Year 12 Definitions

Physical quantity

numerical value and a unit

6 Base Units

mass, length, time, current, temperature, amount of substance

Scalar

has magnitude

Vector

has magnitude and direction

Displacement

the distance travelled in a particular direction (vector)

Instantaneous speed

the speed at the moment it is measured (greater the gradient, greater the instantaneous speed)

Average speed

rate of change in distance over a complete journey (speed = distance/time)

Velocity

rate of change of displacement (vector) (velocity = change in displacement/time)

Acceleration

rate of change of velocity (vector)

Reaction time

time taken for a person to react after an event e.g. seeing a hazard

Thinking distance

distance travelled by a vehicle between the driver seeing a hazard and applying the brakes

Braking distance

distance travelled by a vehicle after the brakes are applied until it comes to a complete stop

Stopping distance

distance covered by a vehicle in the time between the driver spotting a hazard and the vehicle coming to a complete stop

Drag force

frictional force experienced by an object travelling through a fluid

Terminal velocity

the constant speed reached by an object when the drag force (and upthrust) is equal and opposite to the weight

Moment of a force

moment of a force = force x perpendicular distance from pivot. It is the turning effect of a force around a turning point

Couple

pair of forces of equal size which act parallel to each other, but in opposite directions

Torque

the turning effect caused by a force couple (only turning effect, no resultant linear force)

Principle of moments

for a body to be in equilibrium, the sum of the clockwise moments acting about any point must equal the sum of the anticlockwise moments about the same point

Centre of mass

a point through which any externally applied force produces straight-line motion but no rotation

Centre of gravity

an imaginary point at which the entire weight of an object appears to act

Equilibrium

all forces acting on it are balanced and cancel each other out, there is no net force on the object so it is not accelerating

Upthrust

the upwards force experienced by an object partially or fully immersed in water

Archimedes' Principles

when a body is completely or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid it has displaced

Work done

the amount of energy transferred from one form to another when a force moves an object through a distance or when a charged particles moves through a potential difference

Joule

one joule is the work done when a force of 1 newton moves an object through a distance of 1 metre

Principle of conservation of energy

energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.

Power

rate of doing work, amount of energy transferred from one form to another per second

Watt (W)

rate of energy transfer equal to 1 joule per second (Js-1)

Tensile deformation

deformation of an object caused by balanced outwards forces, which leads to extension

Compressive deformation

deformation of an object caused by balanced inwards forces, which leads to compression

Extension

stretching

Compression

squashing

Hooke's law

The extension of a stretched object is proportional to the load or force applied to it. This applies up to the limit of proportionality.

Force constant (k)

force needed to extend an object that obeys Hooke's law by 1m (depends on material, length, shape)

Limit of proportionality (Hooke's law limit)

point beyond which the force is no longer proportional to extension

Ultimate tensile strength

maximum stress that a material can withstand before breaking

Breaking stress

stress experienced by material at point it breaks

Elastic limit

force (or stress) beyond which a material will be permanently stretched

Elastic deformation

material returns to its original shape once the forces are removed (up to elastic limit)

Plastic deformation

material is permanently deformed (when material stretched past elastic limit)

Electric current

rate of flow of charge

Elementary charge (e)

1.6x10-19C

No. electrons = charge/1.6x10-19

Current

movement of electrons in metals and movement of ions in electrolytes

Conventional current

travels from positive to negative(direction positive charges travel)

Electron flow

travels from negative to positive (direction electrons travel)

Conservation of charge

electric charge can neither be created or destroyed

Kirchhoff's First Law

At any point in an electrical circuit, the sum of currents into the point is equal to the sum of currents out of that point, electrical charge is conserved

Conductors

very high number density

Semiconductors

between the two

Insulators

very low number density

Potential difference

energy transfer per unit charge by the charges from electrical energy to other stores (e.g. heat, light)

1 volt

the potential difference across a component when 1J of energy is transferred per 1C passing through the component

Electromotive force (EMF)

energy transfer per unit charge to the charges from a chemical store to electrical energy

Resistance

a property of a component calculated by dividing the potential difference across it by the current in it

Ohm

One ohm is the resistance of a component when a p.d. of 1V is produced per ampere of current

Ohm's Law

for a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends

Resistivity

a property of a material defined as the product of the resistance of a component made of the material and its cross-sectional area divided by its length

Kilowatt-hour (kWh)

energy transferred by a device with a power of 1 kW operating for a time of 1 hour

Kirchhoff's Second Law

In a closed loop of an electrical circuit, the sum of the electromotive forces is equal to the sum of the potential differences

Internal resistance

the resistance of a source of e.m.f. (e.g. a cell) due to its construction, which causes a loss in energy/voltage as the charge passes through the source

Source of e.m.f

battery or power supply

Terminal p.d.

the potential difference across an electrical power source, no current = e.m.f. of the source, is current in the source = e.m.f. minus the lost volts

Progressive wave

a moving wave that carries energy from one place to another without transferring any material

Longitudinal waves

vibrations (oscillations) parallel to the direction of energy transfer

Transverse waves

Vibrations (oscillations) perpendicular to the direction of energy transfer

Displacement (wave)

distance from equilibrium position in a particular direction

Amplitude

the maximum displacement from the equilibrium position (+ or -)

Wavelength

the length of one whole wave oscillation or wave cycle (metres)

Period (waves)

the time taken for one complete wavelength to pass a given point OR time taken for one oscillation (seconds)

Phase difference (phi)

an angle in radians between 0 and 2π which indicates the extent to which two different waves or two different points in the same wave are out of step

Phase

angle in radians (0 and 2π) or degrees (0˚ and 360˚) which indicates a fraction of a completed cycle

Frequency

the number of waves passing a given point in 1 second (Hertz- Hz) (f = 1/T)

Speed of a wave

the distance travelled by the wave per unit time (ms-1)

Oscilloscope

displays electrical signal as a voltage against time trace on a screen

Reflection

the change in direction of a wave at a boundary between two different media, so that the wave remains in the original medium

LAW OF REFLECTION

angle of incidence = angle of reflection

Refraction

the changes in direction of a wave as it changes speed when it passes from one medium to another

Polarisation

oscillations of a transverse wave are limited to only one plane

Diffraction

waves passing through a gap or around an obstacle spread out

Plane polarised wave

a wave in which all the vibrations are in one direction or plane

Refractive index

the ratio between the speed of light in a vacuum and the speed of light in a material

Optical density

property of a medium that describes how fast light travels through it, more optically dense = more light slows down, measured by refractive index

Angle of incidence

the angle that incoming light makes with the normal of a boundary

Angle of refraction

the angle that refracted light makes with the normal of a boundary

Snell's Law

n1sinθ1 = n2sinθ2

Critical angle

the angle of incidence at which the angle of refraction is 90˚

Total internal reflection (TIR)

when all light is completely reflected back into a medium at a boundary with another less optically dense medium, instead of being refracted. It only happens at angles of incidence greater than the critical angle

Principle of superposition of waves

when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements

Interference

superposition of coherent waves

Coherence

sources (or waves) that have the same wavelength and frequency and a fixed phase difference between them are coherent

Path difference

the amount by which the path travelled by one wave is longer than the path travelled by another wave

Phase difference

amount by which one wave lags behind another or difference in phase between two points on a wave

Constructive interference

whole number of wavelength, in phase

Destructive interference

half a number of wavelength, antiphase

Two-source interference

when two waves from two sources interfere to produce a pattern

Stationary (standing) wave

a wave created by the superposition of two progressive waves with the same frequency (or wavelength), moving in opposite directions.

Node

a point of zero amplitude on a stationary wave

Antinode

a point of maximum amplitude on a stationary wave