Subject Knowledge - Energy

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Subject Knowledge - Energy

• This document can be freely copied and amended if
  used for educational purposes. It must not be
  used for commercial gain. The author(s) and web
  source must be acknowledged whether used as it
  stands or whether adapted in any way.
• Subject Knowledge Energy (Download K4.5_1.0a)
• Authored by Keith Ross, University of
  Gloucestershire
• accessed from http//www.ase.org.uk/sci-tutors/
• date created Nov 2005

2
Energy the big ideas

• Can we use energy up? yes??
• Do Fuels contain energy - no!!
• Is Heat the same as temperature - no!!
• Does turning the ignition key provide energy to
  make the car go no!
• Replenishable energy and the environment.

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Two major misconceptions

• The two major misconceptions about energy arise
  because of these two confusions. We need to
  ensure out students distinguish between
• 1. Energy itself (joules) and usefulness of
  energy.
• 2. Fuel (made of matter) and energy (measured in
  joules).
• At this stage (key stages 1-4) dont worry about
  emc2 (mass energy equivalence)

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1. Useful energy?

• I was always confused by the word energy. I
  always knew it was useful but could never explain
  precisely to anyone what it is.
• I now realise that energy can be described in
  two ways ... that its usefulness can be used up
  ... yet the amount of energy remains the same.

Extracts from a learning log written by a BEd
Primary student where they attempt to identify
changes in their understanding as a result of
their study.
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Using energy

• High grade energy resources are used up
• .. but joules in joules out

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2. Do Fuels contain energy - no!!

• The following questions were put, electronically,
  to a group of 120 year one trainee primary
  teachers who all achieved a grade c or better
  at GCSE science two or more years ago. Figures
  show who chose each option.
• Red figures suggest misconceptions
• Green figures suggest useful understanding
• The questions were re-administered at the end of
  year one, and the slides show changes in choices
  following a years science subject study
• Some slides are extracts from learning logs
  written by the same group of students where they
  attempt to identify changes in their
  understanding as a result of their study.

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Many people say that fuels and food contain
energy.

• How do you imagine energy to be associated with,
  or 'in', fuels and food?
• (a) When fuels burn or food is respired the
  energy in them is released.
• (b) Fuels and food contain energy-rich bonds
  which release energy when the bonds break.
• (c) Energy can only be obtained by making new
  bonds. This happens during burning and
  respiration.
• (d) The energy is not in the fuel or food itself,
  but is associated with both the fuel and oxygen.

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Many people say that fuels and food contain
energy.

• How do you imagine energy to be associated with,
  or 'in', fuels and food?
• (a) When fuels burn or food is respired the
  energy in them is released.
• (chosen by 80 before study - 17 after)
• (b) Fuels and food contain energy-rich bonds
  which release energy when the bonds break. (85
  to 35)
• (c) Energy can only be obtained by making new
  bonds. This happens during burning and
  respiration. (17 to 65)
• (d) The energy is not in the fuel or food itself,
  but is associated with both the fuel and oxygen.
  (50 to 95)

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Conservation of Matter

• When something is dumped on the rubbish tip, goes
  up the chimney or down the drain, its atoms
• (a) may eventually cease to exist
• (chosen by half before study - 5 after)
• (b) may remain harmlessly in the
  environment
• (c) may be used by living things to help them
  grow
• (d) may remain in the environment and cause
  pollution

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Fate of food we eat

• Consider the material (stuff, matter, atoms...)
  in our food that enters our blood and which we
  have used as a fuel. How does this material leave
  our body?
• (a) The atoms are all used up and only energy is
  left.
• Chosen by 37 at start and by 2 at end
• (c) We breathe a lot of it out as carbon dioxide
  and water vapour.
• (38 to 77)

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Primary ITT student with C at GCSE - learning log
extracts

• I found the principles of burning very
  enlightening. The constructive process of oxygen
  forming oxides resulting in an increase in weight
  now seems very obvious.

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Primary ITT student with C at GCSE - learning log
extracts

• I gradually came to realise how much of a part
  atoms do play in matter.
• I have always assumed that as materials go
  though the process of change, the atoms from
  which they are made up, change too.
• However, this unit helped me to understand that
  atoms are indestructible and I can look at any
  substance now and judge that ..
• .. regardless of what change it goes through the
  atoms will remain the same.

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Demonstrations

• Char and flame
• Syrup tin
• Candle and methane
• Match and methane

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The stretched oxide model
In the following slides Oxygen or processes
involving oxygen are coloured red. Fuels, food,
etc not oxidised are green

• The model shows oxygen being dragged away from
  carbon dioxide and water to form free oxygen and
  leave behind carbohydrate.
• This is the process that happens in
  photosynthesis, driven by solar energy

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An Ecosystem?
Oxygen used
Fuel line
Fuel cell
Electrolysis of water
Oxygen produced
Electric motor
Solar cell
Respiration
Photosynthesis
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The two meanings of burn

• The next few slides show a vital distinction
  between digestion and respiration processes
  that get confused in the minds of children.
• The same distinction needs to be made between the
  two meanings of burn char and combust

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Adding energy to Food and fuel

• Heating decomposes it into (char)coal, oil and
  inflammable smoke (gas)
• Digestion decomposes it to faeces, blood sugar
  and inflammable gas (flatulence)
• These are all still fuels and can flame-burn
  these are endothermic processes, requiring an
  input of energy

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Getting energy from Food and fuel

• Combustion and Respiration add oxygen to form
  carbon dioxide and water
• These are oxides. The processes are exothermic,
  transferring useful energy.
• The oxides can be re-charged by solar energy
  during photosynthesis to reform fuel/food and
  oxygen

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The rich Gambian Languages

• burn English
• flame-burn char-burn English
• maala jani Mandinka
• taka laka Wolof

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Emphasise the distinction between matter and
energy
In the following slides The everyday life-world
view is coloured blue Suggestions for teaching
approaches follow

• Life-world view - biomass (eg. cornflakes, wood
  ) is energy.
• Call it fuel which combines with oxygen (matter)
• The energy associated with them is like a spring
  -
• stretched (energy stored as fuel and oxygen kept
  apart)
• when relaxed (energy released as oxides re-form).

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Emphasise that gases are the material products of
combustion

• Life-world view - flames, hot air, exhaust gases
  are forms of energy they are weightless.
• Account for matter during evaporation and
  burning.
• Collect exhaust gases from above a flame and see
  the water condense

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Use the atom concept primarily to explain the
conservation of matter

• Life-world view - atoms come and go like bulk
  materials which burn, evaporate react.
• Distinguish between indestructible atoms and bulk
  materials
• water evaporates but the atoms are still there
• Introduce chemical equations with ball and stick
  models or drawings to allow the actual atoms can
  be counted on each side.

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Count the atoms

• carbohydrate oxygen water carbon dioxide

carbohydrate oxygen water carbon dioxide
CHOH O2 H2O
CO2
CHOH O2 H2O
CO2
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Emphasise digestion as the process by which food
enters the blood.

• Life-world view - food goes from mouth to anus
  along the alimentary canal
• Food's path is mouth digestive system blood
  cells. Then
• 90 is a fuel - joins with oxygen, and leaves the
  body at the lungs (and as water in urine).
• 10 is for growth - adds to body biomass by
  helping to construct new cells.

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Emphasise digestion as the process by which food
enters the blood.

• Where does the food which you eat leave the
  body
• Life-world view it leaves as faeces.
• but faeces are still fuel (food), and have
  never really entered the body and have not yet
  joined with oxygen.
• Encourage them to say it leaves from the lungs
  and in urine or (for the 10 of our food used to
  replace body tissue) as house dust

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Emphasise the constructive side to burning
(combustion) and respiration

• Life-world view burning is a destructive process
  - only ashes remain.
• Encourage pupils to see exhaust gases as massive,
  an increase in mass as oxides are built up
• See Water and carbon dioxide we breathe out as
  oxidation products of the food we eat
• See the ashes left after a wood fire, as oxides
  of the metals that were taken in as minerals
  during the lifetime of the tree.

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Emphasise the role of oxygen in burning
(combustion) and respiration

• Life-world view air keeps us alive.
• Emphasise panting starving the brain of oxygen
  smothering a fire etc. all as evidence for the
  constructive role of oxygen.

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Avoid the use of the term high energy bond in
ATP (at A level)

• The third phosphate group in ATP is weakly bonded
  to the rest of the molecule.
• It costs much less than the usual 400 kj per mole
  to break.
• ATP is therefore a reactive molecule.
• By breaking this weak bond and replacing it with
  a stronger bond, energy is made available to do
  useful things in the cell.
• Energy from the glucose-oxygen system is used to
  to re-set the system this energy
• breaks the strongly bonded -OH group from ADP and
  replaces it with the weakly bonded third
  phosphate of ATP again.

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2 other confusions

• 3. Heat and temperature
• 4. Control systems

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Heat and temperature

• You add 10 kettle-fulls of boiling water to some
  cold water in a bath to make the bath water warm.
  
• true or false the bath water now has more heat
  energy in it than a kettle full of boiling water.

38 said the bath had less heat in it than the
kettle
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Temperature
i. when water at 600C in a jar is shared between
two cups
       
ii. when the water from the two cups are mixed
Figure 4.2 (From Stavy and Berkovitz 1980)
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5. Energy and the environment

• Renewable or replenishable energy?
• Since energy becomes degraded (as matter is
  cycled) we cannot actually renew energy, though
  the matter in which the energy is stored can be
  renewed, or re-charged.
• Thus it is better to call energy sources that can
  be re-charged replenishable

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• Use of man power to move a statue in Egypt, about
  2000 B.C. Today we each have the services of more
  energy slaves than are pulling the statue in this
  picture.
• (Wilson A.W. (1976) What is Energy Exeter
  Wheaton)

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Human muscles

• A well-nourished slave produced about 3 Mj of
  energy a day through respiration
• We each use an average of about 400 Mj a day in
  our industrial society
• To produce 400 Mj a day we would therefore each
  need 160 slaves pedalling away at cycle
  generators.
• To grow food for this muscle power (even if they
  were draught animals) would need the land areas
  of several Earths (our global footprint)

http//www.footprintnetwork.org/
million joules
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