Physics Y13 - Looking Inside The Atom

Front 1

How did JJ Thomson first suggest the structure of an atom might be like?

Back 1

He suggested electrons may be embedded throughout a positive sphere - this is nicknamed the "plum pudding" model.

Front 2

Describe Rutherford's experiment to investigate whether Thomson's model was accurate using alpha radiation:

Back 2

A narrow beam of alpha particles was fired at thin gold foil, and this was deflected onto a zinv sulfide screen viewed with a microscope to count the alpha particles at certain angles.

Front 3

What would be the expected result if the Thomson model was accurate?

Back 3

The alpha particles would shoot through the foil with only small deflection.

Front 4

What was the actual result?

Back 4

Many particles were scattered at various angles, with a small proportion deflected at angles more than 90 degrees.

Front 5

What does this suggest?

Back 5

Rutherford suggested that the core of the atom must be relatively massive to deflect alpha particles at large angles and the core of the gold atoms must be very small in order for so few of them to be deflected back.

The alpha particles were deflected because of electrical repulsion between the positive core and the positive alpha particles.

Front 6

How might the size of a nucleus be estimated from an experiment like this?

Back 6

If the alpha particle is fired with 5MeV of kinetic energy, head on at the nucleus, at its closest position to the nucleus its kinetic energy is 0 and its electric potential energy is equal to the kinetic energy a large distance from the nucleus.

The electric potential energy is equal to kQq/r,

where r is the distance. Therefore r can be used as the upper bound for the size of the nucleus.

Front 7

When accelerating electrons through a voltage V, what are the accurate equations to find the speed of the electron at low speeds?

Back 7

Lost electric potential energy = kinetic energy

qV = 1/2mv^2

Note that 1/2mv^2 is no longer accurate at high speeds due to relativistic effects.

Front 8

What are the accurate equations for kinetic energy, momentum, and total energy/rest energy taking into account relativistic effects using λ.

Back 8

p = λmv

ke = ( λ-1)mc^2

total energy/rest energy = λ

Front 9

Why does electron scattering off a nucleus have a minimum point in the curve of number of electrons against angle?

Back 9

This is due to the minima and maxima of diffraction, due to the de Broglie wavelength of an electron being large enough to experience diffraction when fired at a nucleus.

Front 10

What is the equation of this first minimum in terms of the angle and diameter of the nucleus?

Back 10

where sin(a) = 1.22 λ/d

Front 11

As the electrons have to be accelerated to very high speeds for this diffraction to occur, what is a good approximation for the momentum and therefore the wavelength of the electrons?

Back 11

p = E/c and therefore λ = hc/E

as v is close to c so kinetic energy is approx mc^2

Front 12

Name the four type of quark and their charge:

Back 12

up quark (u) - charge 2/3e

down quark (d) - charge -1/3e

anti up quark (u dash) - charge -2/3e

anti down quark(d dash) - charge -1/3e

Front 13

Describe the composition of protons and neutrons and their antimatter equivalent:

Back 13

Proton - uud

Neutron - udd

Antiproton and antineutron have the same composition but with antiquark equivalents.

Front 14

Describe the composition of delta particles:

Back 14

The delta particles are composed of three of the same quark:

Δ- particle - ddd - charge -1

Δ ++ particle - uuu - charge +2

Front 15

Describe the composition of mesons:

Back 15

Mesons are composed of one quark and one of the opposite antiquark:

π + = up quark and antidown - charge +1

π - = down quark and antiup - charge -1

Front 16

Describe the process used to prove the existence of quarks using electron acceleration:

Back 16

Electronsc an be accelerated to high energies ( around 20GeV) and be used to probe nucleons. The electrons are scattered and a jet of new particles, mainly mesons, are created. The scatter angle suggested that there were three quarks that made up nucleons with charges of a fraction of e.

Front 17

How do quarks interact with one another, and what is this force called?

Back 17

Quarks interact by exchanging particles known as gluons which cause the force known as the strong interaction.

Front 18

What is antimatter?

Back 18

Antimatter are made up of antiparticles, there is one for each subatomic particle. They have the opposite charge to their matter equivalent, but the same mass.

Front 19

What two groups are particles and antiparticles divided into?

Back 19

Leptons - These are fundamental particles that interact through the weak interaction. Leptons include: electrons and neutrinos.

Hadrons - Composite particles made up of quarks that interact using the strong interaction. Hadrons include mesons, protons, neutrons.

Front 20

What happens when a particle and its antiparticle equivalent collide?

Back 20

The particles are destroyed, producing a pair gamma photons in their place.

Front 21

What is the energy and the minimum energy that these photons have?

Back 21

Their energy is equal to the rest energy of the particles plus the kinetic energy of the particles.

Therefore the minimum energy is the rest energy when kinetic energy is 0: 2mc^2

Front 22

How are a particle - antiparticle pair created? (and how is momentum conserved)

Back 22

A gamma ray photon of sufficient energy can create a pair near a massive nucleus. Momentum is conserved due to the nucleus moving in the opposite direction.

Front 23

What is the process of beta decay?

How is lepton number conserved during this decay?

Back 23

In beta- decay a neutron decays into a proton and an electron and the high energy electron B- particle is emitted from the nucleus. However, since an electron is a lepton, an anti electron neutrino is emitted with a lepton no of -1 to conserve this.

With B+ decay, a positron is emitted after a proton decays into a positron and neutron. A neutrino with a positive lepton number is emitted to conserve lepton number.

Front 24

What are the characteristics of a neutrino?

Back 24

Neutrinos have no charge, a very small mass and interact little with other matter.

Front 25

As well as lepton number, how else did scientists conclude that a neutrino must be emitted in beta decay?

Back 25

Due to the differences between the mass of neutron and proton, they expected the kinetic energy of every electron emitted to be the same as it is carrying this missing energy each time. However ke varied, this excess energy was carried away by the neutrino.

Front 26

What was the Frank and Hertz experiment to discover discrete energy levels in atoms?

Back 26

Electrons are boiled off a filament in a gas filled tube and accelerated through a voltage, the electrons then have to pass through a negative potential to enter an anode. Only electrons with sufficient ke will reach the anode.

The experiment suggested electrons have discrete energy levels as the voltage increase there were sudden peaks followed by troughs in current at intervals of a certain voltage instead of a gradual increase.

These peaks were where an electron has sufficient energy to raise 1,2,3 etc electrons of a gas atom by an energy level during a collision. This causes a trough in current above this as these electrons therefore lose energy and cannot generate a current.

Front 27

How does this idea of discrete energy levels relate to De Broglie's wavelength and the size of an atom?

Back 27

An atom can be thought of a box in which electrons are trapped in a "potential energy well".

The wavelength of the electrons must be of a certain set of values to create standing waves in the atom. This limits electron energy to certain discrete levels.

Front 28

What equation links the energy of electrons of energy level n?

Back 28

e = e1 * 1/n^2

Where e1 is the energy where n =1.

Energy is shown as a negative value, the greatest negative value when n = 1.

Front 29

How do you calculate the kinetic energy of an electron trapped in an atom?

Back 29

For a standing wave of λ = 2d where d is diameter.

momentum p = h / λ

kinetic energy 1/2mv^2 = p^2/2m

Front 30

How do you calculate the potential energy of an electron trapped in an atom?

Back 30

Ep = -e^2/4πε0r

Front 31

Therefore how do these equations limit how small atoms can be?

Back 31

The smaller λ is, the greater the kinetic energy proportional to 1/λ^2

The smaller λ is, the greater the potential energy is, proportional to 1/λ.

Therefore as atoms decrease in size the electrons would have enough kinetic energy to escape the atom.