Transcript Energy

Energy &
energy resources
Why & how teach energy?
In small groups, discuss:
 Why and how is energy taught at KS3?
 What do students gain from it?
 Is this a useful preparation for GCSE & A-level studies?
Jot a few things down so that you can report back.
5 minutes
2
It’s easy to go wrong
In what way is each of these statements wrong?
1. ‘The moving pencil uses kinetic energy.’ (QCA)
1. ‘The steam [from a volcano vent] is converted into energy and
transported to Europe via a 1,200-mile sea-floor cable.’ (a London
newspaper)
1. ‘Carbonaceous matter is converted to heat or other forms of
energy.’ (Physics World)
2. ‘Energy makes things happen.’ (ASE Big Ideas)
1. ‘The bulb lights because energy flows from the battery to the bulb.’
(Sophie, Year 9)
3
Energy historically
Steam engines:
• 1712 Newcomen,
efficiency ~1%,
• 1775 Boulton &
Watt, efficiency ~7%
Energy and power
by analogy
energy  power  time
water volume  flow  time
measured in joules, MJ or kWh
measured in litres
energy
time
measured in joules per second,
W, MW, GW, TW, kWh per day
volume
time
measured in litres per minute
power 
water flow 
Energy today
How many Mars bars would I need to climb a mountain?
How much coal/oil do I use in a day?
Will UK electricity production match peak demand?
Can the Sun supply the world’s energy needs?
What is the efficiency of a motor?
How high can a projectile go?
Answering questions like this requires a calculation.
6
Learning outcomes
•
describe physical processes in terms of energy stores and transfers
•
distinguish between temperature and internal energy
•
explain thermal transfers (conduction, convection, radiation)
•
calculate the specific thermal capacity of metal objects
•
discuss energy changes associated with change of state (latent heat)
•
explain the law of conservation of energy, calculate efficiency in
energy transfers and recognise dissipation
mv 2
E p = mgh and Ek =
2
•
relate work done by a force to
•
describe rate of mechanical working as power
•
apply energy concepts to decision-making about energy policy
•
use a variety of experiments to convey key ideas about energy
Teaching challenges
•
‘Energy’, an everyday word, in science is an abstract quantity.
•
Energy conservation defies common sense, as everyday
things ‘run out of energy’.
•
Simply naming different types of energy, or energy chains,
provides no explanation for physical processes.
•
Students find work done (force exerted over a distance) more
difficult than impulse (force exerted over an interval of time).
•
Although temperature is a very familiar and tangible property,
it needs to be associated with the random thermal motion of
particles inside a body (internal energy).
What is ‘energy’?
‘A certain quantity that does not change’
‘It is not a description of a mechanism, or anything concrete; it
is just a strange fact that we can calculate some number and
when we finish watching nature go through her tricks and
calculate the number again, it is the same.’
Richard Feynman
The law of conservation of energy
Energy stores & pathways
Practical Physics guidance notes
• Helpful language for energy talk
• What’s wrong with ‘forms of energy’?
• Does energy make things happen?
SEP energy diagrams
A circus of energy experiments
Working in pairs, do as many experiments as
you can.
~30 minutes
[please do NOT write on instruction sheets]
Mechanical energy
For an object starting from rest,
impulse = force ´ time = mv
Work done
W = F ´ displacement = F ´ (average velocity ´ time)
v
v mv 2
W = Ft ´ = mv( ) =
2
2
2
Therefore no surprise that, in some mechanical systems,
mv 2
mgh +
= constant
2
e.g. roller coaster, or pendulum
‘Mechanical equivalent of heat’
Early in 19th C, ‘heat’ was thought to be a fluid, called ‘caloric’.
Many experiments, mainly 1840 – 1900, showed conversions of
heat to/from mechanical or electrical sources, leading to the
concepts of energy & energy conservation.
specific heat capacity of water, c  4200
J
kg C
o
James Prescott Joule, Manchester brewer & amateur scientist.
40 expts! e.g. Swiss honeymoon: thermometer to measure temperature
of water at top & bottom of a waterfall.
m
m
gh  cT
t
t
1850 article, Philosophical Transactions (Royal Society)
Clausius, Thomson (Lord Kelvin), Helmholtz, Rankine
Students often confuse …
Temperature
Average energy per particle
e.g. a sparkler is hot (high
temperature) because each
particle of metal has lots of
energy
e.g. a bath full of water is
not as hot (lower
temperature) because each
particle has less energy.
Directly measurable.
Thermal energy (store)
Total energy of the system
Depends on number of particles
in a body and the energy of
each one e.g. a sparkler has
less energy than a bath full of
water because there are many
more particles in a bath.
Heating (pathway)
A process or pathway which
changes the energy store of an
object
15
Energy efficiency
Sankey diagrams
product labelling
Thermal transfers
Energy transfer from one store to another because of a
temperature difference.
conduction: Ek transferred from atom to atom
convection: bulk movement of a fluid caused by localised
thermal expansion and hence differences of density in the fluid.
radiation: warm body emits a continuous spectrum of
electromagnetic radiation, with peak frequency related to
absolute temperature.
Heat capacity
thermal store associated with a temperature
change but no change of state.
Start simple: Why is a bite of hot potato more likely to burn the tongue
than a bite of cabbage at the same temperature? Which foods stay
hot longer on your plate?
Thermal (heat) capacity of an object: energy stored or released by
an object per degree of temperature change, in J oC-1
‘Specific’ thermal capacity: energy stored or released by a kg of
material per degree of temperature change, in J kg-1 oC-1
Materials used as coolants have a high specific thermal capacity.
Table
Q  mcT
Experiments to determine c
• Electrical method
electrical energy supplied  energy gained by object
IVt  mcT
• Method of mixtures e.g. solid placed in water
energy lost by hotter object = energy gained by cooler object
m c (T  T )  m c (T  T ), where T is the equilibriu m temp
1 1
3
1
2
2
3
2
3
NOTE: Both the equations above assume no heat loss. Insulate
calorimeter & include it in calculations.
• Using a cooling curve
ref: Nelkon & Parker Advanced level Physics
Practical Physics Energy collection ‘Thermal physics’
Latent heat
thermal store associated with a change of state
but no temperature change.
Term ‘latent’ introduced ~1750 by Joseph Black [derived
from the Latin latere, to lie hidden].
Q  mass  specific latent heat
Fusion: reversible change solid to liquid
Vaporisation: reversible change liquid to vapour
Table
Cooling or heating curves
previously schools used naphthalene (now hexadecanol)
Cooling or heating curves
Latent heat
Experiments
• Fusion: Melting ice in a calorimeter
• Vapourisation: passing steam through a calorimeter;
electrical method.
References: Nelkon & Parker Advanced level Physics,
Practical Physics Energy collection ‘Thermal physics’
Applications releasing energy as liquid changes to solid
•
hot pad hand-warmer
•
thermal energy storage in buildings
UK Energy futures
David MacKay, Sustainable energy – without the hot
air
Concerns about UK energy policy:
• Fossil fuels are a finite resource
• Security of energy supply
–
–
–
for the UK population and the economy
not reliant on foreign energy sources
diversified sources mean more robust
• CO2 and climate change
‘Numbers, not adjectives’
A balance sheet
Energy consumed
Energy produced
Cars
Wind
Planes
Solar
Heating and cooling
Hydroelectricity
Lighting
Offshore wind
Gadgets
Waves
Food and farming
Tide
Stuff – materials from cradle to grave
Geothermal
Public services
Fossil fuels - coal, oil and gas
Energy industries
Nuclear
Ben Goldacre, Bad science
12 December 2009
Climate change? Well, we’ll be dead by then
‘Zombie arguments survive, immortal and
resistant to all refutation, because they do not
live or die by the normal standards of mortal
argument.’
In the science classroom?
Carefully structured discussions to develop skill in
policy-related argument, based on
• science, if possible including quantitative estimates
• social values
• a basic understanding of how collective (social,
political and economic) decisions are made in the UK
… simplifying the breadth and depth of science to
match pupils’ age and ability
Useful websites
Climateprediction.net follow links Support -> Schools
Realclimate.org
climate scientists’ blog and archive
Google Earth v5 time series images show impact of climate change
UK Energy Flows comprehensive Sankey diagram published
every 3 years
The Guardian pages on Climate change - plus related
Environment pages … and weblinks
Support, references
www.talkphysics.org
SEP Energy now! cdrom, 3 booklets Energy storage,
Solar power, Wind power
Energy topic, Practical Physics website, including Guidance pages
David Sang (ed, 2011) Teaching secondary physics ASE / Hodder