# Physics U1 Matter and radiation

1) inside the atom

2)stable and unstale nuclei

3)photons

4)particles and antiparticles

5) how particles interact

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## Inside the atom

Every atom contains a positively charged nucleus - (protons and neutrons), and electrons which surround the nucleus.

Electrons are held in the atom by an electrostatic force of attraction between them and the nucleus.

The electron has a much smaller mass than the proton or the neutron

The proton and the neutron have almost equal mass

The electron has equal and opposite charge to the proton. The neutron is uncharged.

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## Isotopes

An isotope of an element: atoms of an element with different numbers of neutrons and the same number of protons.

The proton number is usually called the atomic number

The total number of protons and neutrons in an atom is called the nucleon number or the mass number of the atom.

The atomic number is to the bottom left of the chemical symbol. The mass number is to the top left of the chemical symbol.

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## Specific charge

The specific charge of a charged particle is its charge divided by its mass.

Eg: a nucleus of Hydrogen has a charge of 1.6x10-19 C and a mass of 1.67x10-27 kg.

Its specific charge would therefore be 9.58x10 7 Ckg-1

( Note: The electron has the largest specific charge of any particle.)

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## The strong nuclear force

A stable isotope is held together by a strong nuclear force that overcomes the electrostatic force of repulsion between protons and neutrons and keeps them together.

Range: 304 fm (1fm = 10^-15)

It has the same effect between 2 protons as it does between 2 neutrons or a proton and a neutron.

It is an attractive force.

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This is where an unstable nucleus of an element emits an alpha particle ( which contains 2 protons and 2 neutrons). This means the nucleon number will decrease by 4 and the atomic number will decrease by 2. As a result of the change, the product nucleus belongs to a different element.

This is when an unstable nucleus emits a beta particle ( which consist of fast moving electrons). This happens when a neutron changes into a proton and a beta particle is emitted immediately. In addition,  an antineutrino is emitted as well and this has no charge. The atomic number increases by 1 but the nucleon number stays the same.

This is an electromagnetic radiation emitted by an unstable nucleus. It can pass through thick metal plates and has no mass or charge. It is emitted by a nucleus with too much energy , following an alpha or beta emission.

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## Photons- Electromagnetic waves

In a vacuum, all electromagnetic waves travel at the speed of light.

You can work out the wavelength by using the equation speed of light divided by frequency.

Electromagnetic Spectrum

Radio, Microwave, Infrared, Visible, UV, X-rays, Gamma Rays

These waves consist of an electric wave and a magnetic wave which travel together and vibrate:

1) at right angles to each other and in the direction

2) in phase with each other

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## Photons

Electromagnetic waves are emitted as short 'bursts' of energy, each 'burst' is referred to as a photon.

Electromagnetic waves are emiied by a charged paricle when it loses energy. This can happen when a fast-moving electron is stopped or slowed down, or an electron in a shell of an atom moves to a different shell of a lower energy.

Equation: Photon energy = h f

( where h is Planck's constant and f is the frequency)

A laser beam consists of photons of the same frequency. You can work out the power of the beam by multiplying the number of photons through a fixed point each second, by frequency and by Plancks constant.

the power of the beam =nhf

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## Animatter - annihilation and pair production

Annihilation

This is when a particle and its corrosponding antiparticle meet and explode and this then forms 2 photons are produced. With and electron and a positron, 2 gamma photons are produced. Positron emission takes place when a proon changes into a neutron in an unstable nucleus with too many protons. The positron has a posiive charge as it is the antiparticle of an electron. In addiion a neutrino which is uncharged, is also emitted.

Pair production

Pair producion is the opposite of annihilation. In this process, a photon with sufficient energy  could suddenly change into a paricle- antiparticle pair which would then separate from each other

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## The Electromagnetic Force

When a single force acts on an object, it changes the momentum of the object.  To work out the momentum - mass multiplied by its velocity.

When two forces interact, they exert equal and opposite forces on each other. Eg if two protons approach each other, they repel and move away from each other.

Richard Feynmann stated that the electromagnetic force between two charge object is due to the exchange of virtual photons.

Figure- The Feynmann diagram for the                                                           electromagnetic force between two protons.

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## The Weak Nuclear Force

The weak nuclear force causes changes a neutron to change into a proton in beta decay.

In beta decay a neurino or antineutrino is formed and they hardly interact with other particles apart from:

1) a neutrino interacts with a neutron and makes it change into a proton and an electron is emitted as a result

2) an antineutrino interacts with a proton and makes it change into a neutron and a positron is emitted as a result

Theses interactions are due to the exchange particle known as W bosons. They have a non-zero rest mass, have a very short range and are positively charged.

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## Feynmann Diagrams

A neutron -neutrino interaction

A proton- antineutrino interaction

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## Beta Decay

W Bosons in beta decay

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## Electron Capture

Electron capture is where the W+ boson changes the electron into a neutrino.

The same change can occur when a proton and an electron collide a very high speed. In addition for an electron with sufficient energy, the overall change could also occur as a W- exchange from the electron to the proton.

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