# Physics Unit 5 Definitions

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• Created by: Lauren
• Created on: 05-05-14 09:22
Absolute Scale
Temperature scale in Kelvins defined in terms of absolute zero and the triple point of water which is the point at which ice, water and water vapour are in thermal equilibrium.
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Absolute Zero
The lowest possible temp. the temp. at which an object has minimum internal energy.
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Activity
Of a radioactive isotope, the number of nuclei of the isotope that disintegrate per second. Unit = Becquerel (Bq) equal to one disintegration per second.
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Alpha Decay
Change in an unstable nucleus when it emits an alpha particle which is a particle consisting of two protons and two neutrons.
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Particles that are each composed of 2 protons and 2 neutrons. An alpha particle is emitted by a heavy unstable nucleus which is then less unstable as a result. Easily absorbed by paper, range of few cm and is most ionising.
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Atomic Mass Unit (u)
Correctly referred to as the unified atomic mass constant, one twelfth of the mass of an atom of the carbon isotope carbon-12, equal to 1.661 x 10^27 kg
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Atomic Number
Of an atom of an element is the number of protons in the nucleus of the atom. It is also the order number of the element in the Periodic Table.
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The number of atoms in 12g of carbon-12 is used to define the mole. 6.02 x 10^23 mol^-1
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Beta Decay
Change in a nucleus when a neutron changes into a proton and a beta-minus particle and an antineutrino are emitted if the nucleus is neutron rich or a proton changes to a neutron and a beta-plus particle and a neutrino are emitted.
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Electrons emitted by unstable neutron-rich nuclei. Absorbed by aluminium, range of a couple of metres and less ionising than alpha.
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Positrons emitted by unstable proton-rich nuclei. Positrons emitted in solids or liquids travel no further than about 2mm before they are annihilated.
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Binding Energy of a Nucleus
The work that must be done to separate a nucleus into its constituent neutrons and protons. Binding energy = mass defect x c^2 Binding energy in MeV = mass defect in u x 931.3
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Binding Energy per Nucleon
The average work done per nucleon to separate a nucleus into its constituent parts. Binding energy per nucleon = binding energy of a nucleus / mass number
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Boiling Point
The temperature at which a pure liquid at atmospheric pressure boils.
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Boyle's Law
For a fixed mass of gas at constant temperature, its pressure x its volume is constant. A gas that obeys Boyle's Law is said to be an ideal gas.
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Boltzmann Constant k
The molar gas constant divided by Avogadro's Number
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Brownian Motion
The random and unpredictable motion of a particle such as a smoke particle caused by molecules of the surrounding substance colliding at random with the particle.
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Chain Reaction
A series of reactions in which each reaction causes a further reaction. A steady chain reaction is when one fission neutron on average from each fission goes on to produce on fission event.
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Charles' Law
For an ideal gas at constant pressure, its volume is directly proportional to its absolute temperature.
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Control Rods
Rods made of a neutron-absorbing substance such as cadmium or boron that are moved in or out of the core of a nuclear reactor to control the rate of fission events in the reactor.
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Coolant
A fluid that is used to prevent a machine or deice from becoming dangerously hot. The coolant in a nuclear reactor is pumped through the core to transfer thermal energy to the heat exchanger.
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Count Rate
The number of counts per unit time detected by a Geiger Muller Tube. Count rates should always be corrected by measuring and subtracting the background count rate.
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Critical Mass
The minimum mass of the fissile isotope in a nuclear reactor necessary to produce a chain reaction.
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Decay Constant
The probability of an individual nucleus decaying per second.
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Decay Curve
An exponential decrease curve showing how the mass or activity of a radioactive isotope decreases with time.
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Diffraction
The spreading of waves when they pass through a gap or round an obstacle. High energy electron scattering is used to determine the diameter of the nucleus.
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Dose Equivalent (Unit = Sievert Sv)
A comparative measure of the effect of each type of ionising radiation, defined as the energy that would need to be absorbed per unit mass of matter from 250k of X-radiation to have the same effect.
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Electron Capture
A proton-rich nucleus captures an inner-shell electron to cause a proton in the nucleus to change into a neutron. An electron neutrino is emitted.
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Excited State
An atom which is not in its ground state.
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Fission
The splitting of a nucleus into two approx. equal fragments. Induced fission is caused by an incoming neutron colliding with a nucleus.
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Fission Neutrons
Neutrons released when a nucleus undergoes fission and which may collide with nuclei to cause further fission.
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Fusion (Nuclear)
The fusing together of light nuclei to form a heavier nucleus
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Fusion (Thermal)
The fusing together of metals by melting them together.
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Electromagnetic radiation emitted by an unstable nucleus when it becomes more stable.
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Half Life
The time taken for the mass of a radioactive isotope to decrease to half the initial mass or for its activity to halve. This is the time taken for the no. of nuclei of the isotope to decrease to half the initial number.
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Heat Q
Energy transfer due to a difference in temperature.
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Heat Capacity
The energy needed to change the temp. of an object by 1K
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Heat Exchanger
A steel vessel containing pipes through which a hot coolant in a sealed circuit is pumped, causing water passing through the vessel in separate pipes to turn to steam to drive turbines.
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Ideal Gas
A gas under conditions such that it obeys Boyle's Law.
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At a surface is the radiation energy per second per unit area at normal incidence to the surface.
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Internal Energy
Of an object is the sum of the random distribution of the kinetic and potential energies of the molecules.
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Radiation that produces ions in the substances it passes through. Destroys cell membranes and damages vital molecules such as DNA.
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Isotopes
Atoms which have the same number of protons in each nucleus but different numbers of neutrons.
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Latent Heat of Fusion
The energy needed to change the state of a solid to a liquid without change of temperature.
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Latent Heat of Vaporisation
The energy needed to change the state of a liquid to a vapour without change of temperature.
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Mass Defect
Of a nucleus is the difference between the mass of the separated nucleons and the nucleus.
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Melting Point
The temperature at which a pure substance melts.
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Moderator
A substance in a thermal nuclear reactor that slows the fission neutrons down so they can go on to produce further fission.
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Nucleus
A relatively small part of an atom where all the atom's positive charge and most of its mass is concentrated.
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Pressure
The force per unit area that a gas or liquid or a solid at rest exerts normally on a surface. Unit = Pascals Pa
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Pressure Law
For a fixed mass of an ideal gas at constant volume, its pressure is directly proportional to its absolute temperature.
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Reactor Core
The fuel rods, control rods, and the absorber rods of a nuclear reactor which together with the moderator substance are in a steel vessel through which the coolant is pumped.
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Rutherford's Alpha Scattering Experiment
Demonstrated that every atom contains a positively charged nucleus which is much smaller than the atom and where all the positive charge and most of the mass in the atom is located.
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Specific Heat Capacity c
Of a substance is the energy needed to raise the temperature of 1kg of the substance by 1K without change of state.
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Specific Latent Heat of Fusion
Of a substance is the energy needed to change the state of unit mass of a solid to a liquid without change of temp.
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Specific Latent Heat of Vaporisation
For a substance is the energy needed to change the state of unit mass of a liquid to a vapour without change of temp.
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Strong Nuclear Force
Force that holds the nucleons together.
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Sublimation
The change of state where a solid changes to a vapour directly.
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Temperature
The degree of hotness of an object.
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Thermal Energy
The internal energy of an object due to temperature.
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Thermal Equilibrium
When no overall heat transfer occurs between two objects at the same temp.
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Thermal Nuclear Reactor
Nuclear reactor which has a moderator in the core.
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## Other cards in this set

### Card 2

#### Front

The lowest possible temp. the temp. at which an object has minimum internal energy.

Absolute Zero

### Card 3

#### Front

Of a radioactive isotope, the number of nuclei of the isotope that disintegrate per second. Unit = Becquerel (Bq) equal to one disintegration per second.

### Card 4

#### Front

Change in an unstable nucleus when it emits an alpha particle which is a particle consisting of two protons and two neutrons.

### Card 5

#### Front

Particles that are each composed of 2 protons and 2 neutrons. An alpha particle is emitted by a heavy unstable nucleus which is then less unstable as a result. Easily absorbed by paper, range of few cm and is most ionising.