OCR A A level Physics- Exploring Physics Definitions

Electric current

Rate of flow of charge

Kirchoff's first law

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

Order of magnitude for n of conductors

28

Order of magnitude for n of semi-conductors

approx 17

Order of magnitude for n of insulators

approx 15

Potential difference

The energy transferred from electrical energy to other forms per unit charge

Electromotive force

The energy transferred from chemical energy to electrical energy per unit charge

Resistance

The ratio of p.d across a component to the current in the component

Ohm's Law

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

I-V Characteristics - Filament Lamp

Non-ohmic component, resistance not constant, behaves the same way regardless of polarity

I-V Characteristics - resistor

Ohmic component, resistance constant, behaves the same way regardless of polarity

I-V Characteristics - Diode

Non-ohmic component, resistance not constant, behaviour depends on polarity

Order of magnitude resistivity of insulators

16

Order of magnitude resistivity of semi-conductors

4

Order of magnitude resistivity of conductors

-8

I-V Characteristics- thermistor

Non-ohmic component, resistance not constant (decreases with temperature)

I-V Characteristics- LDR

Non-ohmic, resistance not constant (decreases with light intensity)

Kilowatt-hour

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

Kirchoff's Second Law

In any circuit, the sum of the electromotive forces is equal to the sum of the p.ds around a closed loop

Progressive wave

Oscillation that travels through matter or a vacuum, and transfers energy from one place to another

Transverse wave

Oscillations are perpendicular to the direction of energy transfer, made up of peaks and troughs

Longitudinal wave

Oscillations are parallel to the direction of energy transfer, made up of compressions and rarefactions

Displacement

Distance from the equilibrium position in a particular direction

Amplitude

Maximum displacement from the equilibrium position

Wavelength

Minimum distance between two points in phase on adjacent waves

Period of oscillation

Time taken for one oscillation

Frequency

Number of wavelength passing a given point per unit time

Wavespeed

The distance traveled by the wave per unit time

Phase difference

Difference between displacements of particles along a wave, or the difference between the displacements of particles on different waves

In Phase

Particles oscillating perfectly in step with each other

Antiphase

Particles oscillating completely out of step with each other

Reflection

When a wave changes direction at a boundary between two different media, remaining in the original medium (speed, wavelength, frequency unchanged)

Law of reflection

the angle of incidence is equal to the angle of reflection

Refraction

When a wave changes direction as it changes speed when it passes from medium into another

If a wave slows down....

refracts towards the normal, wavelength decreases and frequency remains unchanged

If a wave speeds up...

refracts away from the normal, wavelength increases and frequency remains unchanged

Diffraction

When waves pass through a gap or travel around an obstacle they spread out (speed, wavelength, frequency unaffected)

Polarisation

Particles oscillate along one direction only, so the wave is confined to one plane

Intensity

Radiant power passing through a surface per unit area

Conditions for total internal reflection

1) Light must be travelling through a medium with a higher refractive index as it strikes a boundary with a lower refractive index 2) The angle at which light strikes the boundary must be above the critical angle

Principle of superpostion

When two waves meet at a point, the resultant displacement at that point is equal to the sum of the displacements of the individual waves

Interference

When two progressive waves pass through each other and superpose to produce a resultant wave with displacement equal to the sum of individual displacements of the two waves

Constructive interference

Two waves in phase superpose to produce a resultant wave with increased amplitude

Destructive interference

Two waves in anti phase superpose to produce a resultant wave with decreased amplitude

Coherence

Waves emitted from two sources with a constant phase difference and the same frequency

Formation of a stationary wave

Two progressive waves with same frequency travelling in opposite directions superpose

Node

Point on a stationary wave where displacement is zero

Antinode

A point on a stationary wave with maximum amplitude

Wavelength of stationary wave

Distance between two adjacent nodes or antinodes

Difference between progressive and stationary waves

No net transfer of energy in stationary waves whereas progressive waves transfer energy in the direction of the wave. In progressive waves phase changes whereas in stationary waves all parts of wave between nodes are in phase and in anti phase on different sides of node

Fundamental frequency

Minimum frequency of a stationary wave for a string

Harmonics in tubes open at one end

Only odd multiples of fundamental frequency

Photon

Quantum of electromagnetic energy

eV

Energy transferred to or from an electron when it moves through a p.d of 1V

Work function

Minimum energy required to free an electron from the surface of a metal

Photoelectric effect observations

1) Photoelectrons only emitted if incident radiation above threshold frequency 2) Above threshold frequency emission of photoelectrons is instantaneous 3) Only way to increase max. kinetic energy is to increase frequency, increasing intensity just increases number of photoelectrons emitted

How photons interact with electrons

one to one interaction

Capacitors

Electric components in which charge is separated

Capacitance

Charge stored per unit p.d

Electric field strength

Force experienced per unit positive charge

Coulomb's law

The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of their separation

Electric potential

Work done per unit positive charge in bringing a positive charge from infinity to a point

Faraday's law

the magnitude of the induced e.m.f is directly proportional to the rate of change of flux linkage

Lenz's law

the direction of the induced e.m.f or current is always such to oppose the change producing it

Transformer

Consists of a laminated iron core, a primary coil and secondary coil. An alternating current is supplied to the primary coil. This produces a varying magnetic flux in the soft iron core. The secondary coil is linked by this changing flux.

Step up transformer

More turns on secondary coil than primary coil

Step down transformer

More turns on primary coil than secondary coil

Isotopes

Nuclei of the same element that have the same number of protons, but different number of neutrons

Hadrons

Particles and antiparticles affected by the strong nuclear force

Leptons

Particles and antiparticles not affected by strong nuclear force

Baryons

Hadrons made with combination of three quarks

Mesons

Hadrons made with a combination of a quad and anti-quark

Alpha radiation

Positively charged particles with two protons and two neutrons

Beta radiation

Consists of fast moving electrons or positrons

Gamma radiation

Consists of high energy gamma photons, travel at speed of light and carry no charge

Random nature of nuclear decay

We cannot predict when a particular nucleus in a sample will decay or which one will decay next. Each nucleus has the chance of decaying per unit time

Spontaneous nature of nuclear decay

Decay of nuclei not affected by presence of other nuclei in the sample or external factors such as pressure

Half life

The average time it takes for half the number of active nuclei in a sample to decay

Activity

Rate at which nuclei decay or disintegrate

Decay constant

The probability of decay of an individual nucleus per unit time

Annihilation

When particle and its corresponding antiparticle meet their entire mass is transformed into energy in the form of two identical gamma photons

Pair production

Photon energy transformed into a particle and corresponding antiparticle

Binding energy

The minimum energy required to completely separate a nucleus into its constituent protons and neutrons

Mass defect

The difference between the mass of the completely separated nucleons and the mass of the nucleus

Induced nuclear fission

When a nucleus absorbs a slow moving, thermal neutron it becomes unstable and splits into smaller daughter nuclei and fast moving neutrons

Attenuation

the decrease in the intensity of electromagnetic radiation as it passes through matter

Simple scatter

X-ray photon doesn't have enough energy to remove electron from atom so is scattered elastically by the electron

Photoelectric effect

X-ray photon absorbed by electron in atom which allows electron to escape atom

Compton scattering

X-ray photon interacts with electron of atom, ejecting it from the atom and the photon is scattered with reduced energy

Acoustic impedance

The product of of the density of a substance and the speed of ultrasound in that substance.