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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
ONE
(a)
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.
Term “nucleus” not accepted.
Identification of negative charges /
electron and positively charged sphere
/ cloud.
Identification of negative charges
/ electrons and positive charge.
AND
The atom as a whole is neutral.
(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
straight through.
OR
A very small number / a few bounced
back / were reflected.
Most of the alpha particles went
straight through.
AND
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.
OR to direct the alpha particles
OR ensure alpha hits perpendicular to gold foil
OR to avoid stray alpha particles affecting the results
Correct statement.
Excellence
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”).
OR
Alpha becomes a helium atom / alpha particles ionise
the air.
Correct statement.
(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”
Correct statement.
TWO
(a)
Alpha particles are deflected up the page.
Beta particles are deflected down the page.
Gamma rays go straight through.
Correct diagrams showing alpha going
up, beta going down and gamma going
straight through.
OR
Incorrect diagrams showing alpha
going down, beta going up and gamma
going straight through AND Beta
curving more than alpha.
Correct diagrams showing alpha
going up, beta going down and
gamma going straight through.
AND
Beta curving more than alpha.
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
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.
ONE correct reason for alpha
OR
ONE correct reason for beta.
OR
Correct comparison of alpha and beta
based upon opposite charges.
OR
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.
NCEA Level 2 Physics (90256) 2011 — page 4 of 5
(c)
Alpha:
• Alpha particles are helium nuclei / two protons and
two neutrons.
• 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:
THREE of nine descriptions are
correct.
• 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
(a)
(b)
226
222
4
88 Ra ® 86 Rn + 2 a
(allow a as well as α)
The time taken for half the (number of) radioactive
nuclei / atoms to decay.
OR the time for the rate of decay to halve.
OR the time for the activity / count rate to halve
OR the time taken for half the radioactive mass to
decay*
OR the time taken for half the radioactive sample to
decay*
OR the time taken for half the radioactive substance to
decay*
(OR similar but cannot accept any of these * type of
answer without the term radioactive)
The time taken for radioactive nuclide not accepted.
Correct answer including numbers for
Alpha.
Correct answer.
SIX of nine descriptions are
correct.
EIGHT of 9 descriptions are correct.
NCEA Level 2 Physics (90256) 2011 — page 5 of 5
(c)(i)
(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.)
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
OR
AND
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).
An attempt at an Exponential curve
is made
AND
Answer matches graph.
(Construction lines preferred but not
required.)
Judgement Statement
Achievement
Achievement with Merit
Achievement with Excellence
5A
3M+6A
2E+2M+6A