Science notes
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Using a marble run as a model for the electron transport chain and chemiosmosis Paula
Child
Diffusion: a challenging approach for more able students at key stage 4 and above Andy
Markwick and Satvinder Nandhra
Practical ideas to help explain the expanding universe Alan Trusler
Convection currants? Using scientific models to challenge pupils’ understanding Lisa
Tilbury
Using a turntable to measure the force necessary for circular motion and to demonstrate
frequency, amplitude and phase J. C. E. Potter
Using a marble run as a model for the electron transport chain and chemiosmosis
Paula Child
In the final stages of aerobic respiration, the
energy for the phosphorylation of ADP to ATP
comes from the transfer of electrons along a
series of electron carrier proteins (the electron
transport chain), each at a lower energy level
than the previous protein. The energy released is
usually ‘lost’ to the atmosphere as heat energy, but
some is used to actively transport protons from
the mitochondrial matrix into the intermembrane
space and set up an electrochemical gradient
of these H+ ions. As these protons diffuse back
down this gradient, they must pass through
protein channels in the inner membrane of the
mitochondria. Associated with each channel is
the enzyme ATP synthase and, as the protons
pass through this channel, the electrical potential
energy is used to synthesise ATP. This movement
of protons is known as chemiosmosis.
How the process of ATP synthesis via
chemiosmosis is linked to the electron transport
chain can be a difficult concept for A2 students
(17–18 years) to visualise. Some textbooks written
for this stage have diagrams such as Figure 1
that show the series of electron carriers each at
a lower energy level, with the entry points for
NADH and FADH2. However, few, if any, show a
link between the energy released via this electron
transport chain and the pumping of protons to
generate the gradient in chemiosmosis.
The use of a commercially available marble
run (Quercetti; widely available), with only minor
modifications, can help students to understand the
separate processes of the electron transport chain:
how the release of (electrical potential) energy
from the electrons as they are transferred through
a series of carrier proteins can be used to pump
hydrogen ions into the intermembrane space to
produce an electrochemical gradient, and how this
gradient is used to synthesise ATP.
The model
An initial assessment of the model was carried out
by setting up the run and discussing its relevance
and limitations with a colleague. The model used
five bridges, the first two representing the two
hydrogen carriers and the remaining three the
electron carriers within the electron transport
chain. After the fourth bridge, a longer drop
SSR September 2010, 92(338)
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Science notes
Figure 1 Oxidative phosphorylation: the electron transport chain (reproduced with permission from Jones
and Gregory, 2001)
was introduced that allowed the marbles to hit a
see-saw (made by modifying a spinner section;
see Figure 2) and propel ‘protons’ up and into the
funnel. As these ‘protons’ exited the funnel they
passed through a complete spinner that represented
ATP synthase. The marble ‘electron’ carried on
down along a final bridge (cytochrome oxidase),
and the ‘protons’ along a separate channel, before
both falling into the same pot where they would
‘combine’ with oxygen to produce water.
Figure 2 Modification of a spinner unit to create a see-saw
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There were a number of difficulties in building
the see-saw as it was important that the marble
‘electron’ could continue on its path through the
electron carriers, and that the ‘protons’ could be
flicked up into the receiving funnel. A working
see-saw was provided when the students were given
the marble run to evaluate as a revision exercise.
A cocktail stick held in place with cotton
thread was used as the pivot and a cradle made
of paper was added to hold the ‘proton’ in place.
It was also necessary to shorten the see-saw at
the ‘inner’ end and add a paper tab to allow the
marble ‘electrons’ to continue through the system.
Using the model
A group of five students were given the relevant
parts of the marble run as a revision exercise for
the electron transport chain. With a little guidance,
they quickly assembled a five-bridge run to
represent the carrier proteins within the chain.
However, the students had difficulty in linking this
electron transport chain model to chemiosmosis,
which prompted teacher–student discussion as to
how the gravitational potential energy could be
used to represent electrical potential energy and
hence how the see-saw could be used to set up a
proton gradient. They were keen that a different
ball be used to differentiate the electrons and the
protons, and an 8 mm plastic ball was found to
be suitable as it was light enough to be propelled
by the see-saw but heavy enough to turn the
‘ATP synthase’ spinner. The students were also
prompted to discuss the fate of the protons and
electrons, and they then included a pot into which
both dropped, to be combined with oxygen to
produce water. Figure 3 shows their final model.
Science notes
possible to reliably flick the ‘protons’ this high
and, when there was sufficient height, they rarely
ended up in the funnel.
It would have been good to be able to show
that the hydrogen carriers are oxidised by removal
of hydrogen by the first hydrogen carrier, and
that the hydrogens are not ionised until the third
carrier, splitting into electrons that continue along
the electron transport chain and protons that enter
the mitochondrial matrix at this stage. I am sure
that it would be possible to engineer removal of
a marble ‘hydrogen atom’ from an ‘NADH’ or
‘FADH2’ carrier model, and then represent the
subsequent split to H+ and e−.
The greatest drawback of the model is that
the most important product of the process, ATP,
is not physically seen – only the turning of ‘ATP
synthase’ is visible to remind students of this
phosphorylation.
Limitations identified by the students during
the revision session
The students discussed limitations of the model,
which showed that they had a good understanding
of some of the concepts within the electron
Evaluation
General limitations of the model
The major limitation with the model came
from not being able to find a reliable method
of ‘flicking’ the ball representing the protons
high enough for it to be seen to move across
the ‘membrane’ – a limitation that the students
identified soon after they had incorporated the
see-saw. If the bridges represent the carrier
proteins, then the protons ought to be moved from
the level of the lowest bridge to above the top of
the model (into the intermembrane space) before
coming down through the ATP synthase back
into the ‘mitochondrial matrix’. Although several
modifications to the see-saw had been made
before giving the model to the students, it was not
Figure 3 The final marble run model, labelled with
the key components and ending up at the final
‘electron acceptor’ to make water
SSR September 2010, 92(338)
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Science notes
transport chain, particularly the transfer of
electrons between carriers and that each carrier was
at a lower energy level. The marble run kit included
an elevator that they wanted to incorporate but they
decided that it was inappropriate as the process is
not cyclical.
The arrangement of the ‘carrier protein’ bridge
units was mentioned in the discussion and the
students recognised that these might have been
better arranged linearly as this more closely
represented their visualisation of the arrangement
of the proteins within the inner membrane. A
misconception arose when the group spent time
trying to recycle ‘protons’ onto the see-saw, but
they eventually realised that the ‘protons’ needed
to end up at the same point as the ‘electrons’ in
order for water to be produced. They thus settled
on the simplified model shown above. One idea
was to have the marble and another small ball
drop into a beaker of water to remind them of the
final product of the electron transport chain.
When prompted to look for other limitations
of the model, one member of the group wanted
it to show that three protons were required to
pass through the ATP synthase to produce one
molecule of ATP from ADP and Pi and there was
much discussion within the group as to how the
model could be modified to show this.
could propose improvements to the model, and
how working within a group had contributed to
their learning.
All of the students thought that the model was
useful in helping them to visualise the electron
transport chain and how it links to chemiosmosis
and the production of ATP via ATP synthase. They
were able to critically evaluate the model and to
suggest specific improvements. All students used
the correct terminology and also appreciated the
opportunity to work within a group to construct
the model, commenting that working with the
most-able students had helped middle-ability
students to improve their terminology and
understanding, while the most-able students
commented on the physical problem-solving skills
of other members of the group.
Conclusion
Written feedback from the students
The students were asked to fill in a brief
evaluation of this lesson. The form asked for
their opinion of the effectiveness of the model
in improving their understanding of the electron
transport chain and chemiosmosis, how well the
model worked as a demonstration, whether they
Use of an easily available marble run works well
to demonstrate the electron transport chain, how
the energy is used to drive chemiosmosis, and the
link to ATP synthase. The students found that the
model enabled them to visualise the processes
and it reinforced understanding. It also prompted
discussion of aspects of the process that the
model could not represent. The limitations of the
model meant that the students had to recognise
these limitations and use problem-solving skills
to overcome them if this could be done within
the constraints of the kit. The level of discussion,
with concomitant use of vocabulary, within the
group was high and there was strong evidence of
peer teaching. All the students indicated that the
model should be used as a teaching tool for future
classes.
Acknowledgements
Reference
I would like to thank Pam Goody for her input
during the trial phase of model building.
Jones, M. and Gregory, J. (2001). Cambridge Advanced
Sciences Biology 2. Cambridge: Cambridge University
Press.
Paula Child was a biology teacher at Charters Comprehensive School, Sunninghill, Berkshire, when
this work was developed, and is now at Tomlinscote School, Camberley, Surrey.
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Science notes
Diffusion: a challenging approach for more able students at key stage 4 and above
Andy Markwick and Satvinder Nandhra
Diffusion and its applications are encountered by
students at key stages 3 and 4 (ages 11–16) owing
to its importance in many physical, chemical and
biological processes. Examples include gaseous
exchange, digestion, corrosion and rates of
reacting gases. As with many processes that are
usually invisible to the naked eye, understanding
is aided greatly by experiments and models.
Demonstrations such as allowing perfume to
diffuse across a room, or observing hydrogen
chloride and ammonia gases slowly react to
form the familiar white ammonium chloride
solid, are familiar to most science teachers and
are often effective in helping students to grasp
the underlying concepts of diffusion. However,
diffusion rates are generally not quantified. The
following method demonstrates how, by adapting
the familiar ‘hydrochloric acid plus ammonia
in a tube’ demonstration, a highly visual and
quantifiable activity can be created. We also show
how experimental data can be compared with
theoretical predictions.
Comparing experimental data with a model for
diffusion
The sequence in which the following activity
takes place is flexible but we suggest that it begins
with a discussion of a simple model for diffusion,
followed by the diffusion demonstration and
culminating in a comparison between experiment
and model.
Modelling diffusion
Using a simplified kinetic theory model, gases can
be considered as solid spheres that do not interact
with each other, i.e. no intermolecular forces exist
and molecules have an overall random motion
through a vacuum. A simple mathematical model
for gaseous molecules can be obtained as follows.
The kinetic energy of the moving molecules
in a gas (providing what is described as its
internal energy) can be shown to give the
gas its measurable properties of pressure and
temperature. The kinetic energy of a single
molecule can be represented as (1/2)mv².
However, molecules do not all have the same
speed and thus root mean square speed has to
be used in considering the kinetic energy of the
whole gas, and the kelvin temperature T.
The internal energy for 1 mole of gas is
expressed as (3/2)RT, where R is the universal gas
constant. For this work, we can also consider the
internal energy per molecule to be (3/2)(R/N)T or
(3/2)kT, where N is the Avagadro constant and k is
the Boltzmann constant, which can be described
as the universal gas constant per particle, i.e.
k = R/N.
When considering the average energy per
molecule, (1/2)mv² = (3/2)kT, where v is the root
mean square speed and T is the temperature of the
gas.
For two different gases (ammonia and
hydrogen chloride) at the same temperature, the
value (3/2)kT has to be the same for both, i.e.
2
2
= (1 2) mNH3 vNH
. Rearranging this
(1 2) mHCl vHCl
3
yields
mNH3
mHCl
=
vHCl
vNH3
The ratio of masses per molecule is the same
as the ratio of molar masses, and for these two
gases the mass ratio mNH3 mHCl is 17/36.5 = 0.46.
From the above equation, the reciprocal of the
square root of the mass ratio is equal to the ratio
of the root mean square molecular speeds.Thus,
vHCl vNH3 = 0.68. This can be rearranged to give
vHCl = 0.68vNH3
This ratio can now be compared with
experimental diffusion rates.
Experimental procedure
The experimental part of this investigation
requires the ratio of the mean molecular gas
speeds to be obtained. To do this, the rate at which
each gas diffuses must be estimated. Measuring
rates of diffusion through the tube can be easily
achieved by using universal indicator paper
to indicate progress along the tube. Hydrogen
chloride will turn moistened indicator paper red as
it interacts with it and the ammonia gas will turn
the paper blue.
Method
l Clamp a transparent plastic or glass tube (at
least 50 cm in length) at each end.
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Science notes
l Cut universal indicator paper into squares
(1 cm × 1 cm).
l Moisten the universal indicator paper squares
and place them in the tube at equal intervals (3 or
4 cm). Allow at least 4 cm from each end of the tube
openings to the nearest paper. The indicator squares
can be positioned easily using a long glass rod. The
moistened paper should stick well to the tube.
l Carefully soak a ball of cotton wool in
concentrated HCl [corrosive] and another in an
NH3 solution (density 0.88 g cm−3) [corrosive]. The
balls should be of equal size and soaked in equal
volumes of acid and alkali; 5 cm3 is suggested,
although this can be varied as required. It is
recommended that this be carried out in a fume
cupboard (also wear safety goggles and gloves).
l Synchronise the placing of the HCl- and
NH3-soaked cotton wool in each end.
l Start the stopclock and record the times
at which each paper changes colour. Making
different students responsible for this timing will
aid accuracy. Several students could be watching
each end and the average of their recordings
taken. Figure 1 shows the initial experimental
set-up and the result.
(a)
(b)
Health and safety
Concentrated HCl and NH3 should be handled
with care as they are both very corrosive
(CLEAPSS Hazcard 47A and Hazcard 6,
respectively). Goggles and gloves must be
worn and if the experiment is to be carried out
in a small or poorly ventilated laboratory it is
recommended that the demonstration takes place
in a fume cupboard. Students can participate in
setting up the apparatus, including preparing the
indicator paper and measuring the diffusion rates,
but they should not handle the chemicals.
Experimental results
Table 1 presents real data collected from a student
investigation. It can be seen that the averaged
(c)
Figure 1 Stages of the diffusion experiment: (a) initial
set-up showing the arrangement of universal indicator
paper; (b) final student result showing the range of
indicator colours obtained; (c) formation of NH4Cl(s)
Table 1 Data from diffusion experiment (averaged from three runs)
Distance/cm
4
8
12
16
20
14
NH3
HCl
Speed ratio
Time/s
Diffusion rate/cm s−1
Time/s
Diffusion rate/cm s−1
3
7
11
21
48
1.33
1.14
1.09
0.76
0.42
6
16
26
46
107
0.67
0.50
0.46
0.38
0.19
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0.50
0.44
0.42
0.46
0.45
speed ratio is less than the ratio obtained by the
theoretical model. Students might be asked to
analyse their data points more closely to see how
the diffusion rate for each gas changes along the
tube length. Using this data, diffusion rates are
shown to decrease. Again, students might be asked
why this might be so (concentration decreases,
greater volume, increased collisions with air
molecules, etc.).
The data in Table 1 have been plotted in
Figure 2.
There are some interesting points to note:
l the averaged ‘cumulative’ experimental
diffusion speed ratio vHCl vNH3 = 0.45
l the model value for vHCl v NH 3 = 0.68
l from final values in Table 1,
vHCl = 1.9 × 10−3 m s−1 and vNH3 = 4.2 × 10−3 m s−1
(these represent diffusion speeds or rates)
l it is interesting to note that root mean square
speeds of gas molecules at room temperature are
in the range 102–103 m s−1 (NH3 ≈ 550 m s−1 and
HCl ≈ 375 m s−1)
l between distances of 8 and 20 cm, vHCl
decreases by 40% and vNH3 by 62%.
Questions that might be asked in order to
initiate discussion include:
l What might have contributed to the
difference between the model speed ratio and the
experimental value?
l How good is the theoretical model at predicting
diffusion rates? How could it be improved?
Figure 2 Plot of time against distance for the two
gases in the tube diffusing in opposite directions
Science notes
l Why do the diffusion rates slow down as the
gases move through the tube?
l What might affect the accuracy of the data?
l Could it be that the model is useful only for
predicting a ratio? (Errors are cancelled out.)
l Are the data precise? Are the results
anomalous? Are there sufficient readings?
l How might we further test the validity of the
investigation?
l How might the concentration affect the rate of
diffusion of each gas?
l What other factors might influence the
diffusion rates?
It can be seen that the diffusion rate for the
heavier HCl molecules decreases relative to NH3
as the experiment progresses. Students might be
able to offer an explanation. At this stage, they
can be asked to think about the assumptions that
have been made in the model and whether these
are realistic in view of the experimental data and
whether they apply equally for each gas.
Students should consider the significant
difference between the molecular speeds and
the rates of diffusion (molecular speeds are far
greater than rates of diffusion as molecules tend
to zig-zag in the tube, colliding with each other
and with molecules in the air). A useful activity
is the ‘random walk’ model (Box 1), which
demonstrates how random collisions affect the
path of the diffusing molecules.
Deepening student understanding through
formula interrogation
The diffusion equation can be manipulated to
obtain an expression for temperature that can
then be used to calculate temperatures from the
individual gas speeds.
Using either v = 3kt m or v = 3RT M ,
where R is the universal gas constant
(8.314 J K−1 mol−1) and M is the mass of 1 mole of
gas, by rearranging we can obtain T = v 2 m 3k
for one molecule or T = v 2 M 3R for 1 mole of
gas molecules.
From their data, students could consider how
appropriate it is to calculate temperature from
gaseous diffusion experiments: can they explain
why the results are in poor agreement? (Hint:
The assumptions state that the gases are rigid
spheres that do not interact and diffusion occurs
in a vacuum, i.e. no collisions with gas molecules
occur. What other assumptions are being made by
using these formulae?)
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Science notes
BOX 1 Method for the random walk model
l Provide students with some graph paper and
a die.
l Ask students to place a dot in the centre of
one of the boxes.
l Explain that they must throw the die to move
the dot, but they must move the dot as follows:
1 = move a square up the paper; 2 = move
a square down; 3 = move a square right;
4 = move a square left.
l If a student throws a 5 or a 6, they must
repeat the throw. (If a three-dimensional
model is used, a 5 would represent ‘out of
the paper’ and a 6 ‘into the paper’.)
Figure 3 shows the outcome from two separate
two-dimensional games.
offers a good introduction to the use of scientific
models and shows how the assumptions we make
can limit the confidence we might have in our
interpretations.
Other considerations
The root mean square is not the same as the mean.
The mean of 6 and 8 is 7 but the root mean square
value is
( 6 2 + 82 )
2 = 50 = 7.07
This is a simple way to illustrate that the root
mean square is dominated by larger values. You
might persuade students to try all the numbers
from 1 to 10 (mean 5.5; root mean square 6.2).
Could this affect the modelling? NH3 will have
more high-speed molecules than HCl.
There are more air molecules (principally N2
and O2) in the tube than NH3 or HCl. Compared
with N2 and O2, each HCl molecule has greater
mass, but each NH3 molecule has smaller mass.
The lighter molecules will in general be moving
faster but have smaller momentum, resulting
in greater rebound when colliding with air
molecules. Perhaps air in the tube inhibits the
movement of NH3 more than HCl.
Other possible experiments that could be
explored
l Observe diffusion starting with the same gas at
each end.
l Observe diffusion of each gas independently
using the same method but placing just one sample
of cotton wool in one end of the closed tube.
l Observe for comparison the use of each gas
independently, with the far end of the tube open
(carrying out this experiment with the gases
concerned is not recommended but, if attempted,
the use of a fume cupboard is essential).
In conclusion
Figure 3 Outcomes from two random walk
experiments
This demonstration not only provides a visual
insight into the process of diffusion and challenges
students to consider a range of factors that might
affect diffusion rates, but the comparison of
experimental data with the diffusion model also
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Unlike many school demonstrations, this experiment
leaves most questions unanswered. However, it
does illustrate that in science there are often more
variables than we can control and that simple
models do not always match experimental results.
Acknowledgements
We thank Felix Ogbogoh, Maria Dudys and Tshidiso
Lehare at Prendergast Ladywell Fields College for
their helpful comments. Geoff Auty’s perseverance
and knowledgeable input towards the completion of
this article is gratefully acknowledged.
Science notes
Further information
Breithaupt, J. (1995) Understanding physics for advanced
level. Cheltenham: Stanley Thornes. pp. 130–132.
Diffusion in gases: yteach.co.uk/index.php/resources/
diffusion_process_gas_liguid_disolving_state_
dissolution_mixing_gas_liquid_solid_page_0.html
Fick’s law of diffusion: en.wikipedia.org/wiki/Fick%27s_
law_of_diffusion
Andy Markwick is an AST (T&L) and lead G&T teacher at Prendergast Ladywell Fields College,
Brockley, London. Email: [email protected]
Satvinder Nandhra is a year 5 teacher at Stillness Junior School, Brockley, London.
Practical ideas to help explain the expanding universe
Alan Trusler
These three simple practical ideas for use in
the school laboratory can help to explain the
expanding universe. They were developed for
teaching AQA modular science P1B, Radiation
and the universe, and will be useful for other
examination specifications.
Emission lines
It is not possible in a school laboratory to show
absorption lines in a spectrum and the equipment
available is not usually sensitive enough to show
the lines in the solar spectrum. Fortunately, it
is possible to show emission lines from various
sources within the laboratory in a simple way
and to display them effectively to a class. Pupils
quickly make the connection that each element
has its own unique spectrum and, more often than
not, will use the term ‘the element’s barcode’.
Figure 1 The set-up of source, diffraction grating
spectroscope and CCTV camera; the camera is
normally up against the spectroscope
I start with a hand-held mercury vapour
fluorescent tube and elicit from the pupils the fact
that the white light contains more information than
they can see. At key stage 3 (ages 11–14), they
learn that if white light is passed through a prism,
it can be split up into the colours of the visible
light spectrum. At key stage 4 (ages 14–16), there
is still more information that can be gathered
from the white light. The tube is held in front of
a diffraction grating spectroscope (I use one from
Griffin & George) with a small CCTV camera at
the viewing end (Figure 1). The camera is linked
up to the laboratory television screen, producing a
wonderfully large emission spectrum (Figure 2).
We have two CCTV cameras available in
the laboratory for teaching purposes. One is a
very useful general-purpose colour camera by
Micromark, purchased from B&Q for about £40.
It makes it very simple to demonstrate optical
experiments, how to lay out the axes on a graph,
Figure 2 Emission spectrum from the hand-held
fluorescent lamp (mercury source)
SSR September 2010, 92(338)
17
Science notes
Figure 3 Emission spectrum of sodium
Figure 4 Emission spectrum of neon
or anything else where a class of 32 pupils needs
to see – far easier than having them crowded
around my teaching bench. The other CCTV
colour camera can be focused, and was designed
to project a magnified image from a microscope
on to a television screen. There are two available,
TeachCam or FlexCam, priced about £450 and
£230, respectively.
If a piece of A4 white paper is put against the
television screen (held in place by electrostatics –
good teaching point) and the major emission lines
marked, the paper can be moved from side to side
to explain the concept of red and blue shift of light
with regard to the recession or approach of distant
galaxies and the consequent increase or decrease
in the wavelength of light.
Twinkle twinkle little star
The distortion of light by the thermal disturbance
of the atmosphere can be shown using a
low-power laser and the yellow flame from a
Bunsen burner. The necessary precautions are
taken when using the laser, which is part of an
infrared thermometer that I use in the laboratory.
The beam is projected across the laboratory
onto a white paper screen. A small CCTV camera
Example of a timeline
Years ago
Exemplar events
Today –10
arrive at Grays Convent School
10–100
man on the Moon, first television, first aeroplanes
100–1000
Newton, Galileo
1000–10 000
Jesus Christ, Roman Empire, Stonehenge
10 000–100 000
end of last Ice Age, humans start to colonise other continents
100 000–1 million
humans start to look as we do today
1–10 million
mammoths appear in the fossil record, hominoids evolve
10–100 million
grasses evolve (50 million), dinosaurs die out owing to probable asteroid impact
(63 million)
100 million – 1 billion
first animals on land (350 million), Cambrian explosion of multicellular life
(550 million), Snowball Earth (650/750 million)
1–10 billion
fossil remains of microorganisms found in Australia (3.5 billion), first simple DNA
cells appear (4 billion), formation of solar system (4.5 billion), supernova triggers
collapse of the future solar system’s dust and gas cloud (5 billion), formation of
Milky Way galaxy (10 billion)
10 billion – Big Bang
first stars evolve, elements beyond hydrogen and helium are built in the cores
of the new stars
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Science notes
is focused onto the screen and the image displayed
on the laboratory television screen. At first, the
spot of light is very stable but when the yellow
Bunsen flame is put in the way of the beam near
to the light source (Figure 5), the distortion can be
seen very clearly on the television screen as the
spot bounces about on the screen.
Timeline for the ‘Big Bang’
I find that my 15- to 16-year-old GCSE pupils
have difficulty trying to imagine the long
‘distance’ back in time to the Big Bang and in
using ‘10 to the power of ...’. To help in both these
aspects and to give them a better understanding of
the past, I use a timeline that starts at the present
and ends with the Big Bang, 15 billion years ago.
I use a 7 cm wide strip of card, 64 cm long,
cut from a sheet of A2 card. It is folded into
sections, 6 cm long. On the last 6 cm fold I write
10 billion years ago and then the further small
section takes it up to the Big Bang.
From the first section, each successive fold
is the previous fold × 10. Within each section,
something relevant can be written, as illustrated in
the panel.
Figure 5 Handheld (infrared thermometer) laser source
with Bunsen flame, directed to a screen 6 m away
Alan Trusler teaches at Grays Convent High School for Girls, Essex.
Convection currants? Using scientific models to challenge pupils’
understanding
Lisa Tilbury
This is a brief description of how I used a simple
demonstration with currants and lemonade to
model convection currents involved in heat
transfer.
The initial idea
Many of us will have shown pupils the effect of
putting a few currants (or raisins) into a beaker
of lemonade. At first they sink to the bottom, but
as more and more bubbles collect on them, the
currants rise up to float on the top. At the top, the
bubbles burst and so the currants sink again.
I recently used this idea with a group of
year 9 pupils (ages 13–14) to challenge their
knowledge of convection. I presented it as a
model of convection and asked them to evaluate
its effectiveness.
Figure 1 An example of a poster produced by
groups of children during this activity
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Science notes
Setting the scene
Initially, I recapped convection with pupils. I
asked them to note down the important keywords
and phrases relevant to describing the transfer of
heat energy by convection. They shared their ideas
and we put them up on the board.
I summarised by saying that a scientific model
is something that helps us understand what is
really happening. For example, It might help
us visualise something we can’t see, it might
be a story (‘It’s a bit like ...’) or it might be a
diagram or physical model. I had a ‘lava lamp’
(see end-note) out as an example of a model of
convection they had seen before, and I showed
them a simple animation of convection from the
Internet by searching for ‘convection animation’
(see Websites).
Pupil group work
My next step was to explain the model of
convection they were going to use. I grouped
pupils to ensure a mix of ability and gave each
group a picture of the demonstration to annotate
– using the information from the board – to help
them explain how the model is like convection. For
extension work, pupils discussed how they might
improve on the model, or design a different model.
The mixed-ability groupings allowed peer
teaching to take place. Pupils were also given
the opportunity to look at the work of other
groups, which they could then feed back into their
own posters. Some examples of the work they
produced are shown in Figure 1–3.
Identifying misconceptions
Misconceptions that would otherwise have
been missed were identified through the peerlearning discussions. One very able pupil thought
that particles expanded and contracted during
convection, a common misconception, and one
that I previously believed I had eliminated.
Another misconception identified was the use
of the term ‘energy particle’, mainly with lowerability pupils. The model allowed students to see
Figure 2 Another example of a poster
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Science notes
that the particles (currants) were separate from the
energy (represented here by bubbles). Being able
to ‘see’ the energy gained by the particles really
helped these pupils move their learning forward.
eventually float to the top. In a heated fluid, the
decrease in density is caused by the particles
taking up more space, making the heated fluid less
dense than its surroundings and causing it to rise.
Limitations of the model
Conclusion
Discussing the limitations of this, and any
model, is an important part of the learning
process. Challenging pupils to consider the
limitations of a model will help to develop their
understanding further. In this model, viewing the
energy (bubbles) as something that can simply
be collected and dropped off is not accurate,
and we must be careful not to embed a new
misconception. To avoid this, discussions should
be extended to the kinetic energy of the particles.
It should be stressed that the gain in energy of the
particles in a heated fluid is due to them moving
faster and therefore increasing their kinetic energy.
The currants in the model gain bubbles of
carbon dioxide, causing the total density (of
currants and gas together) to decrease until they
This is the second year I have tried this activity
in this way. Of the 90 pupils I used it with this
year, almost all said they understood convection
much better following their evaluation of the
‘convection currants’ model.
I think this will now become one of those
lessons I use and refine year after year.
End-note
‘Lava lamp’ is a popular name given to a kind
of table lamp which has been in and out of
fashion. Above the light source, an enclosed glass
tube contains water and oil of similar density
but different expansivity. The two liquids are
coloured with different dyes. When cold, the oil
is at the bottom of the tube beneath the water.
Figure 3 Another example of a poster
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Science notes
When the light is on, the fluids are heated and the
oil becomes less dense than water so that large
coloured globules float slowly to the top. When
they cool at the top of the lamp, their density
increases and the globules sink again.
Websites
Convection animations: www.edumedia-sciences.com/en/
a639-thermal-convection; www.echalk.co.uk/Science/
physics/convection/convection.html
Lisa Tilbury is head of science at St Aidan’s CE Technology College, Poulton-le-Fylde, Lancashire.
Email: [email protected]
Using a turntable to measure the force necessary for circular motion and to
demonstrate frequency, amplitude and phase
J. C. E. Potter
The article on circular motion in the September
issue of SSR (Reigosa, 2009) reminded me of a class
experiment I devised, at Temple Moor High School
in Leeds, to verify the equation F = mrω². A disc of
plywood had a central hole to fit on a turntable. A
spring balance was centrally clamped over it from a
retort stand gantry and a strong fishing line connected
to the spring balance via two fishing line swivels to
prevent line twisting (Figure 1). A figure-of-eight
knot was used to secure the high-breaking-strain line
(Figure 2). A 2″ (50 mm) pulley was screwed to the
turntable so that the fishing line was vertically over
the centre when threaded through the pulley. The
line was then connected to a brass roller (as used
in inclined plane experiments). This arrangement
enabled the spring balance (force F) reading to be
taken during rotations.
5 N spring
balance
swivel
fishing line
ω
turntable approximately 70 cm diameter with pulley
edge centrally mounted
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Figure 1 Assembled
apparatus for measuring
force during circular motion
Science notes
Figure 2 Figure-of-eight knot attached to swivel
A number of revolutions were timed by
stopwatch to measure the angular velocity:
w=
2p ( number of revolutions)
time taken
The distance r of the roller axis from the centre
during rotation was found by pulling the roller when
stationary until the spring balance reading was the
same as during rotation. The mass m of the roller
was known. Readings of F were taken at various
revolutions to compare them with the product mrω².
To remember the formula F = mrω², I
mentioned ‘Mr Omega squared’, a newspaper
headline about a corrupt forceful Greek man, Mr
Omega, who had been bribed, or, to use a colloquial
expression, ‘squared’. (Oxford English Dictionary:
pay, especially bribe, as in ‘he has been squared to
hold his tongue’; secure acquiescence.)
I have no record of any actual readings.
I think the Philip Harris roller had a mass of
0.17 kg. The revolutions were restricted to 33, 45
and 78 revolutions per minute because a recordplayer turntable was used. The plywood disc
was about 70 cm in diameter. The spring balance
probably read up to 5 N. The 2″ pulley was also
from Philip Harris.
Another use for the turntable is to demonstrate
amplitude, phase and frequency (Figure 3).
Dowels of diameter ⅜″ (10 mm) were inserted
in strategically placed tight-fit holes drilled in the
plywood disc. Shadows of the moving dowels were
cast by a projector onto a wall. Dowels at various
distances from the disc centre portray different
amplitudes. Dowels subtending different angles at
the centre indicate phase. Different lengths of dowel
help comparisons. The rotation rate demonstrates
frequency. Rotate slowly by hand initially.
A simple set-up would be a cardboard ‘record’
(or indeed an old ‘78’) with vertical straws stuck
on using Blu-Tack.
The demonstration is useful when teaching
alternating current phase differences. Label one
dowel ‘R’ to represent resistance, for which current
is in phase with the voltage. Label two others ‘L’
and ‘C’ placed out of phase, each to show a 90°
current phase difference to the voltage. (Or ask
which dowel represents inductive current and
which represents capacitive current). ‘Be CIVIL’
is useful to remember that, for a capacitor, current
I leads voltage V and, for an inductance L, the
current I lags voltage V. For this demonstration,
also rotate the disc slowly by hand initially.
Reference
Reigosa, C. (2009) A challenge for students to design an
experiment to measure force in circular motion. School
Science Review, 91(334), 87–91.
ω
Figure 3 Layout for
demonstration of
frequency, amplitude and
phase (for alternating
current and other uses)
John Potter was head of science at Talbot Heath School, Bournemouth, until his retirement in 1985.
Email: [email protected]
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