AQA Physics P7 Answers Key
- 1. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 1 P7 Radioactivity Student Book answers P7.1 Atoms and radiation Question number Answer Marks Guidance 1 a radiation from U consists = particles, radiation from lamp = electromagnetic waves, radiation from U is ionising, radiation from lamp is non-ionising 1 1 1 b radioactive atoms have unstable nuclei whereas atoms in lamp filament do not, decay of radioactive atom cannot be stopped whereas atoms in lamp filament stop emitting radiation when filament current switched off 1 1 2 a i alpha 1 2 a ii beta or gamma 1 2 b gamma 1 3 atoms have unstable nuclei, these nuclei become stable by emitting radiation 1 1 4 a substance emits (ionising) radiation so radioactive 1 1 4 b paper stopped most radiation from substance reaching Geiger counter, paper absorbed radiation, so must be alpha radiation 1 1 1
- 2. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 2 P7 Radioactivity Student Book answers P7.2 The discovery of the nucleus Question number Answer Marks Guidance 1 nucleus much smaller than atom, nucleus positively charged, mass of atom concentrated in nucleus all positive charge of atom concentrated in nucleus 1 1 1 1 2 a B 1 2 b A: attracted by nucleus C: unaffected by nucleus D: repelled in wrong direction by nucleus 1 1 1 3 a i atoms not indivisible, atoms contain negatively charged electrons 1 1 3 a ii any two from: nuclear: all positive charge concentrated in nucleus much smaller than atom, plum pudding: positive charge spread out throughout atom, nuclear: most mass concentrated in nucleus, plum pudding: mass spread out throughout atom nuclear: most atom empty space, plum pudding: no empty space 2 3 b nuclear model explains why some alpha particles scattered through large angles, in plum pudding model such large-angle scattering should not be observed 1 1 4 a similarity: proton and neutron have about same mass (or both found in nucleus) difference: proton is charged whereas neutron has no charge 1 1
- 3. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 3 P7 Radioactivity Student Book answers Question number Answer Marks Guidance 4 b He nucleus contains 4 (neutrons + protons) whereas H nucleus only contains one = a single proton, 2 protons particles in He nucleus because He nucleus has twice as much charge as H nucleus, ∴other 2 particles in He nucleus are neutrons 1 1 1
- 4. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 4 P7 Radioactivity Student Book answers P7.3 Changes in the nucleus Question number Answer Marks Guidance 1 a 6 p + 6 n 1 1 b 27 p + 33 n 1 1 c 92 p + 143 n 1 1 d 4 p 10 n 1 1 2 a 92 p + 146 n 1 2 b 90 p + 144 n 2 2 c 91 p + 143 n 2 3 a 4 2 235 92 Th U 231 90 2 3 b 0 64 30Zn Cu 1 64 29 2 4 0 210 84Po Bi 1 210 83 3
- 5. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 5 P7 Radioactivity Student Book answers P7.4 More about alpha, beta, and gamma radiation Question number Answer Marks Guidance 1 a stops irradiation of nearby people or objects 1 1 b alpha 1 1 c α, β 1 2 a i gamma 1 2 a ii alpha 1 2 a iii beta 1 2 b i gamma 1 2 b ii alpha 1 3 a can knock electrons from atoms, this ionisation damages cell (or kills cell or affects genes in cell which can be passed on if cell generates more cells) 1 1 3 b (place Geiger tube in a holder so it can be moved horizontally,) move tube so end close to source and Geiger counter detects radiation from source, move tube gradually away from source until count rate decreases significantly, distance from end of tube to source is range of α radiation from source 1 1 1 4 very little γ radiation absorbed by foil, it would all pass straight through so thickness of foil would not affect detector reading 1 1
- 6. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 6 P7 Radioactivity Student Book answers P7.5 Activity and half-life Question number Answer Marks Guidance 1 a average time for no. nuclei in sample of isotope to halve 1 1 b 190 cpm 1 2 a i 4 milligrams (= 8 mg 2 ) 1 2 a ii 1 milligram (= 8 mg 23 ) 1 2 b 5% of 8 mg = 0.4 mg so mass < 0.5 mg (= 8 mg 24 ) after 4 half-lives, time taken ∴ just over 4 half-lives → about 65 hours 1 1 1 3 a i 160 million atoms 1 3 a ii 1 32 (= 1 25 ) 1 3 a iii number remaining = 320 million 25 = 10 million atoms 1 1 3 b after 4 half-lives, count rate = initial count rate of 320 cpm 24 = < 37.5 cpm so time taken to drop to 40 cpm from start < 180 minutes (4 half-lives) 1 1 4 after 2 half-lives count rate due to wood = 25% of initial count rate, ∴ the wood is 11 200 years old (= 2 × 5 600 yrs) 1 1
- 7. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 7 P7 Radioactivity Student Book answers P7.6 Nuclear radiation in medicine Question number Answer Marks Guidance 1 a beta or gamma, can be detected outside body 1 1 1 b gamma, radioactive source injected into patient to enter organ to be imaged so needs to do least damage whilst in body, gamma radiation passes through body tissue and detected using gamma camera 1 1 1 2 any two from: food, drink, radon 1 3 a small ‘seeds’ of radioactive isotope placed in tumour, radiation from isotope destroys cancer cells, use isotope with half-lives not long enough to damage normal cells surrounding tumour, half-life not too short or unstable nuclei decay before radiation destroys tumour 1 1 1 1 3 b beta or gamma 1 4 a too short: radioactive isotope decays too much before scan completed, too long: patient exposed to ionising radiation unnecessarily 1 1 4 b too long: after scan radioisotope needs to be stored for a long time until radioactivity insignificant, too short: radioactive isotope decays too much before scan completed 1 1
- 8. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 8 P7 Radioactivity Student Book answers Question number Answer Marks Guidance 5 a any three from: emits radiation detectable outside body (e.g., gamma), non-toxic, short half-life (1–24 hours), decays into stable isotope 3 5 b stable isotope in body (or elsewhere) not dangerous whereas unstable isotope harmful as it emits ionising radiation harmful to body 1 1 1
- 9. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 9 P7 Radioactivity Student Book answers P7.7 Nuclear fission Question number Answer Marks Guidance 1 a nucleus splits into two fragments, releases energy and several neutrons 1 1 1 b nucleus absorbs neutron without undergoing fission, forms unstable nucleus which decays 1 1 2 (in order) B, A, C, D, B 1 3 a to absorb fission neutrons, and keep chain reaction under control by maintaining even rate of fission 1 1 3 b more fission neutrons absorbed so number of fission neutrons in reactor core decreases, rate of release of energy due to fission ∴ decreases 1 1 3 c thick steel withstands very high temperature and pressure in core, thick concrete walls absorb ionising radiation that escapes through steel walls 1 1 1 4 a A and D 1 4 b undergone fission and released neutrons and energy 1 4 c C and E 1
- 10. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 10 P7 Radioactivity Student Book answers P7.8 Nuclear fusion Question number Answer Marks Guidance 1 a formation of nucleus when two smaller nuclei collide and fuse together 1 1 b forms He 3 2 nucleus (with 2 protons and single neutron) 1 2 a so enough KE to overcome force of repulsion between nuclei so they fuse 1 2 b energy output < energy input so does not produce any energy overall 1 3 advantages any two from: nuclear fusion fuel easily available fusion products non-radioactive (or less radioactive than) fission products fusion stops if plasma out of control, disadvantages any two from: very large current needed to heat plasma to start fusion plasma difficult to control, at present, fission reactors produce far more power than fusion reactors 2 2 4 a 1 p and 1 n 1 4 b He p H 3 2 1 1 2 1 1 4 c p p He He He 1 1 1 1 4 2 3 2 3 2 2
- 11. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 11 P7 Radioactivity Student Book answers P7.9 Nuclear issues Question number Answer Marks Guidance 1 a i hazardous and ∴ a danger to people and animals if it escapes 1 1 a ii contains radioactive isotopes with long half-lives 1 1 b absorbed by surrounding tissues and could damage or kill cells in body or cause cancer outside less dangerous as α radiation has no penetrating power 1 1 1 2 a may be more concentrated than outdoors and people could breathe it in, lungs then exposed to α radiation, ionising effect of α particles in tissue damages cells (or kills cells or causes cancer) 1 1 1 2 b install pipes under house and pump radon gas out of ground before it seeps into house, top of outlet pipe from pump needs to be high up outside house 1 1 3 benefits: any two from: no greenhouse gas emissions, reliable and secure electricity supplies, large-scale generation from small sites compared with renewable supplies that take up much larger areas (or other valid points) drawbacks: any two from: long-term storage of nuclear waste, possible escape of radioactive substances into environment, impracticality of fusion reactors, (or other valid points) 2 2
- 12. © Oxford University Press 2017 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original. 12 P7 Radioactivity Student Book answers Question number Answer Marks Guidance 4 any five from: total annual dose ≈2400 units/year so risk of death about 1 in 10 000 /year, some such as cosmic radiation unavoidable, measures reducing total dose by < 10 units/year (e.g. avoiding air travel) have negligible effect, reducing food and drink unlikely to be effective, may be counterproductive due to adverse effects, e.g., cutting food intake by more than half only reduces annual dose by about 3% (∼0.5 × 140 2400 ) so reduce radiation risk very slightly but would harm human health in most cases, could reduce risk from medical X-rays where possible by restricting use of X-rays and using MRI instead 5