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31 Cards in this Set
- Front
- Back
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What does the continuous spectrum consist of and how is it produced? |
All possible frequencies over given range. This depends on the temperature of the material. Low vibration frequency means low frequencies photons are released and hence have a large wavelength. When heated, high vibration frequency means high frequency photons being released and hence small wavelength. Produced by heating solids or dense liquids to incandescence. |
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What does the line emission spectrum consist of and how is it produced? |
Bright, monochromatic lines of light against dark background. Not regularly spaced and can be used to determine the element. Created by putting high p.d. across a gas. Viewed through a spectroscope and a diffraction grating. (Sodium vapour lamp is yellow). The movement of electrons from a higher energy state to a lower state means the emission of a photon. This has specific colour. |
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In what order are the transition sequences? |
Lyman, Balmer, Paschen |
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What is the ionisation energy? |
The minimum energy required to remove a single electron from the atom in the most stable, ground state. |
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How does fluorescence occur? |
If a photon of an exact energy corresponding to the energy transition is incident on a metal, electrons may be excited to upper states. Naturally, they will drop levels, emitting photons. This may occur fully or partially. It may just release the same energy photon or emit two lower energy photons, which is what gives rise to fluorescence. |
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How does line absorption spectra occur? |
If photons (from a white light) of corresponding energy are hitting a gas, the photons with correct energies will be absorbed while the others pass through and diffract to create a continuous emission spectrum but with black bars, indicating the position of the absorbed photons. This can be used to determine the element. |
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What is spontaneous emission? |
Occurs at normal temperatures where electrons are in the ground state. When they are excited, a photon is released which occurs in 10^-8 seconds. |
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What is stimulated emission? |
Occurs when electrons are in a meta stable state. If a photon corresponds to the energy of high energy state to a low energy state, the electrons are initiated, causing the emission of a photon in phase with the original photon. This can cause chain reactions whereby yet more photons are released. |
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What does LASER stand for? |
Light Amplification by Stimulated Emission of Radiation |
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What are the components of a LASER and what do they do? |
- Active medium (the gas) - A pump which supplies energy to the tube - Tube (mirrors) which enable light to be amplified. -Active medium is 85% neon and 15% helium gas. The neon has a meta stable state. -The pump is an electric current which passes the tube. Electrons collide with helium atoms and promotes the atoms to an excited state. Neon has excited state near helium so instead of helium dropping to ground state, it interacts with neon and puts electrons in neon's excited state. Neon now has population inversion where stimulated emission can occur. Electrons drop energy state from non meta stable levels, emitting photons which excite atoms which allow for stimulated emission and hence light. - The tube has two mirrors either side which continually rebound light until it is of a high enough intensity that it passes through the mirror. |
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Applications of LASERs and why are they useful? |
- Can be used to destroy single cells in medicine due to coherent, precise beams. - Used in discs for imaging. -Beams have high intensity and are monochromatic. - Low power output and are safe. |
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In terms of nuclei, what is the atomic number and mass number? |
Atomic number = Z Mass number = A Atomic number = number of photons Mass number = sum of photons and neutrons |
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How are nuclei held together? |
By a strong, attractive nuclear force. - It is short ranged. - Independent of charge. - Can alternate between attractive and repulsive depending on how close nuclei are together. - Force does not exist outside the nucleus. |
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What is the binding energy? |
The minimum amount of energy required to separate the nucleus into its constituent nucleons. (E=mc^2=hf) |
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What 4 things must be conserved in a nuclear reaction? |
- Charge
- Nucleon number - Mass - Momentum |
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Which atoms are unstable? |
Ones with atomic numbers greater than 83. |
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When does alpha decay occur? |
When the atomic number is greater than 83. Daughter nucleus is more stable and may decay further. |
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When does beta minus decay occur? |
Where there are too many neutrons. (Contains an anti-neutrino). |
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When does beta plus decay occur? |
When there are too many protons (too few neutrons). (Contains a neutrino). |
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What properties do neutrinos and anti-neutrinos have? |
- No charge. - No rest mass. - Travel at speed of light. - Can be assigned energy and momentum. |
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What do beta, gamma and alpha decays have in common? |
They are known as 'ionising radiations' by which they ionise the material they pass through. |
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What are the properties of beta, gamma and alpha decays? |
Alpha - Strong ioniser, stopped by paper. Beta - Medium ioniser, stopped by aluminium. Gamma - Weak ioniser, stopped by thick lead or concrete. |
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In magnetic fields, how do radioactive decays move? |
Beta and alpha decays follow uniform circular motion (mv^2/r = qvB). Gamma radiation is unaffected. |
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In electric fields, how do radioactive decays move? |
Both alpha and beta plus decays attract to the negative plate while beta minus attracts to the positive plate. Gamma has no charge and remains unaffected. |
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What is used in PET scans? |
- Fluorine-18 has a half life of 110 minutes and an equation H + O = F + n - FDG is (Fluoro 2 deoxy D glucose) |
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How does a PET scan work? |
When fluorine-18 decays, the following reaction is observed: F = O + B+ + v. After the positron is released, it travels about 1mm before slowing and finally colliding with an electron whereby it undergoes positron-electron annihilation. Two gamma photons are then emitted in opposite directions. These are detected by detector rings where a computer determines high density areas of FDG (where cancers are likely to exist). |
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How does fission occur? |
If a neutron forces itself into the nucleus it can have the effect of distorting its shape. It can increase the vibrational energy of the nucleus causing it to vibrate in an elongated way. Therefore, the nuclear force cannot overcome the coloumbic force and hence the nucleus is forced apart into two approximately equal parts by coloumbic repulsion. |
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How do nuclear reactors work? |
The coolant is then heated in the core from fission and is then converted to steam where a turbine turns to create electricity. |
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What are the advantages and disadvantages of using nuclear reactors? |
Advantages: - No greenhouse effect as there are no toxic emissions like carbon monoxide. - Not much fuel is needed. Disadvantages: - Disposal of spent fuel is a problem as the half lives are quite long. - Heat is produced and is near lakes and oceans. The heat produced is not good for plant or animal life. |
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What is fusion? |
The combination of two very light nuclei to form a heavier nucleus. Requires a great amount of energy and a great amount of energy is created. |
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Why do alpha particles have discrete energies? |
When an alpha particle is emitted, the daughter nucleus produced may be in excited state. The alpha particle's energy is equal to the difference of parent nucleus' energy and excited state of daughter nucleus' energy. Depending on the state of the daughter, the alpha particle will carry discrete energies. When the excited nucleus returns to ground state, the energy is lost as gamma photons. These will have discrete energies equal to the excited energy level. |
