical chemical properties but may differ in nuclear stability.

The spontaneous emission of particles or electromagnetic radiation from unstable atomic nuclei. It changes the nucleus and continues until a stable configuration is reached.

A measure of the average kinetic energy of the particles in a substance. It determines the direction of heat flow: energy moves from hotter objects to colder ones.

The energy required to raise the temperature of an entire object by one kelvin. It depends on the material and the mass of the object.

The energy needed to change the state of one kilogram of a substance without changing its temperature. It covers melting, boiling, freezing or condensing.

The state reached when two objects in contact have the same temperature, so there is no net transfer of heat between them. Their microscopic energy exchanges balance.

The transfer of heat through a material by collisions between particles, without the material itself moving as a whole. Metals conduct well because free electrons carry energy.