Assessment Schedule 2011 Physics: Demonstrate understanding of atoms and radioactivity (90256)

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1 NCEA Level 2 Physics (90256) 2011 page 1 of 5 Assessment Schedule 2011 Physics: Demonstrate understanding of atoms and radioactivity (90256) Evidence Statement Q Evidence Achievement Merit Excellence ONE Thomson s model had a number of negative particles / corpuscles / electrons embedded in a (solid) positive sphere / cloud. Cloud of particles or energy not accepted. The atom as a whole is neutral because the charges are balanced. Identification of negative charges / electron and positively charged sphere / cloud. Identification of negative charges / electrons and positive charge. The atom as a whole is neutral. Term nucleus not accepted. (b)(i) Most of the alpha particles went straight through and a very small number / a few bounced back / were reflected. Most of the alpha particles went Most of the alpha particles went A very small number / a few bounced back / were reflected. A very small number / a few bounced back / were reflected. (b)(ii) Some of the alpha particles were deflected ( deflection is an observation, repulsion is not). Some / few of the alpha particles were deflected. (c)(i) Tube: To provide a narrow (or collimated) beam of alpha particle. to direct the alpha particles ensure alpha hits perpendicular to gold foil to avoid stray alpha particles affecting the results

2 NCEA Level 2 Physics (90256) 2011 page 2 of 5 (c)(ii) Vacuum chamber Alpha particles would not travel far / be stopped / lose energy / path would change / be deviated / collide with the air particles (cannot accept affect, disrupt, react, interfere ). Alpha becomes a helium atom / alpha particles ionise the air. (c)(iii) Alpha particles are not visible to the naked eye. The ZnS coating will show / glow / fluoresce / flash / give off light when alpha particles strike it (this may be implied as opposed to directly stated) but do not allow detected or located TWO Alpha particles are deflected up the page. Beta particles are deflected down the page. Gamma rays go Correct diagrams showing alpha going up, beta going down and gamma going Correct diagrams showing alpha going up, beta going down and gamma going Incorrect diagrams showing alpha going down, beta going up and gamma going straight through Beta curving more than alpha. Beta curving more than alpha.

3 NCEA Level 2 Physics (90256) 2011 page 3 of 5 (b) Alpha: Alpha goes up because: Flemings left hand rule (or other suitable hand rule) predicts this Alpha has a small / smaller deflection because Alpha has much greater mass (8000 compared with beta) so it affected less by the force produced by the magnetic field. Alpha has a much smaller charge / mass ratio (x 1 / 4000 compared with beta) and so is affected less by the force produced by the magnetic field. Beta: Beta changes direction because Flemings left hand rule (or other suitable hand rule) predicts this ONE correct reason for alpha ONE correct reason for beta. Correct comparison of alpha and beta based upon opposite charges. The correct reason for gamma. TWO correct reasons including two of: ONE correct reason for alpha. ONE correct reason for beta. Correct comparison of alpha. and beta based upon opposite charges. The correct reason for gamma. Correct reasons for all THREE radiations including all of: Correct reason for direction for alpha. Correct reason for direction for beta (or correct comparison of alpha and beta based upon opposite charges). Correct reason for shape for alpha. Correct reason for shape for beta (or correct comparison of alpha and beta based upon mass or charge / mass). Correct reason for gamma. Beta has a large / larger deflection because Beta has much smaller mass (1 / 8000 compared with alpha) so it affected more by the force produced by the magnetic field. Beta has a much larger charge / mass ratio (x 4000 compared with alpha) so it affected more by the force produced by the magnetic field. An acceptable alternative for alpha and beta: Positive particles are deflected up and negative particles are deflected downwards by the magnetic field (because of opposite charge). Gamma: Gamma does not change direction / it is unaffected because gamma rays have no charge / neutral so are unaffected by a magnetic field. Direction does not change / it is unaffected by electric field not accepted.

4 NCEA Level 2 Physics (90256) 2011 page 4 of 5 (c) Alpha: Alpha particles are helium nuclei / two protons and two neutrons. THREE of nine descriptions are correct. SIX of nine descriptions are correct. EIGHT of 9 descriptions are correct. During alpha decay, the nucleus loses two protons and two neutrons (this may be implied from above) The atomic number of the nucleus decreases by two and its mass number decreases by four. Beta: Beta particles are electrons (from the nucleus). During beta decay a neutron changes to a proton and an electron and the electron is emitted. The atomic number of the nucleus increases by one, and its mass number stays the same. Gamma: Gamma is electromagnetic radiation / waves / rays / photons. During gamma decay, energy is released from the nucleus. The atomic number and mass number of the nucleus are not affected / stay the same. THREE Ra! Rn " (allow a as well as α) Correct answer including numbers for Alpha. (b) The time taken for half the (number of) radioactive nuclei / atoms to decay. the time for the rate of decay to halve. the time for the activity / count rate to halve the time taken for half the radioactive mass to the time taken for half the radioactive sample to the time taken for half the radioactive substance to ( similar but cannot accept any of these * type of answer without the term radioactive) The time taken for radioactive nuclide not accepted. Correct answer.

NCEA Level 2 Physics (90256) 2011 page 5 of 5 (c)(i) Points plotted on graph: 8 days = 20 16 days = 10 24 days = 5 Points plotted on graph: 8 days = 20 16 days = 10 24 days = 5 Incorrect Exponential

Accurate exponential curve drawn (Joining the dots with a ruled line or joining the dots freehand with multiple curves not accepted).

5 NCEA Level 2 Physics (90256) 2011 page 5 of 5 (c)(i) Points plotted on graph: 8 days = days = days = 5 Points plotted on graph: 8 days = days = days = 5 Incorrect Exponential curve drawn using incorrect points (do not accept joining the dots with a ruled line) BUT must not cross x-axis. Accurate exponential curve drawn (Joining the dots with a ruled line or joining the dots freehand with multiple curves not accepted). (ii) Correct value for cps after 20 days is between 6 8 cps based on graph. (A mathematical solution cannot be accepted as evidence but is neutral.) An attempt at an Exponential curve is made Answer matches graph. (Construction lines preferred but not required.) Judgement Statement Achievement Achievement with Merit Achievement with Excellence 5 A 3 M + 6 A 2 E + 2 M + 6 A

Radioactivity. (b) Fig shows two samples of the same radioactive substance. The substance emits β-particles. Fig. 12.1

Radioactivity. (b) Fig shows two samples of the same radioactive substance. The substance emits β-particles. Fig. 12.1 112 (a) What is meant by radioactive decay? Radioactivity [2] (b) Fig. 12.1 shows two samples of the same radioactive substance. The substance emits β-particles. Fig. 12.1 Put a tick alongside any of the