CSEC Physics key terms

The straight-line distance and direction from a starting point to an ending point. It is a vector quantity, so moving in a closed loop gives zero displacement even though distance travelled is not zero.

The rate of change of displacement with time, including direction. Average velocity is total displacement divided by total time; instantaneous velocity is the value at one moment.

The rate at which velocity changes with time. Because velocity is a vector, acceleration occurs when speed changes, direction changes, or both change.

A scalar has magnitude only, such as mass, time, or temperature. A vector has both magnitude and direction, such as force, velocity, or displacement, and is added using direction-sensitive rules.

Motion under gravity alone, with air resistance ignored. Near the Earth's surface the acceleration is about 9.8 m/s² downward, so all objects fall at the same rate regardless of mass.

An object stays at rest or moves with constant velocity unless a resultant external force acts on it. The law describes inertia, the tendency of matter to resist changes in motion.

The resultant force on an object equals its mass multiplied by its acceleration, written F = ma. A larger force produces greater acceleration, and a larger mass produces less.

For every action force there is an equal and opposite reaction force. The pair acts on two different bodies, which is why the forces do not cancel each other.

The resistance of an object to any change in its state of motion. It depends only on mass, so a more massive object needs a larger force to change its velocity.

The single force that has the same effect as all the forces acting on a body combined. If it is zero, the body is in equilibrium and its velocity does not change.

The turning effect of a force about a pivot, calculated as force multiplied by the perpendicular distance from the pivot. It is measured in newton metres.

For a body in equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that same point. This allows unknown forces or distances to be calculated.

The point at which the whole weight of a body may be considered to act. For a uniform regular shape it lies at the geometric centre; for irregular shapes it can be found by hanging.

The ability of an object to return to its original position after being tilted. It increases when the centre of gravity is low and the base area is wide.

Work is energy transferred when a force moves an object through a distance, measured in joules. Power is the rate of doing work, measured in watts, so power equals work divided by time.

The energy a body possesses because of its motion, calculated as one half of mass times velocity squared. Doubling the speed gives four times the kinetic energy.

Energy cannot be created or destroyed, only transferred or transformed from one form to another. In a closed system the total energy remains constant, though some may spread as heat.

The ratio of useful energy output to total energy input, usually expressed as a percentage. No real machine is 100 percent efficient because friction and other losses waste energy.

The force acting normally per unit area on a surface, measured in pascals. The same force produces greater pressure when it acts over a smaller area.

The mass of a substance per unit volume, measured in kilograms per cubic metre. It explains why objects of equal size can have very different masses and whether they float.

The upward force exerted by a fluid on an object immersed in it, caused by the greater pressure at the bottom than at the top. It equals the weight of fluid displaced.

The total kinetic and potential energy of the particles in a substance. Heating increases internal energy, raising temperature or causing a change of state.

The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius. Water has a high value, so it heats and cools slowly.

The energy absorbed or released when a substance changes state without changing temperature. Latent heat of fusion applies to melting and freezing; latent heat of vaporisation applies to boiling and condensing.

The transfer of heat through a solid by particles passing on vibrations, or by free electrons in metals. Metals conduct well because their free electrons carry energy quickly.

Heat transfer in fluids by the movement of the fluid itself. Warmer, less dense fluid rises and cooler, denser fluid sinks, forming convection currents.

The transfer of heat by infrared electromagnetic waves, which needs no medium and can travel through a vacuum. Dark, matt surfaces absorb and emit it better than shiny ones.

The increase in size of a substance as its temperature rises, because particles vibrate more and take up more space. It is allowed for in bridges, rails, and thermostats.

Two different metals joined together that bend when heated because they expand by different amounts. It is used in thermostats and fire alarms to make or break circuits.

A disturbance that transfers energy from one place to another without transferring matter. Waves may be mechanical, needing a medium, or electromagnetic, which can travel through a vacuum.

The distance between two consecutive corresponding points on a wave, such as crest to crest. It is measured in metres and is represented by the symbol lambda.

The number of complete waves passing a point each second, measured in hertz. It is fixed by the source and does not change when a wave enters a new medium.

The distance a wave travels each second, equal to frequency multiplied by wavelength. When a wave enters a denser medium its speed and wavelength change but frequency stays the same.

The change in direction of a wave as it passes from one medium to another at an angle, caused by a change in speed. It bends towards the normal when slowing down.

A measure of how much a medium slows light, found by dividing the speed of light in a vacuum by its speed in the medium. It also relates the angles of incidence and refraction.

When light travelling in a denser medium strikes a boundary beyond the critical angle, it is completely reflected back inside. This effect carries signals in optical fibres.

A curved transparent object that refracts light to form an image. A converging lens brings parallel rays to a focus; a diverging lens spreads them apart.

The distance from the centre of a lens to its principal focus, where parallel rays converge. A shorter focal length gives a more powerful lens and a larger magnification.

The ratio of the image height to the object height. A value greater than one means the image is larger; a negative value indicates an inverted image.

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

The energy transferred per unit charge as charge moves between two points, measured in volts. It is also called potential difference and is measured with a voltmeter in parallel.

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

The current through a metallic conductor is directly proportional to the voltage across it, provided temperature stays constant. It is written as V = IR and fails for non-ohmic components.

In a series circuit components share one path, so current is the same and voltages add. In a parallel circuit the voltage is the same across branches and currents add.

The rate at which electrical energy is transferred by a component, measured in watts. It equals voltage multiplied by current, or current squared multiplied by resistance.

The production of a voltage in a conductor when it experiences a changing magnetic field. This principle underlies generators and transformers.

A region where a magnetic force acts on a moving charge or magnetic material. Its direction is shown by field lines running from north to south outside a magnet.

A device with two coils on a shared iron core that changes alternating voltage by electromagnetic induction. It works only with alternating current, not steady direct current.

The smallest particle of an element that keeps its chemical properties. It has a nucleus of protons and neutrons surrounded by electrons in shells.

The number of protons in the nucleus of an atom, which identifies the element. In a neutral atom it also equals the number of orbiting electrons.

The total number of protons and neutrons in the nucleus of an atom. It is always a whole number and is used with atomic number to write nuclide symbols.

Atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but may differ in radioactivity and mass.

The spontaneous breakdown of an unstable nucleus, emitting alpha, beta, or gamma radiation. It is random, so we can predict half-life but not the decay of a single nucleus.

The time taken for half the nuclei in a radioactive sample to decay. It is constant for a given isotope and is used in dating and medical treatment.

Radiation carrying enough energy to remove electrons from atoms, forming ions. Alpha, beta, and gamma radiation differ in penetrating power and in how easily they are stopped.

The splitting of a heavy nucleus into two lighter nuclei, releasing energy and neutrons. It powers nuclear reactors and can sustain a chain reaction.

The joining of two light nuclei to form a heavier one, releasing large amounts of energy. It occurs in stars and requires extremely high temperature and pressure.