Transcript Convection

Energy
The ULTIMATE energy source
The sun is the ultimate source of all our energy.
For example, we often get energy from beef:
BEEF comes from COWS,
COWS eat GRASS,
GRASS gets energy from the sun by PHOTOSYNTHESIS
The 9 types of energy
Type
Heat
Kinetic (movement)
Nuclear
Sound
Light
Chemical
Electrical
Gravitational potential
Elastic potential
3 example sources
Energy changes
To describe an energy change for a
light bulb we need to do 3 steps:
1) Write down the
starting energy:
3) Write down
what energy types
are given out:
2) Draw an arrow
Electricity
Light + heat
What are the energy changes for the following…?
1) An electric fire
2) A rock about to drop
3) An arrow about to be fired
Conservation of Energy
In any energy change there is ALWAYS some “waste”
energy:
e.g. a light bulb:
Electricity
Light
+
heat
In this example HEAT is wasted – it’s given to the
surroundings. Waste energy is virtually always
heat and it’s always given to the surroundings.
Heat and Temperature
Heat is a type of energy that will flow from a warm area
to a colder one. For example…
This cup of coffee will ____ ____
because it is _____ ____ heat
energy into the surroundings.
This drink (taken out of the
fridge) will _____ ___ because it
is _____ ___ heat energy from
the surroundings.
Words to use – giving out,
warm up, taking in, cool down
Conduction
Conduction is all about when heat is transferred through a _________.
The heat is passed on by ___________ in the molecules. These
vibrations get BIGGER when the solid has more ENERGY (i.e. when it is
being __________).
http://www.educationusingpowerpoint.org.uk/Animations/heating%
20metal.zip
Metals are _______ conductors than non-metals. This is because the
heat is carried by free ________ that can carry the energy around the
metal and give it to other electrons and ions.
Words – vibrations, electrons, solid, heated, better
Convection
Convection is all about when a gas or liquid (“fluid”) moves and carries heat
with it. When the fluid is heated it ____________. This means that it
will become less __________ than the colder fluid around it. Because of
this the warmer fluid will try to “_______” over the colder fluid, and this
is why warm air rises. This is called a convection ___________. This is
how heat reaches us from the ___________ in this room.
In CONDUCTION the heat was passed on by VIBRATIONS in a
SOLID
In CONVECTION the heat is passed on by the FLUID expanding, rising
and TAKING THE HEAT with it
Words to use: expands, radiators, dense, heated, current, float
Some questions on convection…
1) Freezers in supermarkets are often left open to the
air. Explain why the food does not melt easily.
2) Explain why a hot air balloon rises in the air.
3) Explain why an ice cube floating at the top of a drink
will cool all of the drink.
Radiation
When you wear a black t-shirt on a hot, sunny day you would
feel ______ than if you wore ____. This is because black
_______ infrared radiation (i.e. heat) from the sun better
than white does.
I’m cool!
I’m very hot!
Some examples of radiation
Some examples of radiation
Radiation NOTES
Radiation is when heat moves around in electromagnetic _________ like
light does. Any hot object will emit heat radiation – the hotter it is, the
more radiation it emits. This type of radiation is called __________, and
too much of it will cause _________. Dark, matt colours will absorb AND
emit the _____ infra-red radiation, and light, shiny colours will ________
it.
The main difference with radiation is that conduction and convection could
ONLY happen in solids, liquids or gases, whereas radiation will happen
through an _____ _____. This is just as well, as otherwise we wouldn’t be
able to get any heat from the ___.
Words – sun, reflect, infra-red, waves, most, empty space, sunburn
Anything HOT emits HEAT RADIATION – the hotter it
is, the more infra red radiation it emits
House insulation
Loft insulation
Cavity wall
insulation
Double
glazing
Draught excluders
Energy and Power
The POWER RATING of an appliance is simply how much
energy it uses every second.
In other words, 1 Watt = 1 Joule per second
E = Energy (in joules)
E
P = Power (in watts)
T = Time (in seconds)
P
T
Some example questions
1) What is the power rating of a light bulb that transfers 120
joules of energy in 2 seconds?
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
3) Farhun runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power rating?
4) How much energy does a 150W light bulb transfer in a) one
second, b) one minute?
5) Shaun’s brain needs energy supplied to it at a rate of 40W.
How much energy does it need during a physics lesson?
6) Damien’s brain, being more intelligent, only needs energy at
a rate of about 20W. How much energy would his brain use
in a normal day?
Calculating the cost of electricity
Clearly, this depends on two things: the POWER RATING of
the appliance and HOW LONG you leave it on for. Electricity
is measured in “units”, also called “kilowatt hours” (kWh).
To work out how much something would cost use two steps:
1) Find out how many units have been used:
No. of units = Power rating x Time appliance is on for
in kWh
in kW
in hours
2) Multiply the number of units by how much each one costs:
Cost of electricity = no. of units used x cost of each unit
in pence
in kWh
in pence
Gravitational Potential Energy
To work out how much gravitational potential energy
(GPE) an object gains when it is lifted up we would use
the simple equation…
GPE
(Joules)
=
Weight
(newtons)
x
Change in height
(metres)
GPE
W
H
Some example questions…
How much gravitational potential energy have the following objects
gained?:
1.
A brick that weighs 10N lifted to the top of a house (10m),
2. A 10,000N car lifted by a ramp up to a height of 2m,
3. A 700N person lifted up 50m by a ski lift.
How much GPE have the following objects lost?:
1.
A 2N football dropping out of the air after being kicked up 30m,
2. A 0.5N egg falling 10m out of a bird nest,
3. A 10,000N car falling off its 2m ramp.
4. Mr Richards when bungi-jumping off a 110-metre high bridge in
Zambia in August
Efficiency
Efficiency is a measure of how much USEFUL energy you
get out of an object from the energy you put INTO it.
Efficiency = Useful energy given out by the device
Energy put into it
x100%
e.g. if 2000 joules of electrical energy are put into a
kettle and 500 joules of heat energy are gained from it,
its efficiency is 500/2000 x 100% = 25%
Some examples of efficiency…
1) 5000J of electrical energy are put into a motor. The
motor converts this into 100J of movement energy.
How efficient is it?
2) A laptop can convert 400J of electrical energy into
240J of light and sound. What is its efficiency?
Where does the rest of the energy go?
3) A steam engine is 50% efficient. If it delivers
20,000J of movement energy how much chemical
energy was put into it?
Energy Transfer diagrams
Consider a light bulb. Let’s say that the bulb runs on 100
watts (100 joules per second) and transfers 20 joules per
second into light and the rest into heat. Draw this as a
diagram:
“Input” energy
100 J/s
electrical
energy
“Output” energy
20 J/s
light energy
80 J/s heat
energy (given to
the surroundings)
Example questions
Consider a kettle:
2000 J/s
electrical
energy
Sound
energy
Wasted
heat
Heat to
water
1) Work out each energy value.
2) What is the kettle’s
efficiency?
Consider a computer:
150 J/s
electrical
energy
10 J/s
wasted
sound
20 J/s
wasted
heat
Useful
light and
sound
1) How much energy is converted
into useful energy?
2) What is the computer’s
efficiency?
Renewable vs. non-renewable fuels
A ________ energy source is blatantly one that can be
_______ (“renew = make again”), e.g. _____, solar power
etc.
A ___________ energy source is one that when it has
been used it is gone forever. The main examples are
____, oil and gas (which are called ______ ____, as they
are made from fossils), and nuclear fuel.
Words – non-renewable, coal, renewable,
fossil fuels, wood, renewed
Pollution
When a fuel is burned the two main waste products are
_____ dioxide and ________ dioxide.
Carbon dioxide destroys the ____ layer and helps to
cause the _______ ______.
Sulphur dioxide, when dissolved in ________, causes
______ _____.
Words – sulphur, ozone, carbon, acid rain,
greenhouse effect, rainwater
Power stations
Boiler
Transformer
Turbine
Cooling
tower
Generator
What does
each part do?
The boiler is where the fuel is burnt to boil
water
The steam from the boiler is used to turn a
turbine
The turbine is connected to the generator,
which acts like a dynamo – it generates
electricity out of movement
The steam is cooled down and turned back
into water in the cooling tower
Nuclear power stations
These work in a similar way to normal power stations:
The main difference is that the nuclear fuel is NOT
burnt – it is used to boil water in a “heat exchanger”
Advantages of non-renewable fuels
Relatively
cheap
Reliable
Coal, oil
and gas
Relatively
easy to find
Flexible in
meeting
demand
Cheap fuel
costs
Don’t produce
as much
pollution
Nuclear
The problems…
Power stations using coal, oil, gas or nuclear fuels can certainly
whack out the energy, BUT…
Problem 1 –
Fossil fuels and nuclear sources will RUN OUT
Problem 2 –
Burning fossil fuels will pollute the atmosphere
Problem 3 -
Nuclear plants run the risk of contamination and the
cost of shutting them down (“decommissioning”) is
very high
So what’s the solution?
Other ways of generating electricity…
Can we drive the turbine DIRECTLY?
Other ways of generating electricity…
Wind
Hydro-electric
Wave
Tidal
What are the disadvantages of these
renewable forms of energy?
Source
Wind
Wave
Tidal
Hydroelectric
Disadvantages
Source
Disadvantages
Wind
Depends on weather, visual pollution,
difficult to “store”
Wave
Harms wildlife, depends on size of
waves, depends on weather
Tidal
Depends on size of tides, harms
wildlife, visual pollution
Hydroelectric
Habitats can be destroyed, only good as
a “short term” supply
Also, these sources all have high setting up costs
Other options…
Geothermal energy – water is heated
using heat given off by _________ rocks
underneath the surface of the Earth
Solar energy – using solar panels
______ can be converted straight
into electricity
Although renewable fuels are ____, the energy we get from them is
“_______”. This means that we have to spend a lot of money on
generators to make them worthwhile.
This is why solar panels are only used in small things (like calculators) or
extremely remote locations (like ____________).
Words – dilute, radioactive, free, satellites, light
Matching supply and demand…
Hydroelectric power
station might “kick in” here
“Baseline” power stations
Summary
Renewable sources of energy are cheaper, cleaner and
will not run out. However, they are incapable of
providing all the energy we need, so we will have to go on
burning fossil fuels for now.
Topics we’ve covered…
1) The nine types of energy
2) Conduction, convection and radiation
3) House insulation
4) Working out energy, power and time
5) Working out the cost of electricity
6) Working out weight and GPE
7) Working out efficiency
8) How a power station works
9) Definitions and examples of renewable and non-renewable
10) Advantages and disadvantages of renewable and non-renewable
11) How geothermal and solar sources work