Topic E Definitions

Definitions for topics E1 - E5, (only 4,5 ratings), bold parts important for each definition, some extra notes missing, SL & HL

Nuclear density

… is constant

Angular momentum (for orbiting…)

the product of mass, speed and orbit radius of a particle

Bohr model of hydrogen atom

Said that angular momentum is quantised

hence electron could only take certain orbits

this led to discrete energy levels in the atom

Theory matched with atomic hydrogen spectra very well

(for atomic hydrogen only)

Deviations from Rutherford scattering at high energies

Because alpha particles had got within range of the strong nuclear force

Rutherford scattering based on electrostatic repulsion only

Explanation of absorption spectrum

Continous spectrum shines through gas

Gas atoms have discrete energy levels

So only discrete energy transitions available for electrons in atom

When incoming photons have exactly the energy corresponding to a transition they are absorbed by electron, as it goes to higher energy level

Electron then de-excites, re-emitting photon but these photons go in random directions so absorption spectrum observed

Explanation of emission spectrum

(Isolated) atoms have discrete energy levels, so only discrete transitions available between them when electron de-excited (goes from high energy level to low energy level) a photon with the corresponding energy is emitted.

Energy is related to wavelength/frequency hence discrete (line) emission spectrum

Geiger-Marsden experiment

also known as Rutherford Alpha Particle Scattering or Gold Foil Experimet

What:
alpha particles fired in vacuum at thin gold foil
deflection angles detected

Expected:
v minor deviations would reveal fine structure of plum-pudding model
(also OK to say would be undeflected, which is the simplified version.)

Actual:
most undeflected
some deflected through small-medium angles (more than expected)
a few deflected through very large angles (>90degrees)

Conclusion:
most undeflected: most of atom was empty space
some deflected through small angles - atom must have dense positive nucleus
some deflected through v large angles - dense positive nucleus must contain most of mass of atom.

Rutherford also hypothesised that electrons would orbit this dense, massive, positive nucleus.
(Understood weakness that electrons should, according to classical physics, radiate energy and inspiral into the nucleus.)

Nucleon

A proton or neutron

Nuclide

A particular type of nucleus with a certain number of protons and neutrons

Discrete energy

That can take a set of specific values as opposed to a continuous range of values

Photoelectric effect (explanation)

Observation: does not occur with low-frequency radiation, whatever the intensity.
Classical: high-intensity radiation should have transferred enough energy to release elections
Quantum: one photon absorbed by one electron. photon energy below work function so no emission.

Observation: max KE independent of intensity of radiation
Classical: more intense radiation should give electrons greater KE
Quantum: one photon absorbed by one electron. greater intensity does not change this. Max KE is excess photon energy after work function overcome.

Observation: max KE depends on frequency of radiation.
Classical: should not affect max KE
Quantum: one photon absorbed by one electron. Higher frequency photons have more energy, so more excess energy for electron KE.

Observation: photoelectric effect occurs immediately, even with low-intenstiy radiation.
Classical: energy should need time to build up, as spread across many electrons. should be a delay.
Quantum: one photon absorbed by one electron. So effect can start instantaneously.

Work functions (φ)

Minimum photon energy needed to eject electrons from surface of a metal

OR energy required to remove the least tightly bound electron from a metal surface

Photoelectric effect

The emission of electrons from a metal surface when electromagnetic radiation of high enough frequency falls on the surface

de Broglie Hypothesis

All particles can behave like waves, whose wavelength is given by λ = h/p (where h is Planck’s constant and p is the momentum of the particle)

Matter waves

All moving particles have a “matter wave” associated with them whose wavelength is the de Broglie wavelength

Evidence for matter waves

Electron diffraction

Threshold frequency (f0)

Minimum frequency of light needed to eject electrons from a metal surface

Compton scattering

experiments in which X-rays scatter from collisions with electrons.

additional evidence (with photoelectric effect) for particle nature of light.

scattered X-rays have increased wavelength. therefore reduced momentum by p=h/lambda. consistent with CoM calculations.

Evidence for nuclear energy levels

Discrete energy spectra of alpha and gamma radiation

The evidence for the strong nuclear force

Deviations from Rutherford scattering

Role of ratio of neutrons to protons in a nuclide

contributes to stability of nuclides

N/Z=1approx for small nuclides; up to N/Z=1.5 for larger nuclides

electrostatic force repuslive between protons.
strong nuclear force attractive between all nucleons.
but short range.
hence need more neutrons as far side of nucleus becomes out of range for any neutron.

The approximate constancy of binding energy curve above a nucleon number of 60

above this number, additional nucleons have about same number of closest neighbours.
- for strong nuclear force, this will be the maximum range of force.
- for coulomb repulsion, will be dominated by nearby protons.
so additional nucleons do not significantly change average binding energy per nucleon.

Decay constant

Approximates the probability per unit time for a nucleus to decay (but only if lambda x t sufficiently small)

Activity

Number of radioactive disintegrations (decays) per unit time

Alpha particle (α)

Helium nucleus (2 protons + 2 neutrons) emitted from decaying nucleus

Radioactive half-life

the time taken for ½ the number of radioactive nuclei in sample to decay
OR
the time taken for the activity of a sample to decrease to ½ its initial value

Atomic mass unit

A unit of mass equal to 1/12 of the mass of a neutral atom of carbon-12

Nuclear binding energy

the (minimum) energy required to completely separate the nucleons of a nucleus OR the energy released when a nucleus is assembled from its separate nucleons

Mass defect

Difference between the mass of the nucleus and the sum of the masses of its individual nucleons (mass equivalen of binding energy)

Isotopes

Nuclei of the same element containing the same number of protons but different numbers of neutrons

Beta particle

An electron

Beta-minus decay

A decay producing an electron and an anti-neutrino

Beta-plus decay

A decay producing a positron and a neutrino

Range of strong force

~10-15m (might just need to know ‘short-range’)

Range is also finite

Strong nuclear force acts on….

Nucleons

I.e. protons & neutrons

Not electrons

Alpha particle absorption characteristics

Absorbed by: piece of paper, few cms of air

Beta particle absorption characteristics

Absorbed by: few mms of aluminium, 10s of metres of air

Gamma ray absorption characteristics

Absorbed by: few cms of lead, 100s of kms of air

Random

Cannot be predicted

Spontaneous

Not caused by anything (not induced)

Unified atomic mass unit

1/12th of the mass of a carbon-12 atom

Binding energy per nucleon

Energy released per nucleon when a nuclide is assembled from its individual nucleons (OR: energy required per nucleon to separate a nucleus into its individual nucleons)

Moderator

Slows neutrons to increase chance of fission

Therefore absorbs energy, energy must be extracted from moderator

Nuclear fission

A heavy nucleus splits into two smaller nuclei

Control rods

Absorb neutrons; raised/lowered to control rate of reaction

OR a rod that regulates the rate of energy release in a nuclear fission reactor by regulating the absorption of neutrons

Fuel rods

Containers of nuclear fuels in a nuclear fission reactor

Heat exchanger (in nuclear reactor)

System that extracts thermal energy from the moderator of a nuclear reactor

OR This allows the nuclear reactions to occur in a place that is sealed off from the rest of the environment. Reactions increase temperature in the core and this thermal energy is transferred to water and the steam that is produced turns the turbines.

Astronomical unit

Average earth-sun distance

Parsec

Distance at which 1AU subtends an angle of 1 arc second

Determining distance in parsecs

Determine maximum angle between apparent position of star relative to background stars 6 monzhs apart. Parallax angle is half that angle

Stars have a stable radius when:

Outward radiation pressure in equilibrium with inward gravitational pressure

(this determines the size of the star, when equilibrium is reached)

Light year

Distance light travels in a vacuum in one year

Main sequence star

• Stars are on main sequence while they fuse hydrogen into helium in their cores

• spend most of their lives on main sequence

Lifetime on main sequence

High mass stars spend a much shorter amount of time on main sequence than low mass stars

Instability strip

Region of HR diagram in which stars are unstable and pulsate

Red giant

Stage in life - cycle of mid-mass star after main sequence

Large, cool star

Helium (and heavier) fusion

Red supergiant

Stage in life-cycle of high-mass star after main sequence

Very large, cool star

Helium (and heavier) fusion begins

White dwarf

Small, hot star in which fusion has stopped

Final stage of mid-mass stars

Cooling core of star after planetary nebula ejected

Gravitational pressure balanced by electron degeneracy pressure (rather than radiation pressure)

(First 2 of these for 1-mark definition)

Nuclear fusion

Two light nuclei join to form a heavier nuclei

Nuclear fusion in stars

A source of energy for stars

Needs very high temperature and pressure (to overcome Coulomb repulsion)