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physics > P7 > Flashcards

P7 Flashcards

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1
Q

what is used to detect radioactivity

A

a geiger counter that clicks each time a particle of radiation from a radioactive substance enters the geiger tube

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2
Q

what is a radioactive substance

A

something that has an unstable nuclei that becomes stable by emitting radiation

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3
Q

what were the results when geiger and marsden investigated how a thin metal foil scatters a beam of alpha particles

A

most alpha particles passed straight through the metal foil
as angle of deflection increased, the number of alpha particles deflected per minute decreased
1 in 10,000 alpha particles were deflected by more than 90 degrees

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4
Q

what did rutherford deduce from the alpha particle scattering experiment

A

that there is a positively charged nucleus at the centre of every atom that is much smaller than the atom (because most alpha particles pass through without deflection)
most of the mass is located at the nucleus

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5
Q

why was rutherfords nuclear model of the atom accepted so quickly

A

agreed with the measurements geiger and marsden made in their expriments
explained radioactivity in terms of changes that happen to an unstable nucleus when it emits radiation
predicted the existence of the neutron (later discovered)

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6
Q

what was bohr’s model of the atom

A

electrons in an atom orbit the nucleus at specific distances and energy values

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7
Q

what is atomic number

A

number of protons in the nucleus

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8
Q

what is mass number

A

number of protons and neutrons

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9
Q

what are isotopes

A

atoms of the same element with different numbers of neutrons

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10
Q

what happens when an unstable nucleus emits an alpha particle

A

atomic number goes down by 2 and mass number goes down by 4

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11
Q

what is an alpha particle identical to

A

a helium nucleus

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12
Q

what happens when an unstable nucleus emits a beta particle

A

atomic number goes up by 1 and mass number doesn’t change

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13
Q

what happens when a gamma particle is emitted

A

no change as it is uncharged and has no mass

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14
Q

how do you carry out an absorption test

A

place the material between the geiger tube and the radioactive source then measure the count rate. add more materials until the count rate reaches 0 which is when the radiation has been stopped by the absorber material

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15
Q

what is ionisation

A

when radiation from radioactive substances knocks electrons out of atoms (atoms become charged as they lose electrons)

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16
Q

what is radioactive contamination

A

the unwanted presence of material containing atoms on other materials

17
Q

what is the absorber material and range of alpha

A

thin sheet of paper

5 cm

18
Q

what is the absorber material and range of beta

A 
aluminium sheet (5mm thick) or lead sheet ( 2-3mm thick) 
1m range in air
19
Q

what is the absorber material and range of gamma

A

thick lead sheet ( several cm thick) or concrete ( more than 1m thick)
unlimited range in air

20
Q

in what situation is alpha the most dangerous

A

in the body as it has the strongest ionising power

21
Q

how do smoke alarms work

A

contain a radioactive isoptope that sends out alpha particles into a gap in a circuit in an alarm. alpha particles ionise the air in the gap so there is a current across the gap. in a fire smoke absorbs the ions so the current across the gap drops and the alarm sounds

22
Q

why couldn’t beta or gamma radiation be used in a smoke alarm

A

they do not have strong enough ionising power to create enough ions to make the air in the gap conduct electricity

23
Q

what is th activity of a radioactive source

A

the number of unstable atoms in the source that decay per second measured in becquerel

24
Q

what is the half life of a radioactive isotope

A

the average time it takes for the number of nuclei of the sample to halve and for the count rate to halve its initial value

25
Q

how do you calculate the count rate after x half lives

A

initial count rate / 2^x

26
Q

what does a nucleus release when it undergoes fissions

A

2 or 3 neutrons at high speeds

energy in the form of gamma radiation plus the ke stored in the fission neutrons and the fragment nuclei

27
Q

what is nuclear fission (induced)

A

nucleus of an atom of a fissionable substance is struck by a neutron causing the nucleus to split into 2 fragment nuclei and release several neutrons

28
Q

what is spontaneous fission

A

fission that happens without a neutron being absorbed

29
Q

how does a chain reaction of fission events occur

A

when a fission event releases several neutrons which cause other fissionable nuclei to split

30
Q

what happens in a nuclear reactor

A

control rods absorb fission neutrons to ensure that on average only one neutron per fission goes on to produce further fission

31
Q

what is nuclear fusion

A

2 small nuclei release energy when they are fused together to form a single larger nucleus. some of the energy is released as nuclear radiation from the larger nucleus formed

32
Q

what are the difficulties of making fusion a useful source of energy

A

plasma of light nuclei has to be heated to very high temperatures and pressures before the nuclei will fuse. this is because 2 nuclei approaching each other will repel each other because of their positive charge but if they are moving fast enough they can overcome the force of repulsion and fuse together

33
Q

what happens in a fusion reactor

A

plasma is heated by passing a very big electric current through it
plasma is contained by a magnetic field so it doesn’t touch the reactor walls as if it did it would go cold and fusion would stop

34
Q

what are 8 sources of background radiation

A 
cosmic rays
ground and buidlings
food and drink
natural radioactivity in the air 
medical applications
nuclear weapons tests
air travel
nuclear power
35
Q

what is the main cause of background radiation in the air

A

radon gas that seeps from the ground from radioactive substances in rocks underground

physics (12 decks)

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