Unit 1 Flashcards
What is an field?
A region of space where an object will experience a force without being touched
Where do electric fields exist?
Around charged particles and between parallel charged plates
In what direction do electric fields always point?
Positive to negative
For electric fields between parallel plates, field lines must:
Point from + to -
Be parallel and at right angles to the plates
Be equally spaced
Touch the plates
End bits usually ignored
How do electric fields work?
Each particle experiences an unbalanced force due to the electric field and so it will accelerate
What is the same as work energy?
Kinetic energy
What is potential difference?
The work energy gained by 1C of charge (due to an electric field)
What produces a magnetic field?
Moving charges or a flow of charges
What experiences a force in a magnetic field?
Moving charged particles
Right hand rule
For electrons
thumb-thrust
index-field
middle-electron flow (original)
Three main parts of particle accelerators
Electric fields-used to accelerate the particles across a potential difference (work done=QV=gain in kinetic energy)
Magnetic fields-used to change the direction of (or deflect) the charged particles
Collision chambers and detectors-where high energy collisions of charged particles take place to produce other particles which can be studied
Three types of particle accelerators
Linear accelerators
Cyclotron
Synchrotron
what is not needed in a Linear Accelerator?
No magnetic field needed as particles are accelerated in a straight line across more than one voltage
Linear accelerator- Why is it important that an ac supply is used?
To keep the particles accelerating in the correct direction. This is done by keeping the electric fields in the correct direction
Linear accelerators- Why do the ‘drift tubes’ get longer?
Because the particles keep getting faster so they can travel further in the same time. The time is the same to ensure they stay in sync with the alternating supply.
Cyclotrons-what fields?
Use both magnetic and electric fields to accelerate particles
When it reaches the outer edge of the cyclotron the particle beam is extracted and used in other experiments such as collisions
Cyclotron - why is an ac power supply used?
as the direction the particle enters the accelerating gap changes, the electric field also needs to change so it is always pointing in the correct direction to accelerate the particle
How do synchrotrons work?
They use a magnetic field which gets stronger as the particles get faster, allowing the radius of the path to stay constant
They have accelerating cavities at many different points around the ring to increase the speed of the particles they go around
What are the two main groups of fundamental particles?
Fermions
Bosons
What are Fermions?
Particles of matter
Eg quarks and leptons (fundamental particles)
(hadrons)
quarks-up, down, strange, charm, top, bottom
leptons-electron, muon and tau, together with their neutrinos
How was the existence of quarks proved?
By high-energy collisions between electrons and nucleons (neutrons and protons) in particle accelerators
What are bosons?
Force exchange particles
examples of bosons
Photon-carrier of electromagnetic force
Gluon-carrier of strong nuclear force
W boson-carrier of the weak nuclear force (can have - or + charge: antiparticles of each other)
Z boson-carrier of the weak nuclear force (has no charge - is its own particle)
What are Hadrons?
composite particles made of quarks?
Examples of hadrons
Baryon-3 quarks or 3 antiquarks
Mesons-made of a quark and antiquark
Anti matter
Each particle in the standard model has a corresponding anti-particle
Identical in every way apart from electric charge
Bar above symbol
How do we know anti particles exist?
When a particle and its antiparticle meet they perfectly annihilate each other
Their combined mass is converted into energy
The energy produced from the annihilation is our evidence for the existence of antimatter
What was the first evidence for the neutrino?
Beta decay
when a nucleus decays in beta minus decay, an anti-electron neutrino is also given out alongside the electron
1ev=
1.6x10^-19J (energy an electron gains when accelerated through 1V
W=QV
Isotopes
Atoms of the same element but with a different number of neutrons in the nucleus
examples of elements which can undergo spontaneous fission
Plutonium 239
Uranium 235
Radioactive decay
An element can decay to become more stable by alpha, beta or gamma emission
Induced nuclear fission
The splitting of an unstable nucleus from neutron capture into 2 smaller parts (releasing energy and more neutrons)
Nuclear fusion
Two nuclei joining together to form a larger nucleus (releasing energy and sometimes more particles)
Why is energy released in a fusion or fission reaction?
Mass is lost and converted into energy
What does E=mc^2 represent?
Equivalence of mass and energy
mass and energy are interchangeable
In many nuclear reactions, there is a very small decrease in the overall mass of the particles involved
This lost mass is converted into energy
Why is containing nuclear fusion challenging?
Due to extremely high temperatures required
H2 used (which is now plasma) is too hot to come into contact with any material
Solution for containment of nuclear fusion?
To contain H2 plasma within toroidal magnetic field. The device that does this is called a tokamak
Issues and challenges with fusion reactors
Energy-huge amounts of energy need to heat H2 to plasma hot enough to undergo fusion
Containment-very large magnetic fields needed to keep the plasma (full of charged particles) inside the tokamak
Cooling-Fusion produces very hot neutrons which are not contained by the magnetic field. These heat up the walls of the reactor which will need to be cooled. This is where the extracted energy comes from
What is irradiance?
The irradiance of a source of radiation is the power (energy per second) incident on a unit area of surface (1m^2)
sphere area
4pir^2
What is irradiance directly and indirectly proportional to?
directly- 1/d^2
indirectly- d^2
charging the gold leaf electroscope
charging by induction-electroscope is charged oppositely to charge rod
bring rod->touch->take finger away->take rod away
don’t let rod touch
Conclusions of gold leaf electroscope practical
only uv light has an effect
only negatively charged electroscopes can be discharged
the effect is only apparent in some metals
explanation of the photo-electric effect
energy needs to be passed onto an electron to make it escape. Light is regarded as a stream of ‘particles’ called photons. Which carry ‘packets’ of energy depending on the frequency of the light
each electron can absorb a single photon
If a photon has a high enough energy, an electron is freed. If not, then it won’t be freed
What is the photoelectric effect evidence for?
The particle model of light. This shows that each photon contains a fixed (discrete) amount of energy. Each individual photon will remove one electron.
E=hf
frequency
planck’s constant-6.63x10^-34J
energy of photon
each photon will have an energy dependent on its frequency
What is the work function
E^o
Each electron has a minimum energy to ‘just’ free it.
This means that each photon must have a minimum energy and minimum frequency associated with it, called the threshold frequency
E^o = hf^o
What if a photon energy e=hf where f>f^o?
Then E^o of that energy frees it and the rest gives the e electron kinetic energy
Ek=hf-hfo
kinetic energy of electron
photon energy
work function (of electron)
What effect does increasing the irradiance of a light source have on the maximum kinetic energy gained by the electrons when they leave the surface of the metal
no effect
When are two waves coherent?
If they they are produced by the same source.
They will have the exact same frequency,wavelength, velocity, and they have a constant phase relationship
Constructive interference
If 2 coherent waves meet completely in phase with each other
Amplitude gets louder
Peak to peak
trough to trough
Destructive interference
If 2 coherent waves meet completely out of phase with each other by half a wavelength
Amplitude is zero
peak to trough
What is interference?
When 2 waves combine to make a resultant wave. This wave can have a smaller, larger or same amplitude as original waves
Path difference
Difference in distance to one point by 2 waves
If equal to a whole number of wavelengths—>waves will meet in phase and interfere constructively
If equal to odd number of half wavelengths—>waves will meet completely out of phase and interferendestructively
how to find d in dsinx
d is difference between gratings
you need to divide 1mm by the number of lines per mm to find ‘d’
Prism v grating patterns (white light)
Prism
-spectrum produced by refraction
-a single spectrum produced
-short wavelengths refract most
Grating
-spectrum produced by interference
-central white fringe
-spectra occur in pairs either side of central fringe
-long wavelengths in the outer edge
Bohr model of the atom
Neil Bohr proposed that
1-the nucleus contains protons and neutrons and is positively charged
2-electrons orbit the nucleus in discrete energy levels
energy levels are of an exact value-electrons can only be in one of these allowed energy levels
Absorption spectrum
produced by the gases in the sun’s atmosphere
provides evidence for the elements that exist in the sun’s atmosphere
Black lines of colour missing
cool gas absorbs the same lines of light
Emission line spectrum
Lines of colour on black
hot gas emits lines of light
The coloured lines correspond to the colour of wavelength (wavelength of photon) that an excited electron emits as it falls to a lower energy level.
Different transitions produce different lines/frequency of photon
What is the ionisation level?
The level where the electrons have 0 potential energy due to the nucleus
What does it mean if a line in an emission line spectrum is brighter than the others?
More photons of that frequency/wavelength are given off
This means more electron transitions must have taken place between those energy levels and are more common
what is sinxair directly proportional to?
sinxperspex
What is the refractive index?
It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium (n)
What happens to the refractive index of a medium when the frequency of light increases?
Increases
What is the critical angle?
The angle of incidence which produces an angle of refraction of exactly 90 degrees (theta c)
When does total internal refraction occur?
If light meets a boundary at an angle greater than the critical angle total refraction occurs and the light does not refract out of the block.
What is a continuous spectrum?
consists of all visible wavelengths
produced by sunlight, filament lamps etc (‘white’ light sources)
What is a line spectrum?
produced by elements (in gas form)
consists of very specific wavelengths of light which are emitted or absorbed by the element
How can you increase the spacing between maxima on the interference pattern? Why would you want to do this?
increasing wavelength
decreasing d (smaller slits, less distance between slits)
increasing d(distance of grating to screen)
(smaller percentage uncertainty as angles are bigger)
How is an line emission spectrum produced?
If an electron is in an excited state, it can return to a lower energy level. When it does this, it emits a photon. Different transitions produce different lines/frequencies (of photons)
How is a line absorption spectrum produced?
When photons of light pass through a gas, the photons with the same energy as the energy gaps in the atoms can be absorbed. This causes an absorption spectra, as shown below.
Because the energy levels are the same, the lines in the emission spectra of an element are in the same position as the lines in the absorption spectra of the same element, as shown below.
Evidence for the wave-model of light?
double-slit experiment
maxima and minima are produced due to interference
if grains of sand were shot through then they would end up in a line in the same place (same angle)
photoelectric effect
frequency of light determines energy of photons and therefore kinetic energy of emitted electrons
Evidence for the particle-model of light?
photoelectric effect
How to improve irradiance experiment?
-smaller lamp will be more like a point source (needs to be a point source to prove inverse square law)
-black cloth on bench to reduce reflections
why may a wire have a large diameter?
-reduce resistance
-prevent overheating/prevent wires melting
What are absorption lines (Fraunhofer lines) in the spectrum of sunlight evidence for?
the composition of the Sun’s outer atmosphere
Bohr model of the atom
How can you improve the interference grating experiment?
-repeat measurements
-use additional gratings
-move screen further away
-use second order maxima to determine angle
-measure angle from first order to first order
