Module 6: C24 - Particle Physics Flashcards
What did Rutherford carry out his Alpha-Scattering Experiment
A narrow beam of alpha particles, all of the same kinetic energy, from a radioactive source were targeted at a thin piece of gold foil which was only a few atomic layers thick. The alpha particles were scattered by the foil and detected on a zinc sulphide screen mounted in front of a microscope. Each alpha particle hitting this fluorescent screen produced a tiny speck of light. The microscope was moved around in order to count the number of alpha particles scattered through different values of the angle Θ per minute, for Θ from zero to almost 180°.
What Observations were made from Rutherford’s alpha-scattering experiment?
- Most of the alpha particles passed straight through the thin gold foil with very little scattering. About 1 in every 2000 alpha particles were scattered.
- Very few of the alpha particles (about 1 in every 10,000) were deflected through angles of more than 90°.
What does this conclusion from Rutherford’s alpha-scattering experiment:
Most of the alpha particles passed straight through the thin gold foil with very little scattering. About 1 in every 2000 alpha particles were scattered
This shows most of the atom is empty space, and most if the mass is concentrated in a small nucleus
What does this conclusion from Rutherford’s alpha-scattering experiment:
Very few of the alpha particles (about 1 in every 10,000) were deflected through angles of more than 90°.
This statement shows the nucleus has a positive charge because it repelled the few positive alpha particles.
Example Question:
Describe how the alpha-particle scattering experiments provide evidence for the existence, charge, and size of the nucleus.
In the alpha particle scattering experiment, most of the alpha particles passed straight through the gold foil with very little scattering. This highlights that most of the atom is made up of empty space. The fact that 1 in every 2000 alpha particles were scattered/deflected shows that there is very a very small mass concentrated within a particle, suggesting there is a nucleus/centre of mass in the atom. Additionally, 1 in every 1000 were deflected by angles of more than 90°, showing that the nucleus does have a positive charge, as it has repelled the positively charged alpha particles (two positives repel).
The size of the nucleus is about 10^-14 m
Exam Questions:
Rutherford used alpha particles of kinetic energy 1.2x10^-12 J in one of his experiments. Using the idea of conservation of energy, calculate the distance d of the closest approach between an alpha particle and the gold nucleus.
Energy = Qq/4πεοr
r = Qq/4πεοEnergy
1.2x10^-12 = (2x1.6x10^-19) x (79x1.6x10^-19) / (4πx8.85x10^-12 x r)
r = 3.029x10^-14m
Example Question:
Suggest why only a small number of alpha particles were scattered through large angles.
Only a small number were scattered, as the nucleus of an atom is very small compared to its size, making it less likely for head-on collisions to occur.
Example Question:
State the approximate radii of the atom and its nucleus
Nucleus: 10^-15 m
Radius: 10^-10m
Example Question:
Alpha particles of kinetic energy 8.8MeV are fired at lead atoms. The charge on the nucleus of lead is 82e. Calculate:
a) the minimum distance the alpha particles approach to the nucleus of lead
b) the maximum electrostatic force experienced by the alpha particle.
a)
8.8x10^6 x 1.6x10^-19 = Qq / 4πεοd
1.408x10^-12 = 82 x 2 x (1.6x10^-19)^2 / 4π x 8.85x10^-12 x d
d = 2.68x10^-14m
d = 2.7x10^-14m
b)
F = Qq / 4πεοr^2
F = 82 x 2 x (1.6x10^-19)^2 / 4π x 8.85x10^-12 x (2.68x10^-14)^2
F = 53N
Example Question:
A tiny droplet of oil diameter 1.0mm is placed on water. The oil spreads out as a circular disc of thickness approximately one atom thick. Estimate the radius of this oil disc.
Initial Volume = Final Volume
4/3π x (0.5x10^-3)^3 = 10^-10 x (π x r^2)
r = 1.29m
r = 1.3m
Nucleon Definition
The term nucleon is used to refer to
either a proton or neutron.
What is an Isotope?
Isotopes are nuclei of the same element that have the same number of protons but different numbers of neutrons.
What is an Atomic Mass Unit
The masses of atoms and nuclear particles are often expressed in atomic mass units (u).
One atomic mass unit (1u) is one-twelfth the mass of neutral carbon-12 atom.
The experimental value of 1u is about 1.661x10^-27 kg.
Equation for Radius of a Nucleus
R = ro ∛A
(A = nucleon number)
(Ro = 1.2 fm (10^-15m)
How do you derive the equation for density of a nucleus
ρ = m/v
ρ = m / (4/3πr^3)
ρ = m / (4/3π(ro∛A)^3)
ρ = m / (4/3π ro^3 A
Calculate the (a) the mass, (b) the radius, (c) the volume, (d) the density of a nickel nucleus containing 28 protons and 36 neutrons, and (e) the density of the atom. The mass of both protons and neutrons to 3sig.figs is 1.00u (1u = 1.66x10^-27 kg).
a)
28 + 36 = 64
64u = 64 x 1.66x10^-27 = 1.06x10^-25 kg.
b)
R = ro^3 ∛A
R = 1.2x10^-15 ∛64
R = 4.8x10^-15 m
c)
V = 4/3 πr^3
V = 4/3 x π x (4.8x10^-15)^3
V = 4.632x10^-43 m^3
d)
ρ = m/v
ρ = 1.06x10^-25 / 4/3π ro^3 A
ρ = 2.29x10^17 kgm^-3
e)
ρ = m/v
ρ = 1.06x10^-25 / 4π (10^-10)^3
ρ = 2.53x10^4 kgm^-3
Describe the nature and range of the three forces acting on the protons and neutrons in the nucleus
Gravitational - very small attractive force, will be stronger the closer two objects are.
Electrostatic force repelling the protons - very big relative to these forces, works when two charges are in close proximity
Strong Nuclear Force - very small range (<3fm). When too close they repel, otherwise this force is attractive and keeps the nucleus together
What keeps protons together in the Nucleus
The attractive gravitational force between the proton is far too small (about 10^-34) to keep them together, so there must be another, much stronger force acting on the protons. This force is the strong nuclear force.
What is the Strong Nuclear Force (what does it act on, what is its range?)
The strong nuclear force acts between all nucleons. It is a very short range force, effective over just a few fe to meters. Figure 3 shoes the variation of the strong nuclear force F between two nucleons with separation r. The force is attractive to about 3fm and repulsive below about 0.5fm.
When is the Strong Nuclear Force attractive and repulse
The force is attractive to about 3fm and repulsive below about 0.5fm.
What are Fundamental Particles?
Fundamental particles are a particle that have no internal structure and bend can’t be divided into smaller bits.
Protons and neutrons are not fundamental particles, because they are made up of quarks.
Quarks, electrons, and neutrinos are considered as fundamental particles.
What 3 things are considered a fundamental particles?
Quarks, electrons, and neutrinos are considered as fundamental particles.
What are the 4 Fundamental Forces
Magnetic Force
Gravitational Force
Strong Nuclear Force
Weak Nuclear Force
Strong Nuclear Force:
• Effect
• Relative Strength
• Range
Effect:
Experienced by nucleons
Relative Strength:
1
Range:
Only acts within 3fm
