Mr Palmer's AQA GCSE Physics Notes
Nuclear Physics
Atomic Structure
The nucleus of an atom is made of protons and neutrons. Compared to the whole atom, the nucleus is a tiny fraction of the size.
The number of protons in an atom is the atomic number. The number of protons plus the number of neutrons is the mass number.
number of protons + number of neutrons = mass number
number of protons = number of electrons
number of protons = atomic number
An element as it would appear on the periodic table.
Protons have a charge of +1, electrons have a charge of -1, and neutrons have no charge. In a neutral atom, the number of electrons and the number of protons is equal so that the overall charge is 0 (neutral). If an atom loses an electron it becomes charged (+1) and is now called an ion.
Protons and neutrons have approximately the same mass, while an electron's mass in comparison is so small that we just say it's zero.
All atoms in an element have the same number of protons. If the number of protons changes, the element also changes.
Elements with different numbers of neutrons are called isotopes.
Rutherford's Experiment
The `Plum Pudding Model'
One of the earlier ideas of what an atom looked like was the `plum pudding' model. This theorized that an atom was a ball of positive charge, with the negatively charged electrons suspended inside.
A scientist named Rutherford set up an experiment to test the plum pudding model. He took a layer of gold just a few atoms thick and fired α-particles at it. He then used a Geiger counter to detect where the α-particles went after passing through the gold.
Rutherford expected that all the α-particles would pass through the gold undisturbed because the atoms' positive charge was distributed over a large area and couldn't resist the kinetic energy of the α-particles.
The actual result of the Rutherford experiment showed that most of the α-particles went straight through the gold as expected, but a few were deflected off a straight path and very rarely some were repelled backward.
This forced scientists to come up with a new model of the atom that could explain the results of the experiment. In the new model the electrons orbit the outside of a small, dense and strongly positive center.
The modern atomic model explains the results of Rutherford's experiment in the following way:
- Most α-particles travel in a straight path through the gold because they are too far from the positive nucleus to be affected.
- A few α-particles come close enough to the nucleus to be deflected by its positive charge.
- Very rarely an α-particle happens to head straight towards the nucleus. It is repelled backwards by the force of the positive nucleus
Rutherford's Experiment
α-particles passing through, and being deflected by, gold in the Rutherford Experiment
The Three Kinds of Radiation
Alpha Radiation
- Name: Alpha Radiation
- Symbol: α
- What it is: A Helium nucleus (2 Protons and 2 Neutrons)
- Absorption: Most easily stopped. Absorbed by centimeters of air or thin paper.
- Ionization: Highly ionizing
Alpha radiation leaves a large atom
In α-radiation, the nucleus of a large atom ejects a Helium nucleus. The atom, because it has lost two protons, becomes a different element.
Beta Radiation
- Name: Beta Radiation
- Symbol: β (sometimes drawn as: e-)
- What it is: A high energy, high-speed electron.
- Absorption: Absorbed by millimeters of metal. Not absorbed by centimeters of air or paper.
- Ionization: Highly ionizing
When a beta particle leaves a nucleus, one of the neutrons changes into a proton
In β-radiation, one of the neutrons transforms into an electron and a proton. The electron is ejected from the nucleus. The atom now becomes a different element because it has an additional proton.
Gamma Radiation
- Name: Gamma Radiation
- Symbol: γ (sometimes drawn as a wave)
- What it is: short wavelength, high frequency electromagnetic radiation.
- Absorption: Least easily stopped. Takes many centimeters of lead or meters of concrete to be absorbed.
- Ionization: weakly ionizing
Gamma radiation does not change the structure of the nucleus
In γ-radiation the nucleus gives off a short wavelength, high frequency burst of electromagnetic radiation. The atom does not become a new element as no protons have been gained or lost.
Nuclear Fission
A large atom splitting into two smaller atoms when hit by a neutron. This is Nuclear Fission.
Nuclear fission is when a large nucleus is hit by a neutron and splits into smaller nuclei and releases energy. The original nucleus is called the parent nucleus. The smaller parts it breaks into are called daughter nuclei.
A chain reaction. You are expected to be able to reproduce this diagram (without the electrons).
A chain reaction is when the fission process becomes self-sustaining. Neutrons released during the fission of the first parent nucleus hit the nuclei of other atoms, causing them to undergo fission as well. They release more neutrons and cause further fission.
The process of fission releases a large amount of energy and is used in nuclear power stations. The two fissionable substances used in most nuclear reactors are Uranium-235 and Plutonium-239.
Nuclear Fusion
Nuclear fusion is when two small nuclei join together to form a larger one and release energy. Because the positive nuclei will normally repel each other, they must have a large amount of kinetic energy to overcome the repulsion.
When two smaller nuclei join together, they release heat and light while they form a larger nucleus.
Nuclear fusion is the process that takes place inside the Sun. There, Hydrogen and Helium nuclei collide and provide us with light and heat.
Scientists are trying to build fusion reactors, to mimic the process in the Sun to provide people with cheap, clean energy. To do this they create a plasma\footnote{Plasma means that the Hydrogen atoms do not have any electrons.} of Hydrogen. The plasma is made extremely hot by passing a large current through it. The plasma is suspended inside a magnetic field to prevent it from touching the walls of the reactor.
If fusion can be made to work, we could get the fuel necessary from the Hydrogen in sea water and the only pollution made would be Helium gas.
Background Radiation
Background radiation is the normal level of radiation that is always present in an environment. It comes from these sources:
- ionizing radiation that comes from outer space. We sometimes call it `cosmic rays'.
- Background radiation also comes from radioactive isotopes in the environment. For example granite is a material that is natural radioactive.
- Some foods, such as bananas, are also mildly radioactive.
- Radon gas from the Earth also contributes to background radiation.
Physics P1 Topics
Physics P2 Topics
- Motion
- Forces
- Energy
- Static Electricity
- Current Electricity
- Mains Electricity
- Momentum
- Nuclear Physics
Other