Transcript 1. Heat Transfer - science

REVISION
• Y10 Revision of Energy and Electricity
How does heat travel through space?
The Earth is warmed by heat energy from the Sun.
How does this heat energy travel from the Sun to the Earth?
?
infrared
waves
There are no particles
between the Sun and the
Earth, so the heat cannot
travel by conduction or by
convection.
The heat travels to Earth by
infrared waves. These are
similar to light waves and
are able to travel through
empty space.
What are infrared waves?
Heat can move by travelling as infrared waves.
These are electromagnetic waves, like light waves,
but with a longer wavelength.
This means that infrared waves act like light waves:
 They can travel through a vacuum.
 They travel at the same speed as light – 300,000,000 m/s.
 They can be reflected and absorbed.
Infrared waves heat objects that absorb them and are also
known as thermal radiation.
Investigating thermal absorption
Absorbing thermal radiation
Infrared waves heat objects that absorb (take in) them.
Certain surfaces are better at absorbing thermal radiation
than others. Good emitters are also good absorbers.
worst emitter
best emitter
matt
black
best absorber
white
silver
worst absorber
Matt black surfaces are the best absorbers of radiation.
Shiny surfaces are the worst emitters because they reflect
most of the radiation away.
Why are solar panels that are used for heating water
covered in a black outer layer?
Why does heat transfer happen?
Heat is a type of energy called thermal energy.
Heat can be transferred (moved) by three main processes:
1. conduction
2. convection
3. radiation
During heat transfer, thermal energy always moves in the
same direction:
HOT
COLD
Heat energy only flows when there is a temperature
difference from a warmer area to a cooler area.
Why do objects get hotter or colder?
Temperature is a measure of how hot an object is.
Heat transfer only takes place when there is a temperature
difference. The heat energy flows from a warmer area to a
cooler area.
Why does an ice lolly melt on a warm tongue?
There is a temperature difference between
the tongue and the lolly, so heat energy
flows from the warm tongue into the
cold ice lolly.
This heat transfer means that the ice lolly
melts as it gets warmer, and the warm
part of the tongue touching it gets cooler.
How might climate change cause the polar ice caps to melt?
What is conduction?
How are the particles arranged in a solid, a liquid and a gas?
solid
liquid
gas
Particles that are very close together can transfer heat
energy as they vibrate. This type of heat transfer is called
conduction.
Conduction is the method of heat transfer in solids but
not liquids and gases. Why?
What type of solids are the best conductors?
How do non-metals conduct heat?
How do metals conduct heat?
Metals are good conductors of heat. The outer electrons of
metal atoms are not attached to any particular atom. They
are free to move between the atoms.
When a metal is heated, the free
electrons gain kinetic energy.
This means that the free electrons
move faster and transfer the
energy through the metal.
This makes heat transfer in
metals very efficient.
heat
Insulators do not have free
electrons and so they do not
conduct heat as well as metals.
What is convection?
Warmer regions of a fluid are
less dense than cooler
regions of the same fluid.
The warmer regions will rise
because they are less dense.
The cooler regions will sink
as they are more dense.
This is how heat transfer
takes place in fluids and is
called convection.
The steady flow between the
warm and cool sections of a
fluid, such as air or water, is
called a convection current.
What is payback time?
Payback time is the time it takes for the cost of installing
insulation to be equalled by the savings made from reduced
energy costs.
payback time (in years) =
cost of insulation
saving each year
Example:
Adding silver reflectors behind radiators costs £25 and
saves £50 per year.
payback time = 25
50
= 0.5 years (6 months)
Calculating payback time
What is the payback time for these types of insulation?
How heat
escapes
Cost of heat
escaping
per year
Cost of
insulation
Payback
time
roof
£80
£240
3 years
windows
£40
£3,200
80 years
draughts
£50
£50
1 year
walls
£100
£500
5 years
Why is double glazing popular if the payback time is so long?
Energy transfer in a television
An energy transfer diagram shows the input and output
energies for a device. This includes all the useful and wasted
forms of energy.
For example, in a television:
light
electrical
sound
heat
Law of conservation of energy
All energy transfers follow the law of conservation of energy:
Energy cannot be created or destroyed,
just changed in form.
 This means that energy never just ‘disappears’.
 The total amount of energy always stays the same,
i.e. total input energy = total output energy.
 In most energy transfers, the energy is transferred to
several different forms, which may or may not be useful.
 Energy that is transferred to unwanted forms of energy is
wasted.
What happens to wasted energy?
What happens to the wasted energy produced when energy
is changed from one form to another?
Remember that energy cannot be
created or destroyed.
Wasted energy spreads out into
the surroundings – this is called
dissipated energy.
This dissipated energy is too
spread out to do useful work and
so cannot be reused.
For example, the heat and light
energy produced by this light bulb
are too spread out to be reused.
How is energy efficiency calculated?
The energy efficiency of a device can be calculated using
this formula:
useful output energy
energy efficiency =
total input energy
 Useful energy is measured in joules (J).
 Total energy is measured in joules (J).
 Energy efficiency does not have any units.
It is a number between 0 and 1 which can be
converted into a percentage by multiplying by 100.
Calculate the efficiency of a bulb
This filament bulb is supplied
with 100 J of electrical energy,
which it converts to 45 J of
light energy.
a) How much energy
is wasted? 55 J
Wasted energy = Total – Useful
= 100 J – 45 J
= 55 J
b) In what form is the
energy wasted? heat
c) What is the efficiency
of the bulb? 0.45 or 45%
Efficiency = Useful
Total
= 45 J
100 J
= 0.45 or 45%
Non-renewable or renewable?
What happens in a coal/oil power station?
What waste do fossil fuels produce?
Burning fossil fuels creates waste
products that can act as pollutants and
have harmful environmental effects.
 Carbon dioxide – This greenhouse
gas is the main waste product of
burning fossil fuels. Increased levels
of carbon dioxide due to human
activities are thought to be
connected with global warming.
 Sulfur dioxide and nitric oxides – These gaseous
pollutants contribute to the formation of smog and acid rain.
 Ash – This waste solid is disposed of in landfill sites.
What can be done to reduce the problems caused by
burning fossil fuels?
What is the greenhouse effect?
What happens in nuclear power station?
How does a greenhouse capture solar energy?
What is a solar power station?
Solar power stations use the Sun’s energy to heat water and
make steam, which then drives a turbine to produce electricity.
Some solar power
stations use a series
of mirrors, called
heliostats, to reflect
light onto a boiler.
© Sandia National Laboratory/NREL
This solar power
station in California
consists of about
1800 heliostats, with
an electrical output
of 10 megawatts.
Why use solar cells to power satellites?
Solar cells (or photocells) turn light energy from the Sun
directly into direct current electricity.
Manufacturing solar cells is very expensive and requires the
use of highly toxic materials. However, once the solar cell is
built it produces no pollution and requires little maintenance.
Solar cells are ideal for use
in remote locations where
maintenance is difficult and
other sources of electricity
would be expensive.
Satellites have been
powered using solar cells
since the 1950s.
Pros and cons of solar cells
What is biomass?
Biomass, also known as biofuels
or bioenergy, is material from
living sources. The simplest
biomass energy sources are
plants, which can be burnt to
produce steam to turn a turbine.
Traditionally, wood is burnt to
give heat but trees grow slowly
and require a lot of land. Other
materials, such as waste from
poultry farms, can also be burnt.
Biomass fuels are renewable as
more plants can be grown,
producing yet more biomass.
The carbon dioxide
released during burning
biomass is absorbed by
the replacement plants.
Biomass is said to be
‘carbon neutral’ and does
not add to global warming.
Using biofuels – true or false?
How can hot rocks generate electricity?
A geothermal power plant can be built to utilize steam and
hot water that rise naturally to the Earth’s surface.
The steam and hot water
provide a way of turning a
generator, which then
produces electricity.
© David Parsons/NREL
This is the largest producer
of geothermal power in the
world. It is in California and
has an output of 750 MW
of electricity.
Sometimes, the hot water is trapped in natural underground
reservoirs and does not reach the Earth’s surface. This water
can be reached by drilling to depths of several kilometres.
Pros and cons of geothermal energy
How do wind turbines work?
Wind turbines have large
blades that capture the
kinetic energy of the wind.
This kinetic energy is used
directly to turn the turbine
and generate electricity.
Wind turbines are a source
of 'clean energy’ as they do
not produce any polluting
waste.
However, some people consider this source of renewable
energy to be noisy and an eyesore. There is also some
concern that, if wind turbines are poorly located, they could
kill migrating birds.
Wind power – true or false?
How does a hydroelectric dam work?
Pros and cons of hydroelectric power
How does wave power work?
Wave power – true or false?