6.4 Nuclear & Particle Physics Flashcards
what was the alpha particle scattering experiment?
Ernest Rutherford fired a narrow beam of fast moving alpha particles at a thing gold foil to see how the alpha particles were deflected (or a a scattered), the detector was a zinc sulphide screen that would produce a tiny spec of light whenever an alpha particle hit it
What are the two significant observation, which could not support Thomson’s plum-pudding model of the atom
- 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 were the deductions from the alpha particle scattering experiment?
- the vast majority of the mass of the atom is contained within a small volume called the nucleus
- the nucleus has a positive charge
- the nuclear distance is considerably smaller than the diameter of the atom, meaning that the atom is mostly empty space
what was the evidence in the experiment that showed there was a very dense nucleus?
most of the mass must be in the nucleus since the fast alpha particles (with high momentum) are deflected by the nucleus
what was the evidence in the experiment that showed the nucleus was positive?
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, Rutherford named this the nucleus
what was the evidence in the experiment that showed that the atom is mostly made up of empty space?
most of the alpha particles went straight through the foil, so the atom is mainly empty space
what is the charge and mass of a proton (in terms of e for charge and u for mass)?
+1 charge
mass = 1
what is the charge and mass of a neutron (in terms of e for charge and u for mass)?
0 charge
mass = 1
what is the charge and mass of a electron (in terms of e for charge and u for mass)?
-1 charge
mass = 1 / 1836 (negligible)
what is the defintion of nucleon number?
no. of protons and neutrons inside the nucleus (also known as the mass number)
what are isotopes?
isotopes contain the same no. of protons and electrons but different no. of neutrons
what is mass number equal to?
no. of protons and neutrons
what is atomic number equal to?
just no. of protons
what is the strong nuclear force?
the strong nuclear force acts between nucleons and holds the nucleus together against the electrostatic attraction repulsion of the protons
So why do protons not fly apart in a nucleus
For example, In a helium-4 nucleus, the two protons are separated by a distance of about 10^-15m and exert a large repulsive electrostatic force given by equation
F= Qq/kr^2
This is an extremely large repulsive force. Now the attractive gravitational force between the proton is far too small to keep them together, so there must be another, much stronger force acting on the proton. This force is the strong nuclear force
is the strong nuclear force attractive or repulsive?
(trick question) it is BOTH, it is attractive to about 3 x 10^-15 and 0.5 x 10^-15m, it is a repulsive force between nucleons for distances of separation up to around 0.5x10^-15m, it is a SHORT RANGE force - it doesn’t act over large distances
State the approximate radii of the atom and the nucleus
atom ~ 10^-10 m and nucleus ~ 10^-15 m
how much bigger is the diameter of the whole atom compared to the diameter of the nucleus?
roughly 10,000 times greater 10^5
what will a graph of radius of nucleus against nucleon number look like (think about the relationship)?
exponential, starts steep then decreasing gradient until it plateaus
what will a graph of radius of nucleus against cube root of nucleon number look like (think about the relationship)?
constant gradient, straight line upwards
what is the equation that links radius of a nucleus to nucleon number?
R = Ro x A^1/3
where Ro = a constant (1.4 x 10^-15)
R = nuclear radius
A = nucleon number
what is greater atomic density or nuclear density?
nuclear density is much greater than atomic density
what are the two equations needed to calculate nuclear density? to get to ρ = 3Mn / (4πro^3)?
ρ = m / v
and assuming v = 4/3πr^3,
subbing in R^3 = Ro^3A from R = Ro x A^1/3 gets you to a simplified formula to work out the mean density of the nucleus: ρ = 3Mn / (4πro^3)
P = Amnucleon/ 4/3πr^3A
what are fundamental particles?
fundamental particles are particles that cannot be broken down into smaller components (e.g electrons, quarks)