## Definitions for Module 5 – OCR A Physics

### Units

Arcminute

An alternative unit for measuring angles
( $2 \pi \text{[rad]} = 360^{\circ}$)

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;
$1 \ ly = 3 \times 10^{8} \ ms^{-1} \cdot 365.25 \times 24 \times 60 \times 60$
$1 \ ly = 9.46 \times 10^{15} \ m$

Astronomical Unit

Is the average distance between Earth and the Sun;
$1 \ AU = 1.50 \times 10^{11} \ m$

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.
$1 \ pc = 3.09 \times 10^{16} \ m$

### 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

$2 \pi \times \text{frequency}$

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 $1 \ kg$ of a solid into a liquid at constant temperature

Latent heat of vaporisation

The thermal energy required to turn $1 \ kg$ 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 $1 \ K$

Boyle’s Law

Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature

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 $3 \ K$

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 $10^{-43} \ \text{seconds}$ 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