Definitions for Module 5 – OCR A Physics
Units
Arcminute

Radian
An alternative unit for measuring angles
( )
Kelvin
The unit of thermodynamic temperature
(equal to degrees Celsius – 273.15)
Arcminute
An angular measurement equal to 1/60 of a degree
Arcsecond
An angular measurement equal to 1/3600 of a degree or 1/60 of an arcminute
Light year
A unit of astronomical distance equivalent to the distance that light travels in one year;
Astronomical Unit
Is the average distance between Earth and the Sun;
Parsec
Is a unit of length used to measure the large distances to astronomical objects outside the Solar System.
It is defined as the distance at which one astronomical unit subtends an angle of one arcsecond.

Quantities

Net force on a body
Rate of change of its momentum
Centripetal force
The net force on acting on an object moving in a circle; it is always directed towards the centre of the circle
Gravitational field strength
Force per unit mass
Period of an object describing a circle
The time taken by an object to complete one orbit
Displacement
Distance from an equilibrium position
Amplitude
Maximum displacement
Period
The time taken for one complete oscillation of a vibrating object
Frequency
Number of oscillations per unit time
Angular frequency
Phase difference
The fraction of an oscillation between the vibrations of two oscillating particles (expressed in degrees or radians)
Pressure
Force per unit area
Internal energy
The sum of the random distribution of kinetic and potential energies associated with the molecules of a system
Latent heat of fusion
The thermal energy required to turn of a solid into a liquid at constant temperature
Latent heat of vaporisation
The thermal energy required to turn of a liquid into a gas at constant temperature
Intensity
Power per unit cross-sectional area

Concepts

Newton’s law of gravitation
force between two masses is proportional to the product of the masses and inversely proportional to the square of the distance between them
Geostationary orbit
An equatorial orbit with a period of 24 hrs, moving from west to east
Kepler’s third law
The cube of a planet’s distance from the Sun is proportional to the square of its orbital period
Simple harmonic motion
Acceleration is directly proportional to displacement and is directed in the opposite direction to the displacement
Thermal equilibrium
No net heat flow between objects
Absolute zero
The temperature at which a substance has minimum internal energy
Specific heat capacity
The energy required per unit mass of a substance to raise its temperature by
Boyle’s Law
Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature
Olber’s paradox
If the universe is static and infinite, the night sky should be uniformly bright because every line of sight would end on a star
Hubble’s law
The speed of recession of a galaxy is proportional to its distance
Cosmological principle
The universe has the same large-scale structure when observed from any point (homogeneous, isotropic and the laws of physics are universal)
Cosmic Microwave Background Radiation
Gamma radiation released during the Big Bang has stretched as the Universe expanded, causing it to become microwave with an energy equivalent to a temperature of
Critical density
The density for which the universe will expand towards a finite limit (resulting in a flat universe)
Open universe
The universe will expand forever
Flat universe
The universe will expand towards a finite limit
Closed universe
The universe will contract (to a big crunch)

Basic Assumption of the Kinetic Theory of Gases:
- particles move with rapid, random motion
- all collisions are elastic
- volume of atoms is negligible (compared with volume of container)
- no intermolecular forces (except during collisions)
- collision time is negligible (compared to time between collisions)

Useful Applications of Resonance

- cooking: microwaves cause water molecules to resonate
- woodwind instrument: reed causes air column to resonate
- brass instrument: lips cause air column to resonate
- MRI: radio waves in a magnetic field cause protons to resonate
- radios: radio waves cause electrons to resonate
- person on a swing: intermittent pushes cause swing to resonate
Problematic Examples of Resonance
- electrostatic force – repulsive between protons, no effect on neutrons; long-ranged
- gravitational force – attractive; long-ranged
- strong nuclear force – attractive; short-ranged

Formation of a Star:

- gas cloud drawn together by gravitational forces (gravitational collapse)
- GPE is converted to KE, causing temperature to increase
- hydrogen nuclei (protons) fuse to make helium + energy
- a stable star is formed when the radiation pressure is equal to the gravitational pressure
Probable Evolution of the Sun:
- when hydrogen runs out, the outer layers of the star expand
- a red giant is formed
- the outer layers are shed, leaving behind a white dwarf

Probable Evolution of a Star Much More Massive Than the Sun:

- supernova
- core becomes either a neutron star or black hole
Evolution of Universe from
After the Big Bang to the Present:
- hot, dense singularity
- all forces were unified
- expansion led to cooling
- quark and lepton soup
- more matter than antimatter
- quarks combine to form hadrons (protons and neutrons)
- helium formed from imbalance of protons and neutrons
- atoms formed
- gravitational force forms stars/galaxies
- temperature becomes 2.7K and the universe is saturated with microwave background radiation

Evidence for the Big Bang:

- spectra from galaxies show shift to longer wavelengths (redshift), which suggests galaxies are moving away from Earth
- the more distant galaxies are moving faster
- existence of microwave background radiation; the temperature of the universe is 3K; gamma radiation stretched to become microwaves as the Universe expanded
- existence of primordial helium
- temperature fluctuations are predicted and observed
