Download B9: Towards a conceptual understanding of Physics

Language and understanding in Physics
Acknowledgements:
Brian McKittrick, Kim Falloon
Helen McDonald & Geoff Davies
Student explanations and descriptions
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The momentum of the car caused the brick wall to break
The passenger flew through the windscreen because of the accident
When the starters gun went off, I pushed myself forward
The force of the moving ball propelled it through the air
The cup is stationary on the table due to the force of gravity and the
equal and opposite reaction force due to the table
• The cup is stationary because the table is in the way
What’s new to discuss in Unit 1?
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Temperature
internal energy
kinetic energy
heat
work
image courtesy pHet physics
Force
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Forces are pushes and pulls
Pulls are attractive forces
Pushes are repulsive forces
Forces are created in pairs – a pair of attractions, a pair of repulsions; one
force acts on one body the other force acts on the other body
Forces cause objects to speed up, slow down, change direction, remain
stationary and change shape
Many forces can act on an object simultaneously
Forces acting on an object can be added together – the net force as a
mathematical quantity, not a physical quantity
The net force is identical in size to the product mass of object and acceleration
of object ∶
𝐹 = 𝑚𝑎 or better still: 𝑎 =
𝐹
𝑚
energy, kinetic and
potential
An unbalanced force
component acting on an object
over a displacement gives rise
to a change in the kinetic
energy of the object.
momentum
A net force acting on an object
over a period of time gives rise
to a change in the momentum
of the object.
Isolated/non-isolated collisions and elastic/inelastic collisions
What, if any, is the difference in the meaning of the words
“unbalanced” and “net”?
Force
𝑑𝑝
𝑑𝐾𝐸
• The net force acting on an object = =
, that is forces are associated
𝑑𝑡
𝑑𝑥
with momentum and kinetic energy changes which when combined with
conservation rules lead to transfers and transformations of these
commodities, a force or collection of forces acting on an object is now seen
in terms of other dynamical quantities namely momentum and kinetic
energy
• In contemporary physics force is not a significant parameter, but rather an
outcome of statistical averaging of field interactions.
• Force as a word used to describe the result transferring quanta associated
with a field, such as photons for electromagnetic interactions or W-bosons
for the weak nuclear force or gluons for the strong nuclear force.
Forces and objects
• A force acts on one object due to another object.
• For example the weight of an apple is the gravitational force acting on the
apple due to the earth
• The table pushing up on the cup is the force on the cup by the table. This force
is electrical by nature. The earth pulling down on the cup is gravitational in
nature
Force on cup by table
Force on cup by earth
Forces and objects
How many vertical forces act on objects A, B and
C respectively?
What is the size and direction of all identified
forces?
A: 1 kg
On A
On B
On C
By A
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10 N
down
0N
By B
10 N up
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35 N
down
By C
0N
35 N up
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B: 2.5 kg
C: 3 kg
table
earth
mass and charge
• Mass is a measure of the amount of “stuff” a body is made of
• Mass describes the amount of inertia an object has and hence the
kinematic response an object has to a net force
• Mass determines the strength of a gravitational field around an object
• Mass determines the energy content of an object
• Why does inertial mass yield the same numerical value as gravitational
mass?
• Charge is a measure of the amount of “electricity” a body has
• Charge has no discernible inertial effect
• Charge determines the strength of an electrical field around an object
and moving charge is associated with magnetic fields
weight
• The weight of an object is the force of gravity acting on it
• Does an apple weigh as much as the earth?
• What is meant by apparent weight?
What are fields?
How do we teach about fields?
What words can we use?
Fields
• What is a field? A field is a physical quantity that has a value for
each point in space and time.
• How is it used in secondary school physics?
• http://en.wikipedia.org/wiki/Classical_field_theory
• What are examples of fields? In what sense are they real?
• Can a field store potential energy? How does this description stand
with the language used to describe lifting a mass in a gravitational
field?
Student explanations and descriptions
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The voltage passed through the globe causing it to glow
The globe used up all the current
The cell supplied energy to the charge as it passed through the cell
The electricity moved around the circuit at the speed of light
potential difference, current
• Is potential difference the
most difficult physics
concept in secondary
schools?
• The potential difference between two points in a circuit, for example
the potential difference across the LED
• The current in the LED
• The resistance of the LED
• The power absorbed by the LED
Electricity – charge, motion and potential
• The mean speed of a free electrons in thermal equilibrium is given by
𝑣 =
8𝑘𝑇
.
𝜋𝑚
In a room temperature conductor, free electrons have a
mean speed of 1.1 × 105 m s-1. The drift velocity of an typical electric
current is of order 10-2 m s-1; this differs by a factor of 107. How do we
talk of an electric current consistent with this finding?
• The mean speed due to thermal motion results in electrons having a
mean kinetic energy of 0.03 eV at room temperature.
• Potential difference – the energy gained or lost per coulomb of charge
∆𝐸
– an open ended discussion about ∆𝑉 = 𝑞
• Does charge have to pass through a cell to gain potential energy?
• General circuit theorems:
conservation of charge leading to current conservation at a node
conservation of energy transfer leading sum of potential
difference around a closed loop equalling zero.
𝑣 =
8𝑘𝑇
: Bose Enstein statistics, < 𝑣 > =
𝜋𝑚
3𝑘𝑇
(𝑜𝑙𝑑 𝑠𝑐ℎ𝑜𝑜𝑙)
𝑚
light
• What language do we use when we describe reflection and refraction?
• What language do we use when describing diffraction and interference?
• How do we deal with the particle model for light – the photon – for some
interactions and retain a wave model of light for propagation?
• How do we talk about the photoelectric effect?
• How do we resolve G. I. Taylor’s experiment circa 1920’s?
• Is there a consistent language for use in modern physics?
• What would a candle look like to a group of observers if it emitted
electromagnetic radiation at a rate of 1 photon per second?
inside the nucleus
• What is the nucleus composed of?
• Is it reasonable to think of the nucleus as being composed in individual
protons and neutrons?
• Both the proton and the neutron are modelled as being composed of 3
quarks.
• A free neutron has a half-life against beta decay of about 10 minutes.
How can light be both a particle and a wave?
How can matter be both a wave and a particle?
What language can we use when discussing the uncertainty
principle (Unit 4)?
task instruction
Term
Definition
Typical learning examples
Analyse
Identify components, elements,
Consider presented information and clarify concepts and
constituent parts of the whole and identify knowledge; use qualitative and quantitative methods to
the relationships between them
distinguish between components (words, tables, labelled
diagrams, calculations, graphs); recognise patterns;
identify and relate implications; graphical analysis
Apply
Use knowledge (ideas, formulae,
principles, theories, laws, models,
techniques) in a new situation or context
Calculate
Use mathematical formulae and modelling Solve numerical problems by using formulae and
to solve quantitative problems
mathematical processes; find the numerical value of an
unknown variable or constant
Compare
Identify the similarities and differences
List, tabulate or use a graphic organizer to identify
between two or more objects or processes similarities and differences
Describe
Communicate the characteristics and
features of an event, object, procedure or
process
Propose a solution or response to a problem or issue;
show steps; use algebraic and/or graphical methods as
appropriate
Use written or visual representations to communicate
characteristics or features
Term
Determine
Definition
Typical learning examples
Find out, based on reasoning, observations
and information
Make reasoned judgments or decisions on
given or collected information, based on
established criteria
Recognise a quantity (often without the use of
calculations); interpolate; extrapolate; estimate
Assess the merit (strengths and limitations) of ideas,
processes or procedures and reach a conclusion; validate
evidence; choose from options based on reasoned
arguments
Explain
Make clear; account for the reason for
something or the relationship between
cause and effect; state why and/or how
Provide reasons mechanisms and outcomes, incorporate
quantitative data as appropriate
Identify
Recognise particular elements of a whole or Recognise and name/label a specific object, element,
part; select from a number of possibilities;
component or underlying principle or concept;
select relevant information or aspects of key label/annotate components of a system, model or diagram
ideas
Interpret
Take a form of information and make
conceptual meaning from it
Derive meaning from information presented in multimodal
texts (for example, written, aural and diagrammatic), tables,
images and graphical formats
Model
Reproduce conceptual understandings and
principles; physical structures and systems
Construct a visual, physical, algebraic or graphical
representation of concepts, principles or processes
Evaluate
Language, learning and teaching
References
• Physics questions without numbers, Dick Gunstone and Richard
White, Faculty of Education, Monash University, 2012
• Understanding and developing Science Teachers’ Pedagogical
Content Knowledge; John Loughran, Amanda Berry and Pamela
Mulhall
• Physics 1 and 2, Halliday and Resnick, Wiley 1966
• http://en.wikipedia.org/wiki/Classical_field_theory
• Useful applets:
• https://phet.colorado.edu/en/simulations/category/physics
• www.falstad.com/mathphysics.html
• Useful sites: www.khanacademy.org