Download Opening Address by Prof Dick Gunstone

Exploring student difficulties with mechanics
and electricity
Richard Gunstone (Monash University)
Pamela Mulhall (University of Melbourne)
Brian McKittrick (formerly Monash University)
The support of Australian Research Council Large Grants A00104120 and A79800528 is gratefully acknowledged
www.monash.edu.au
THE PURPOSES OF THE RESEARCH
ON WHICH THIS TALK IS BASED
1.
To explore in greater detail some of the learning difficulties we already knew
students had in mechanics and DC electricity,
areas chosen because both are conceptually abstract but in different ways  both involve difficult relationships between concepts and have
disarmingly simple formulae that often “hide” the conceptual difficulty in
these relationships, but
 DC electricity is, put bluntly, much less well understood (by students and
others – including textbook writers)
2.
To explore how reasonable is the common view that alternative teaching
approaches that seek student understanding are just too time intensive to
be viable in senior high school physics
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SOME BACKGROUND
 Already a huge amount of research on student difficulties (alternative
conceptions, conceptual change), and this used in quite a lot teaching
approaches for developing greater student understanding,….
 And lots of closely related research on the impact on students’ understanding
of the conceptions they [students] have of what teaching is, what learning is,
and the appropriate roles they believe teachers and learners should have
 Physics a particular curriculum emphasis for these studies
 Dense senior high school curricula that demand rapid “coverage” mean new
teaching and learning approaches are often seen to “take too much time”
 The existence of a high-stakes external examination is powerful
reinforcement of these beliefs of time and coverage – strong pressure comes
from students, parents, school administration,........
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THE CONTEXT OF THE RESEARCH
 The research was conducted in Victorian schools, in Year 11 physics classrooms
 All teachers were of course informed volunteers (usual ethical requirements
followed)
 Student learning outcomes were considered in terms of
(i) understanding and
(ii) more common question types found on the Year 12 external exams
(but of course at levels appropriate for Year 11 students)
We called the questions used for each (i) “conceptual” and (ii) “traditional”
So 4 types of question
- mechanics: “conceptual” and “traditional”
- electricity: “conceptual” and “traditional”
all given via pencil-and-paper tests
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An example question – mechanics “conceptual”
(a) Sarah is playing on a trampoline.
In this diagram she is MOVING
UPWARDS.
(i)
On the diagram draw and label
all the forces on Sarah.
(ii) Explain, in terms of the forces on
her, why Sarah is slowing down
(b) Now Sarah has fallen again and hit the
trampoline. She has stretched the
trampoline but is STILL MOVING
DOWNWARDS.
On the diagram opposite draw and label
all the forces on Sarah.
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An example question – mechanics “traditional”
A car of mass 800 kg is towed along a straight road so that its velocity changes
uniformly from 10 m/s to 20 m/s in a distance of 200 m. The frictional force
acting on the car is constant at 500 N.
a) What is the acceleration of the car?
b) What is the magnitude of the net force on the car?
c) What is the magnitude of the force exerted on the car by the towing vehicle?
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An example question – DC electricity “conceptual”
Two ammeters, A1 and A2, are connected in series with a cell and a variable
resistor.
A1
A2
The resistance of the variable resistor is INCREASED.
(a) (i)
Will the reading on ammeter A1 increase, decrease or remain unchanged?
(ii) Explain your answer.
(b) (i)
Will the reading on ammeter A2 increase, decrease or remain
unchanged?
(ii) Explain your answer.
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An example question – DC electricity “traditional”
A 9 V battery is connected to a resistor producing a current of 3 A.
One point in the circuit is marked X. 9 V
X
3A
(a)
(b)
Calculate the amount of charge passing point X in 10 s.
How many joules of energy is given to each coulomb of charge as it passes
through the battery?
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THE CONTEXT OF THE RESEARCH
 Both of our research purposes meant it was crucial to have the classrooms
involved in the research as unaffected as possible by the research –
therefore:
• all classes in the study were always taught by their usual Yr 11 physics
teacher, who was not in any way asked to teach anything differently to usual
• as far as we could ensure, students in the classes were not aware of their
involvement in a research study
• the questions used to assess student learning were given as tests, with the
tests administered by the usual class teacher in a manner that made them as
‘real’ for the students as we could ensure (the questions were given as part of
what the students knew to “count”)
• the questions used in these tests were constructed and selected by some of
the teachers involved in the research working with us, so the teachers were
consistently indicating whether or not they saw particular questions as
“conceptual” or “traditional” or neither.
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In the rest of the talk - - 1. The first research purpose (learning difficulties in mechanics & electricity)
 A sense of the sort of information we have by looking at some of the
detail from the trampoline questions
 A summary of what we found from the mechanics questions
 A summary of what we found from the electricity questions
2. The second research purpose (does teaching for understanding take TOO
much time?)
 Comparing data from the two groups of classes (conceptual and
traditional)
 Some teaching implications
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RESEARCH PURPOSE 1: “To explore in greater detail learning difficulties we
already knew students had in the two areas of mechanics and DC electricity”
 The “trampoline” question
(a) Sarah moving up - draw force diagram and explain why she is slowing down,
(b) Sarah landed again, still moving down - draw force diagram:
How we classified common forces given in student answers –
Typical labels given by students
Our interpretation of nature of force
given in student response
Force of trampoline / Force of jump /
Force of rebound / Force of momentum
Force of trampoline on Sarah propelling
her through the air
Force of planet Earth on Sarah arising
from gravitational interaction between
Earth and Sarah
Force of gravity / Weight / Gravity
Air resistance / Friction from air / Force
of air
Force of air on Sarah resisting her
motion through the air
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Trampoline question [cont.]
Answers we judged correct:
part (a)
(i) (force diag) (ii) (explain)
part (b)
(force diag)
all students
22%
52%
61%
conceptual
41%
69%
63%
traditional
12%
44%
60%
Answers including particular forces in part (a)(i) (moving up & slowing down)
Weight
Air resistance
‘Propelling force’
(regardless of direction)
all students
88%
60%
54%
conceptual
90%
69%
31%
traditional
87%
55%
65%
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Trampoline question [cont.]
 Just over half of the students believed there was an upwards force on Sarah
from the trampoline as she moved through the air, even though there was no
contact between trampoline and Sarah – the common “throw” force that is
often seen to be what “makes” a ball keep going though the air.
Sarah is slowing down because as she travels the
downwards forces eat away at the force of the
trampoline causing her speed to decrease.
 Just under 10% had difficulty distinguishing force and energy
Sarah is slowing down due to the gravitational force
pushing down on her, also her upwards movement
caused by kinetic energy is converting into potential
energy which will cause her to fall
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Summary of what we conclude about student understanding
– from all MECHANICS questions
1. Often an inability to describe and/or recognise the scientific view of force
– aalack of the fundamental scientific view of always two forces resulting from
an interaction between two objects; a lot of use of vague (often unhelpful) terms
such as ‘friction’, ‘gravity’ and ‘weight’ part of this.
2. Often an inability to represent or interpret the physicist’s accepted
diagrammatic representation of forces
3. A widely held view of a ‘propelling’ force, seen as an essential force on
an object if it is moving
4. A lack of understanding of the concept net force, and of its use.
5. An inability to identify the physics understanding common to slightly
different but essentially similar contextual situations
6. The limitations of seeing a modelled situation as representative of the
real world equivalent
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Summary of what we conclude about student understanding
– from all ELECTRICITY questions
 Understanding of electric circuits is patchy – for about half the questions less
than 50% gave a correct answer (regardless of correctness of reasons)
 It was common for students who gave correct answers to not be able to give any
reasons other than a formula for their answer – there was a very strong reliance
on algorithms to both answer and “explain”
 There was almost no use of metaphors, models or analogies in
explanations, (and the few we saw ALL came from one class).
 Ambiguity about the meaning of ‘voltage’ and related problems of cause in
electric circuits was central to much of the poor understanding – not just “what
voltage is” but also how voltage links with models for movement of charge and
energy (and that extends beyond students - textbooks & curriculum documents)
 Students lack the RANGE of models, metaphors and analogies that are helpful
understanding and predicting behaviours of electric circuits (especially those that
link mechanical and electrical energy changes, and those involving electric field)
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RESEARCH PURPOSE 2: “Are alternative teaching approaches that seek student
understanding too time intensive to be viable in senior high school physics?”
Some further detail about the classes involved in the research:
 In seeking teachers willing to be involved we looked for either
(i) teachers with a focus on student understanding in their classrooms, or
(ii) teachers who focussed much more on traditional approaches to teaching
how to solve standard [exam-type] problems
This was so we could compare student learning outcomes across the 2 types of
teaching.
 ALL teachers who volunteered were interviewed and their classrooms
observed, in order to determine whether or not they were valid members of the
group we had seen them in (were they really “conceptual” or “traditional”?)
Some teachers were omitted but none was moved to the other group because
of this process – our judgment was made on the criterion of demonstrating that
they really were members of one group; they were not just allocated to one of
the 2 groups
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An example question – mechanics “traditional”
(a) Sarah is playing on a trampoline.
In this diagram she is MOVING
UPWARDS.
(i)
On the diagram draw and label
all the forces on Sarah.
(ii) Explain, in terms of the forces on
her, why Sarah is slowing down
(b) Now Sarah has fallen again and hit the
trampoline. She has stretched the
trampoline but is STILL MOVING
DOWNWARDS.
On the diagram opposite draw and label
all the forces on Sarah.
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Trampoline question [cont.]
Answers we judged correct:
part (a)
(i) (force diag) (ii) (explain)
part (b)
(force diag)
all students
22%
52%
61%
conceptual
41%
69%
63%
traditional
12%
44%
60%
Answers including particular forces in part (a)(i) (moving up & slowing down)
Weight
Air resistance
‘Propelling force’
(regardless of direction)
all students
88%
60%
54%
conceptual
90%
69%
31%
traditional
87%
55%
65%
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Trampoline question [cont.]
Answers we judged correct:
part (a)
(i) (force diag) (ii) (explain)
part (b)
(force diag)
all students
22%
52%
61%
conceptual
41%
69%
63%
traditional
12%
44%
60%
Answers including particular forces in part (a)(i) (moving up & slowing down)
Weight
Air resistance
‘Propelling force’
(regardless of direction)
all students
88%
60%
54%
conceptual
90%
69%
31%
traditional
87%
55%
65%
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A SUMMARY OF DIFFERENCES BETWEEN
THE TWO GROUPS OF CLASSES
1. Mechanics – conceptual
questions
Conceptual classes very clearly
better (trampoline qn.
differences are quite typical)
2. Mechanics – traditional
questions
Conceptual classes better
3. Electricity – conceptual
questions
Conceptual classes clearly
better
4. Electricity – traditional
questions
Conceptual classes perhaps
marginally better
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A SUMMARY OF DIFFERENCES BETWEEN
THE TWO GROUPS OF CLASSES
A. Differences for 1. (mechanics conceptual questions) & 3. (Electrical
conceptual questions) show - greater focus on understanding in the
conceptual classrooms has led to better understanding
B. This has NOT meant lower performance on the traditional problems
more common on Year 12 exams (see 2. [mechanics traditional] & 4.
[electricity traditional])
C. So a view that senior school physics classrooms are inappropriate
locations for a focus on understanding is not valid; better student
conceptual understanding also leads to better (or equivalent)
performance on standard problems.
D. The relatively lesser difference for electricity conceptual questions and
electricity traditional questions ……...
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WHY THE DIFFERENCE BETWEEN MECHANICS & ELECTRICITY?
[two suggestions only here]
1. Significant differences in the nature of the knowledge:
MECHANICS
OBSERVATION
ANALOGIES &
MODELS
almost
always
direct
almost
never
used
ELECTRICITY
always
indirect
completely
central
(and remember how very few students used any analogies, models, metaphors
in the explanations they offered to our questions)
2. Significant differences in the levels of understanding of textbook writers,
curriculum writers (and teachers)
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Teaching mechanics for understanding: a few suggestions
1. The concept of force needs emphasis on
 involvement of two bodies in the description of any force
(emphasise terms ‘agent’ and ‘receiver’;
require all force descriptions be of the form “Force of A on B”;
we would ban vague terms such as ‘friction’ or ‘gravity’ [and its many aliases]
when describing actual forces – we believe these 2 terms should be used only
to refer to forms of interaction and not for specific actual forces)
 relation between an interaction between 2 bodies and the two forces we derive
 learning & using the diagrammatic representation of actual forces, especially
the convention and rationale of representing a single point of application
 the strong alternative conception of a ‘throw’ etc force – this needs to be
explicitly addressed in teaching (and can be – see conceptual classes data),
including by focussing on what the agent of such a force might be, linking with
the potentially confusing idea here of ‘force at a distance’, and seeing the
concept of ‘momentum’ as linked with the alternative conception
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Teaching mechanics for understanding: a few suggestions (cont.)
2. The concept of net force needs emphasis on
 does not exist in its own right as a single force
 is a means of expressing the combined effect of all forces acting on an object
 is collinear with the acceleration that is the consequence of the net force
3. Students very often do not “see across” a dense curriculum, very often do not
link to build up more cohesive understandings. This showed in 2 ways in
particular in this research:
 students often do not realise that one situation (like the trampoline questions)
can be viewed through a number of different physics lenses (most obviously a
“mechanics” lens, an “energy” lens); both this and how to decide the most
appropriate lens need explicitly teaching
 many students do not see similar contexts as, in physics terms, the same; the
transfer of understanding that we often assume frequently does not take
place; linking the understanding of similar contextual situations needs to be
specifically highlighted in teaching
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Teaching electricity for understanding: some suggestions
 Use analogies, metaphors, models – and choose these carefully
(remember that there is NO analogy or model or metaphor that can validly
represent every aspects of DC electricity; use one analogy for flow phenomena,
another for energy phenomena; help students understand when an analogy does
NOT ‘work’, and why)
 And, sadly, don’t rely on the textbook – many of these books focus on the
mathematical relationship between fundamental concepts with little effort to
develop understanding of these concepts; too often textbooks put all their focus
on one analogy (or even none)
 We don’t see how an understanding of potential difference can be developed
without an understanding of electric field – whether field is part of a formal
course or not.
(And the ideas of ‘electric field’ and how this is connected to ‘voltage’ are
important prerequisites for helping students to understand causality in electric
circuits.)
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Teaching electricity for understanding: some suggestions (cont.)
 Students need the opportunity to think about, discuss and explore their ideas in
order to develop their understanding. This research suggests a number of
contexts that students find difficult to explain, and which could very usefully be
used as the focus of discussion to promote better understanding.
1. Open circuits: many students find it very difficult to understand why the
potential difference across the ‘gap’ in an open circuit is equal to the emf of
the battery.
2. Circuits with voltmeters: many students do not understand how a voltmeter
works (including that it actually has a small current through it in normal use)
and that, like ammeters, they are not passive devices - they actually change
the circuit they are connected to (even though we can often consider this
negligible).
3. Circuits where resistance in one component changes (or components are
added or removed): many students do not understand that changes in one
part of a circuit result in changes in other parts of the circuit.
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THANK YOU
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