Physics Definitions IGCSE

Speed

distance travelled per unit time

velocty

speed in a given direction

acceleration

change in velocity per unit time

Gravitational field strength

force per unit mass

density

mass per unit volume

spring constant

force per unit extension

‘limit of proportionality’

the point beyond which the extension is not directly proportional to the load

moment of a force

moment = force x perpendicular distance from the pivot

it is a measure of its turning effect

momentum

mass x velocity

impulse

force x time for which force acts

resultant force

change in momentum per unit time

efficiency

efficiency = (useful energy/power output) / (total energy/power input) x100%

power

work done/energy transferred per unit time

Pressure

force per unit area

specific heat capacity

energy required per unit mass per unit temperature increase (to increase 1kg by 1°C)

normal

an imaginary line drawn perpendicular to the surface at the point of incidence?

angle of incidence

the angle between the incident ray and the normal?

angle of reflection

the angle between the reflected ray and the normal?

angle of refraction

the angle between the refracted ray and the normal?

Refractive index

the ratio between the speeds of waves in two different regions

focal length

the distance between the centre of a lens/mirror and its principal focus?

principal axis

the straight line passing through the centre of a lens/mirror and the principal focus?

principal focus (focus point)

the point where parallel rays of light (parallel to the principal axis) converge after reflection or refraction??

Ultrasound

sound with a frequency higher than 20kHz

electric current

charge passing a point per unit time

Electromotive force (emf)

electrical work done by a source in moving a unit charge around a complete circuit

Potential difference (p.d)

the work done by a unit charge passing through a component

kilowatt-hour

the amount of electrical energy transferred by 1kW appliance in 1 hour

Proton number (atomic number) Z

the number of protons in the nucleus of an atom?

Nucleon number (mass number) A

the total number of neutrons and protons in the nucleus?

Half-life of a particular isotope

time taken for half the nuclei of that isotope in any sample to decay

time taken for number of radioactive nucleu in sample to half

Average orbital speed

orbital speed is the total distance travelled in one orbit divided by the time taken to complete that orbit.

It is the circumference of the orbit (2 x pi x the orbital radius) divided by the orbital period

Hubble constant (H₀)

ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth

Mass

a measure of the quantity of matter in an object at rest relative to the observer

Weight

gravitational force on an object that has mass

Equilibrium

when an object has no resultant force/moment

Centre of gravity

the point where all of the weight seems to act

Critical angle

angle of incidence of light at which the angle of refraction is exactly 90°

Direction of a magnetic field at a point

direction of the force on the N pole of a magnet at that point

Direction of an electric field at a point

direction of the force on a positive charge at that point

Isotope

atoms of the same element with the same number of protons but different number of neutrons

Background radiation

low-level radiation that is always present in the environment from natural sources (cosmic rays, radon gas)

Wave motion

the transfer of energy from one place to another without net movement of matter?

Light year

distance travelled by light in one year in a vacuum

electric field

a region where a charge experiences a force

virtual image

an image that cannot be projected onto a screen

galaxy

collection of billions of stars

magnetic field

region where a magnetic pole experiences a force

Absolute zero

the lowest possible temperature and the temperature at which particles have the least kinetic energy

monochromatic

light/EM radiation of a single frequency

potential divider

Potential divider divides the emf/pd/voltage (of a power source in a circuit) between 2 resistors/components (connected in series to power source) in proportion to the resistances (of the resistors)

evaporation

molecules with enough energy escape from the surface

solid friction

the force between two surfaces that may impede motion and produce heating

Melting point + Boiling point

energy input without a change in temperature

echo

reflection of sound waves

deceleration

rate of decrease of velocity (per unit time)/negative acceleration