www.monash.edu.au exploring student difficulties with mechanics and electricity richard gunstone...

www.monash.edu.au

Exploring student difficulties with Exploring student difficulties with mechanics and electricitymechanics 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

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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 MOVINGUPWARDS.(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.

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.

A1A2

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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.

(a) Calculate the amount of charge passing point X in 10 s.(b) How many joules of energy is given to each coulomb of

charge as it passes through the battery?

3 A

X

9 V

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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

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 trampoline on Sarah propelling

Force of rebound / Force of momentum her through the air

Force of planet Earth on Sarah arising

Force of gravity / Weight / Gravity from gravitational interaction between

Earth and Sarah

Air resistance / Friction from air / Force Force of air on Sarah resisting her

of air motion through the air

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Trampoline question [cont.] Answers we judged correct:

part (a) part (b)

(i) (force diag) (ii) (explain) (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)



traditional 87% 55% 65%12

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 fall13

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.


part (a) part (b)










A SUMMARY OF DIFFERENCES BETWEEN THE TWO GROUPS OF CLASSES

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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

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 ELECTRICITY

almost always

OBSERVATION always indirect

direct

ANALOGIES & almost completely

MODELS never central

used

(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 26

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