8. Nuclear and Particle Physics Flashcards
How has the understanding of atomic structure changed overtime?
Dalton’s ‘Billiard Ball’ Model 1803
J.J Thompson’s Plum Pudding Model 1897
Rutherford’s Gold Foil Experiment 1909-1911
Neil Bohr’s Model 1913
Quantum Mechanical Model 1926
What’s Dalton’s ‘Billiard Ball’ Model 1803?
Imagined all matter was made of tiny solid particles called atoms
No-one believed him for years, as they thought everything was fire, earth and water
His model proposed:
- Atoms are the smallest constituents of matter and cant be broken down any further
- a given element has identical atoms
- different elements have different atoms
- When chemical reactions occur, atoms rearrange to make different substances
What’s J.J Thompson’s Plum Pudding Model 1897?
Discovered the electron
Proposed the Plum Pudding Model:
- atom consists of positive and negative charges in equal amounts, so neutral overall
- spheres which are positively charged with uniformly distributed charge and density, making a ‘plum pudding’
What’s Rutherford’s Gold Foil Experiment 1909-1911?
Hans Geiger and Ernest Marsden set out to test the plum pudding model:
- Walls of zinc sulphate which emitted light when hit, this needed a microscope to see
- Positively charged alpha, α particles were aimed at very thin gold foil
Most went straight through, only some were deflected and only some were completely deflected in other direction
The results stated:
- Atoms have a central, positively charged nucleus containing the majority of mass
- Most of the atom is empty space
- electrons orbit the nucleus
What’s Neil Bohr’s Model 1913?
Bohr improved Rutherford’s planetary model
- Showed electrons occupy shells or energy levels around the nucleus
- these are a particular distance away from the nucleus and they cant exist between
What’s the Quantum Mechanical Model 1926?
Ervin Schrödinger used equations to calculate the likelihood of finding an electron in a certain position
Portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest and vice versa
James Chadwick then discovered the neutron in 1932, which completes the model of the atom we know today
What is an isotope?
An isotope is an atom (of the same element) that has an equal number of protons but a different number of neutrons
There are stable isotopes and unstable isotopes, which are radioactive which decay into other elements
What is thermionic emission?
When electrons gain enough thermal energy to leave the surface of a metal
How does thermionic emission differ from photoelectric effect?
In thermionic emission, electrons only gain energy from thermal energy
In photoelectric effect, electrons only gain energy from absorbing photons
And energy is only carried by photons in electromagnetic radiation
What are the 2 type of particle accelerators?
Linac accelerator
Cyclotron accelerator
Why are particle accelerators used?
In order to discover what is inside of subatomic particles, scientists accelerate the particles to close to the speed of light
These particles break, and particle tracks can show the properties of fundamental particles
What is a linac accelerator?
Particles with a charge (an ion) are accelerated due to electric fields
It accelerates through a tube due to the other side of the tube having an opposite charge to the particle
When it gets there, the A.C leads to the electric field switching, leading to it being accelerated through a longer tube (for equal time) to the next electric field
This process is repeated until the particle is moving almost at the speed of light
However, these tubes can go up to 5km long, meaning you need a lot of space
Also very expensive
What is a cyclotron?
Accelerates ions from a central point, which is a source of the ions, around a spiral path
There are 2 hollow semi-circular electrodes called dees which they travel in
The electric field which is opposite to the ion forces it to go through the next dee
Then the square sine wave A.C changes so it is accelerates to the opposing dee again
This means it carries on to speed up
There is a uniform magnetic field perpendicular to the electrodes
The magnetic field acts perpendicular to the velocity of the ions (it just does)
As the ions are always changing direction, it leads to ions moving in a circular motion, with the magnetic force acting as the centripetal force
What effects the radius of the motion of a charged particle in a uniform magnetic field?
e.g in a cyclotron
The force exerted by the magnetic field on a charged particle is always perpendicular to its velocity
Causing it to follow a circular path as the magnetic force acts as the centripetal force
The magnetic field is given by F=BQv
So… mv^2/r = BQv
r=mv/BQ
Meaning the faster the ion goes, the bigger radius it makes
The mass also increases due to relativistic effects
What happens when an object reaches very high speeds near c?
The mass experiences relativistic effects causing them to increase in mass!
Time dilation - The clock also slows down for them, meaning travelling at such high speeds, they live for much longer
Length contraction
- Moving at faster speeds makes the particle shorter
What’s the Geiger-Muller (GM) Tube?
The GM tube is a particle detector that picks up radiation
The radioactive particles go through a mica window and emits electrons from the argon gas
This leads to the free electrons moving towards the anode (+ve) and also takes other electrons from other argon gas atoms during its journey
When the electron goes onto anode, it creates a pulse of electricity, which is counted by the counter
There’s a 400V p.d between the anode and the cathode
So there’s enough strength to push electrons towards the anode
If the voltage emits too much of a current, it goes into the Earth
