Physics - Comprehensive Key Terms

Displacement

the distance between two points in a specified direction.

Speed

the rate at which distance changes with time.

Velocity

the rate of change of displacement with time.

Average speed

the total distance travelled by the total time taken.

Average velocity

the total displacement divided by the total time taken.

Acceleration

the rate of change of velocity with time.

Scalar

a physical quantity that has magnitude but not direction.

Vector

a physical quantity that has both magnitude and direction.

Balanced forces

occur when there is no resultant force on an object.

Unbalanced forces

occur when there is a resultant force causing an acceleration.

Mass

measured in kg and shows the amount of matter in an object.

Weight

the force of gravity on an object.

Acceleration of free fall

the acceleration of an object twoards the surface of the Earth when the only force on it is the gravitational force.

Gravitational field strength, g

the gravitational force on an object of mass 1kg close to the surface of a planet.

Hooke's law

states that the extension is directly proportional to the applied load, up to a limit known as the limit of proportionality.

Natural length

the natural length of a spring when no stretching or compressing forces are applied.

Extension

the difference between the stretched length of a spring and its natural length.

Elastic

materials return to their original length when the stretching force is removed.

Plastic

materials do not return to their original length when the stretching force is removed.

Pressure

the force per unit area on a surface.

Pascal

the SI unit of pressure and is the same as 1 N/M^2

Centre of gravity

the point through which the entire weight of a body appears to act.

Equilibrium

a state in which opposing forces or moments are balanced.

Neutral equilibrium

a state in which a slight displacement causes the object neither to move very far from its original position nor return to it (e.g. a ball sitting on a bench).

Unstable equilibrium

a state in which a slight displacement causes the object to move a long way from its original position (e.g. a pencil balanced on its end).

Stable equilibrium

a state in which a slight displacement causes the object to return to its original position (e.g. a traffic cone standing on its wide base).

Moment

the moment about a point is the product of a force and its perpendicular distance from the point.

Principle of moments

when a lever is balanced, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that same point.

Density

the mass of an object divided by its voume.

Kinetic theory

explains the properties of solids, liquids and gases according to the arrangement and motion of their molecules.

Energy forms

the different ways in which energy can appear, such as heat, light, sound, etc.

Energy resources

the different ways of supplying a particular energy form such as coal, oil, wind etc.

Kinetic energy

the energy possessed by a mass due to its speed.

Gravitational potential energy

the energy possesed by a mass due to its height above the ground.

Principle of conservation of energy

energy can change from one form to another but can never be created nor destroyed.

Renewable

energy that is being replaced by nature in less than a human lifetime, so we will never run out of it.

Non-renewable

energy that will eventually run out.

Work

the product of the force and the distance moved in the direction of the force.

Energy

what is required to do work.

Power

the rate of doing work.

Atom

the smallest part of an element that can exist on its own.

Radioactivity

the process that occurs when alpha particles, beta particles or gamma waves are emitted from an unstable nucleus.

Alpha

alpha particles are the helium nuclei containing two protons and two neutrons which are emitted by radioactive nuclei.

Beta

beta particles are fast moving electrons emitted by radioactive nuclei.

Gamma

gamma radiation is a high energy electomagnetic wave emitted by radioactive nuclei.

Half-life

the time taken for the activity of a radioactive material to fall by half.

Nuclear fission

the process by which a uranium (or plutonium) nucleus absorbs a slow neutron and then splits to produce two or more lighter nuclei and several neutronsm together with a vast quantity of energy.

Nuclear fusion

the process in which light nuclei such as hydrogen combine togther to produce a heavier nucleus such as helium and emit a vast quantity of energy.

ITER

International Thermonuclear Experimental Reactor', an experimental nuclear fusion device being built in southern France and involving the cooperation of scientists and engineers from 35 nations.

Longitudinal wave

a wave in which the particles vibrate parallel to the direction in which the wave is travelling.

Compressions

are places where the coils (in a slinky) or particles in a longitudinal wave bunch togther.

Rarefactions

places where the coils (in a slinky) or particles in a longitudinal wave are furthest apart.

Transverse wave

a wave in which the particles vibrate perpendicular to the direction in which the wave is travelling.

Frequency

the number of complete waves passing a fixed point in a second.

Wavelength

the distance between two consecutive crests or troughs.

Amplitude

the greatest displacement of the wave from its undisturbed position.

Angle of incidence (i)

the angle between the normal and the incident ray.

Angle of reflection (r)

the angle between the normal and the reflected ray.

Angle of refraction

the angle between the normal and the refracted ray.

Dispersion

the splitting of white light into its component colours.

Converging lens

a lens that is thickest at its centre and least thick at its edges.

Diverging lens

a lens that is thickest at its edges and least thick at its centre.

Principal axis

a straight line joining the principal foci and passing through the optical centre of a convex lens.

Principal focus

a point on the principal axis of a convex lens through which rays of light parallel to the principal axis pass after refraction in the lens.

Focal length

the distance between the optical centre of a lens and the principal focus.

Electrical conductor

a material through which electrical current passes easily.

Insulator

a material through which electrical current cannot pass.

Free electrons

electrons that are not attached to any particular atom.

Conventional current

the imagined current flowing from the positive terminal of a battery to the negative terminal.

Standard symbols

the internationally recognised symbols for electrical components.

Cell polarity

concerned with which end of a cell is positive and which end is negative.

Series circuit

a circuit in which the components are arranged one-after-another, like carriages in a train.

Voltage

the difference in electrical potental between two points that causes a current to flow.

Current

a flow of electrical charge in a circuit.

Resistance

the opposition to the flow of current and is defined as the ratio of voltage to current.

Parallel circuit

a circuit is one in which the current divides to travel independently along two or more separate loops.

Inverse proportion

the mathmatical relationship between quantities in which one quantity doubles when the other halves.

Electrical power

the rate at which electrical energy is used in a circuit.

Joule's law

states that the power generated in an electrical component is the product of the current in the component and the voltage across it (P = IV).

Kilowatt-hour

the amount of energy transferred when 1000 W is delivered for 1 hour.

Neutral wire

the wire in a mains voltage system that is permanently at zero volts.

Live wire

the dangerous wire in a mains voltage system in which the voltage is changing.

Earth wire

the wire that connects the metal frame in an electical appliance to the earth.

Fuse

a short wire, often used inside a plug, which melts to disconnect a circuit when too much current flows.

Double insulation

a safety technique in which an electical appliance is encased in an insulating plastic case to prevent a user coming into contact with a live component and being harmed.

Magnetic field

a region of space within which a magnet experiences a force.

Plotting compass

a small compass used to find he shape of a magnetic field.

Magnetice field lines

lines drawn to represent a magnetic field.

Poles (of a magnet)

the ends of a magnet (north and south) where the magnetic field is the strongest.

Solenoid

a coil of wire carrying an electrical current.

Planets

large heavenly bodies that orbit a star.

Elliptical

the oval shape of the path taken by many heavenly bodies as they orbit the Sun.

Asteroids

lumps of rock up to about 1000 km diameter. In our Solar System most asteroids can be found in a 'belt' between Mars and Jupiter.

Comets

heavenly bodies made of mostly ice and dust that orbit a star, often in very elongated elliptical paths.

Solar System

consists of a star and everything that orbits it.

Stellar nebula

a cloud of gas and dust from which stars are formed.

Gravitational collapse

a process in the evolution of a star in which hydrogen particlesget closer and closer together because of gravity.

Protostar

a very, very hot ball of gas in which nuclear fusion has not yet begun - when fusion begins the protstar becomes a star.

Main sequence

the most stable stage in the mid-life of a star.

Red giant

a star that has used up almost all of its hydrogen and the outward pressure from nuclear fusion makes it expand to several hundred times its normal size.