ch18 Flashcards

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1
Q

Rutherford experiment disproved which model for the atom?

describe the model

A

The Thompson model or plum pudding model

atoms were a plum pudding with electrons embedded throughout a positive sphere

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2
Q

how do you investigate the structure of an atom

A

with scattering experiments

particles are accelerated to a high energy and directed at a target

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3
Q

Describe Rutherford’s experiment and his results

A

A stream of alpha particles from a radioactive source was fired at a very thin gold foil.

The angles at which the particles were scattered were recorded

some passed straight through (so most atom is empty space)

but a few of the alpha particles bounced right back from the foil being scattered at angles greater than 90 degrees

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4
Q

What could Rutherford conclude after his experiment?

A

The atom is mainly empty space.

the core must be massive on an atomic scale to deflect alpha particles through large angles, but its much smaller than an atom as very few particles deflected through more than 90 degrees

alpha particles deflected due to electric repulsion between positively charged alpha particles and positive core in gold atoms.

The centre of the atom must have a large, positive charge. Rutherford named this the nucleus.

The nucleus must be tiny, but massive (mass).

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5
Q

1 in how many alpha particles were turned back and what does this suggest about the size of a nucleus

diameter of atom

diameter of nucleus

A

1 in 10000 turned around so nucleus is 10 times smaller than the atom

diameter of atom is 10-10m

diameter of nucleus therefore is 10-14 m

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6
Q

what happens to the deflection angle if the alpha particles are slowed done

how is scattering affected if the nuclei has less electric charge

relationship between scatter angle and inverse square law

A

if alpha particles are slowed down more would be deflected at greater angles since the nucleus would be able to turn them back more easily

nuclei of smaller electric charge scatter alpha particles less strongly

the pattern of number of alpha particles scattered at different angles fits the pattern expected from the inverse square law for electric repulsion

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7
Q
A

alpha particles are highly ionising and so will lose their energy over a short distance ionising air particles

by evacuating the apparatus (removing air particles) you cen minimise collisions and therefore get an accurate measure of the deflection angles

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8
Q

kinetic energy of alpha particle at its closest approach

A

0 kinetic energy

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9
Q

what is electrical potential energy equal to at a large distance from the nucleus

A

at a large distance from the nucleus, alpha particle electrical potential energy is equal to its initial kinetic energy

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10
Q

equation for initial kinetic energy

charge on alpha particle

charge on gold particle

A

initial kinetic energy = electrical potential energy = kQq / r

alpha: 2e = 2 *(1.6*10-19)
gold: 79e = 79 * (1.6*10-19)

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11
Q

force that a particle experiences near a nucleus

A

F = KQq / r²

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12
Q

What did Rutherford discover about charge at the centre of the atom?

A

Some of the alpha particles were deflected through large angles, so the centre of the atom must have a large positive charge to repel them.

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13
Q

What value do you need when you are estimating the distance of closest approach of an alpha particle that has been fired at a gold nucleus?

A

The alpha particles initial kinetic energy

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14
Q
A
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15
Q

What value do you need to find the charge of the nucleus?

A

The atom‘s proton number, Z

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16
Q

how are electrons similar to alpha particles

A

electrons are scattered by the nucleus in the same way alpha particles are, but the force is attractive this time.

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17
Q

how to produce high energy electrons

A

using particle accelerators

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18
Q

one application of particle accelerators

A

dental x-ray tubes are particle accelerators

electrons boiled off heated wire and accelerated towards positive target

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19
Q

describe the change in energy for electrons in dental x-ray tubes

rearrange to give velocity

A

electrons lose electrical potential energy (qV) and gain kinetic energy (1/2mv²)

qV = 1/2mv²

can rearrange for v (velocity)

V = accel voltage

q = charge

m = mass of electron

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20
Q

limitations of momentum equation

A

accurate at low speeds but not at high speeds

when v << c

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21
Q

einstein momentum equation

A

momentum = relativistic factor * mass * velocity

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22
Q

total energy of free particle

A

kinetic energy + rest energy

= relativistic factor * m * c²

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23
Q

equation for energy when particle at rest

A

relativistic factor is 1 at rest so

Erest = mc²

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24
Q

what is rest energy equivalent to

A

mass

Erest = m

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25
Q

equation to calc relativistic factor using energy

A

rel factor = Etotal / Erest

ymc² / mc²

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26
Q

what is a linear accelerator and how does it work

A

a long straight tube containing electrodes which change charge signs

electrons are acclerated between a pair of electrodes which have an alternating current applied to them

its timed so that the electron is always attracted to the next electrode and repelled from the previous one

length of electrodes increases because electrons are getting faster so they must stay in the tube for the same amount of time so that electrodes pd doesn’t change too early

the max speed of the electrons is only limited by the length of the accelerator, so they can approach the speed of light

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27
Q

momentum approximation equation

wavelength equation using this

A

when v is similar to c we can say momentum = y * m * v goes to momentum = y * m * c

Etotal = y * m * c²

so

p = Etotal / c

λ = h / p

λ = h * c / Etotal

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28
Q

sin theta equation involving nuclear diameter

A

sin theta = 1.22(wavelength) / nuclear diameter

can get wavelength from hc / E

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29
Q

what are quarks

A

quarks are the building blocks for hadrons (protons and neutrons)

30
Q

how are quarks used to make protons and neutrons

A

to make the nucleons you need the 2 types of quark, the up quark (u) and the down quark (d)

the up quark has charge 2/3

the down quark has charge -1/3

so proton = up(2/3) + up (2/3) + down (-1/3) = +1e

neutron = ddu = -1/3 - 1/3 + 2/3

31
Q

how do you use quarks for antiprotons and antineutrons

A

an antiproton is a p with a line above it

antineutron is a n with a line above it

to make these you need antiquarks, quarks with the opposite charge

these are the anti-up (-2/3) and anti-down (1/3)

pic shows how to make them

32
Q

what holds quarks together inside a particle and how does it work

A

gluons are particles that are very strong and hold quarks together

quarks attract one another by exchanging gluons

gluons hold protons and neutrons in one piece keeping the nucleus together

this interaction is known as the strong interaction

33
Q

antiparticle of electron

how do antimatter differ from matter

A

the antiparticle equivalent of electron is the positron

all antiparticles have the same mass as particles just the opposite charge

34
Q

2 ways to represent an alpha particle

A

42α

42He

35
Q

what are leptons

what do they do

example

A

leptons are fundamental particles that interact through a nuclear force called the weak interaction

this weak interaction is reponsible for the beta decay

electrons and neutrinos are leptons

36
Q

what are hadrons

what do they do

examples

A

hadrons are composite particles made up of quarks

they interact through the strong interaction

protons and neutrons are hadrons

37
Q

what are neutrinos

A

neutrinos are particles that you can assume to have 0 charge and 0 mass

38
Q

how are positrons emitted from a nucleus

what happens after its been emitted

what happens to the energy

A

in a process called beta decay (B+)

a positron cannot exist in the world, almost instantly it encounters the outer electron of another particle and the 2 particles annihilate each other

matter is destroyed, but energy can’t be so all the mass of the particles gets converted to energy and is carried away by gamma rays travelling in opposite directions

39
Q

what is conserved during an annihilation event

A

electric charge

linear and angular momentum

energy

lepton number

40
Q

lepton number for electron and positron

A

lepton number for electron: 1

lepton number for positron: -1

41
Q

what is pair creation

what happens in it

requirements

A

the opposite of annihilation

creation of equal amounts of positron and electron by gamma ray photon of sufficient energy near a massive nucleus

gamma ray photon must have energy equal or more to 2mc² (total rest energy of electron and positron) which is 1.022MeV

42
Q

why do you need a nucleus near a gamma photon to create an electron positron pair

A

a 1.022MeV gamma photon has enough energy to create an electron-positron pair but it can’t create this in free space

this is because the photon has momentum p = E / c and momentum must be conserved

pair creation can occur near a nucleus which then carries away some momentum so that both energy and momentum is conserved

lepton number and electric charge are conserved because the remain at 0

43
Q

how to find max lambda of gamma ray photon after electron and positron made from single gamma ray photon

A

E = 2mc²

then E = hc / lambda

rearrange and then calc for lambda

44
Q

what does a nucleus do when it has too many neutrons

what is this process called

A

the nucleus has too many neutrons so it can decay by emitting an electron

neutrins are unstable and decay into a proton

this is beta minus decay (B-)

n -> p + e + antineutrino

45
Q

what is an issue with beta minus decay and what did it lead to the discovery of

use the decay of strontium 90 to ytrrium 90

equation

A

9038Sr -> 9039Y + 0-1e

neutron turns into proton to keep charge conserved and all numbers conserved

electron (lepton) on right but no lepton (electron) on left

difference in rest energy between Sr and Y but every beta particle doesn’t have same energy as the difference

so it was suggested that theres another particle emitted carrying the extra energy, no charge, small mass and weak interactions

this is the neutrino (00v)

so when the Sr decays to Y a beta particle is emitted along with an antineutrino (same as neutrino with a line above v)

equation: 9038Sr -> 9039Y + 0-1e + (00v)line above v

46
Q

why is beta- decay not due to the strong interaction

A

n -> p + e- + antineutrino

electrons and v are leptons

leptons are not affected by strong interaction, the decay can’t be due to the strong interaction

47
Q

how are neon light signs evidence that electrons in atoms have discrete energy levels

A

their light comes in sharp discrete spectral lines

48
Q

how to find energy of photons given energy of 2 discrete energy levels

A

the energies of the photons correspond to the difference in energy levels in the atom

49
Q

describe the franck and hertz experiment

A

F and H experiment gives evidence for discrete energy levels

tube filled with gas atoms that collide with electrons as they travel along the tube

filament on left of tube is heated so electrons ‘boiled off’ and accelerated towards the wire grid at a certain positive potential. the anode will be at a slightly lower potential than the grid

the difference in potential (e.g. 0.2V) is the minimum kinetic energy in electronvolts that electrons need to reach the anode (0.2eV)

the current registered on the sensitive ammeter is a measure of the number of electrons passing through it each second

50
Q

fh exp:

describe and explain the graph and what it gives evidence for

A

describe: increasing accelerating pd increases current but at Vc,2Vc and 3Vc current falls then rises
exp: at low accelerating pds the electrons from the filament make elastic collisions with gas atoms bouncing off without loss of energy

increasing pd increases the current registered on sensitive anode

but at Vc, accel electrons have enough energy to knock an electron in the gas atom to a higher energy level, so some of the accel electron energy is given to the gas atom in an inelastic collision. this means some electrons don’t have enough energy to reach the anode so current falls

at 2 and 3Vc accel electrons can give energy to 2 or 3 atoms

drops in current show inelastic collisions can only happen at specific electron energies. this shows that there are specific energy levels in the atom which the electrons are fixed to

51
Q

are electrons only allowed to have cerain energies

A

not really, because electrons have de broglie wavelengths associated with them

atoms can be thought of having a potential energy well in which electrons are bound

52
Q

explain the standing wave for electrons with equation for energy of electrons

A

as electrons are trapped in the potential energy well of the positive nucleus

standing waves for electrons can form

de broglie wavelengths are limited by the size of the box

we know p = h / lambda

energy of electron is small so use non relativistic expression Ek = 1/2mv² = p² / 2m

Ek = h² / 2mλ²

53
Q

how does the size of the box trapping the electron affect wavelength and energy and momentum

how does it affect the fixed energy values

A

the smaller the box trapping the electron, the smaller the wavelength and so the greater the momentum and kinetic energy

the size of the box determines the wavelength and so the allowed energy values

54
Q

hydrogen atom model for standing waves

A

applying model to hydrogen with a single electron with no potential wells but instead hard walls a fixed distance apart

55
Q

what does energy of electrons depend on in the hydrogen atom standing wave model

what does this produce

A

the width of the box and the number n of half-wave loops in the standing waves

produces a number of alowed energy levels labelled by a quantum number n at the ground state

56
Q

charge, and lepton number of

proton, antiproton

neutron, antineutron

electron, positron

neutrino, antineutrino

A
57
Q

how much energy do electrons need to escape the potential well in the wave model of the atom

A

has to have a total energy (Ek + Ep) of greater than 0

58
Q

using the de broglie wavelength equation how many wavelengths are there in n = 1 and 2 of the electron standing wave model

A

lambda = h / p = h / mv

n = 1, half a wavelength between walls. lambda = 2d

n = 2, one complete wavelength between walls, lambda = d

n = 3, 2/3d

59
Q
A

force at right angles to direction of motion

60
Q

describe the process for annihilation in Positron Emission Tomography and then for brain tumour detecting

A

isotope of O2 gives off positron which interacts with outer electron of another atom, both destroyed in annihilation event, energy taken by gamma ray photons

in brain tumours isotope of O2 injected into blood that gives out positrons while travelling throughout the body but a tumour will have a rich supply of blood so lots of decay there gives lots of gamma ray photons which leave head and are detected

61
Q

why does the graph look like this

A

electrons can show wave like properties

the red line is if electrons act like the particles that they are

the blue line is the actual scattering due to electron diffraction showing the ‘kinks’ in the curve

electrons follow rutherford’s scattering but the curve superimposed on it is the diffraction pattern because electrons act in a wave like manner

62
Q

if given graph similar to this how do I work out nuclear diameter of particles

A

look for lowest kink on diagram and read at what angle that is on the graph

nuclear diameter = 1.22lambda / sin theta

lambda = hc / E

63
Q

describe bubble chambers and what they do

A

in bubble chambers fast particles ionise water vapour creating a trail

there are 2 trails due to the charge on the particle

one due to +ve charge and the other -ve charge

inwards spiralling because they’re losing energy

64
Q

kinetic energy for electron using de broglie wavelength

A
65
Q

as only certain wavelengths can fit in an electron how is ke affected

A

only certain wavelengths of electrons can fit inside atoms at a certain distance from nucleus ke is also limited to certain energies

66
Q

what does En = -13.6eV / n show

find out En when n = 1 and 2

A

this shows the energy required for energy levels for hydrogen where n is the energy level so n = 1 at ground state

when n = 1, En = -13.6eV / 1² = -13.6eV

when n = 2, En = -13.6eV / 2² = -3.4eV

67
Q

what happens when an electron goes from a higher energy orbit to a lower energy orbit

A

electric potential energy gets more negative

the change in energy is the energy of the photon emitted

68
Q

find the wavelength of the photon emitted when an electron falls from n = 3 to 1

A

n = 3 = -1.5eV

n = 1 = -13.6eV

change in energy = 12.5eV * 1.6 * 10-19 = 1.936 * 10-18J photon energy E

E = hc / lambda

lambda = hc / E = 1 * 10-7m

visible light = 400Nm to 700Nm

this is below it so its in the UV range

this is correct as drop from n = 3 to n = 1 is in the lyman series

69
Q

why is an electron better for diffraction compared to alpha particles

A

electrons aren’t affected by strong nuclear force so they can touch the nucleus and turn around whereas alpha particles would be captured by gluons and absorbed into the nucleus

70
Q

quick way to calculate velocity with a known value of relativistic factor

A

v / c = ROOT(1 - 1 / (rel factor)²)