Heat Transfer - St Mary`s College

Module P1a ­ Energy & Energy Resources
Heat Transfer
Year 10 Biology 1 Chemistry 1
+CAU
+CAU
Year 11
Physics 1
+CAU
GCSE: Science
GCSE: Additional Science
B2
+CAU
C2
+CAU
P2
+CAU
B3
+CAU
C3
+CAU
P3
+CAU
GCSE: Biology
GCSE: Chemistry
GCSE: Physics
Energy is the ability to cause change.
Without energy there can be no change.
Energy can be due to motion (kinetic energy), or it can be hidden away or stored (potential energy)
What is heat?
Heat is the vibration of the atoms of a substance.
The hotter the material, the more the atoms vibrate.
Notice that the material expands, but the atoms stay the same size.
The atoms gain kinetic (movement) energy
Heat always flows from hotter regions to cooler regions.
Insulators can reduce the rate (speed) of this heat flow, but they can't stop it.
Illustration:
w
t flo
Hot metal
Hand
Hea
flow
t
a
e
H
Ice
The greater the temperature difference, the greater the rate of heat flow
There are three kinds of Heat Transfer
Conduction
This is where the heat energy/vibrational energy (same thing) of the atoms is passed on to their neighbours:
Heat applied here
These atoms...
...hit these atoms
­ which passes on the heat
Heat flow
Heat flow
Heat flow
Heat flow
Cool & contracted
d
H
de
n
a
p
ex
ot &
• Heat the strips
• Time how long the wax takes to melt
wax blobs
copper
Classic conduction experiment
brass
steel
Fair­testing:
• same length of strips
• same bunsen
• same wax/same heat
apply bunsen flame here
Metal
Copper
Brass
Steel
Time taken to melt (s)
tripod
How Science Works:
Categoric:
A variable that is a label, ie a name
eg, eye colour, school, class, hair colour
How Science Works:
Continuous:
A variable that is a number and it can be any value.
eg, height, mass, speed
Discrete:
A variable that is also a number, but it can only be a very limited number of values.
eg, shoe size, school year
Metal
Time taken to melt (s)
Copper
Brass
Steel
Conclusion:
Copper is a better conductor than brass. Steel is either a poor conductor or an insulator.
Graphing the results
In this experiment the independent variable (the one you change) is the metal.
time taken
(dependent variable)
continuous ­ any value allowed
The dependent variable (the result) was the
time taken for the wax to melt.
Metal (the independent variable)
categoric ­ a label
Time taken (s)
Brass Copper
metal
Since the metals are an example of a categoric variable the chart must be a bar chart or (much more rarely in science) a pie chart
Conduction in metals
inn
ele
ctr er
ons
...because the inner Metal atoms have special properties.
electrons The negatively charged electrons are attracted towards the repel them positively charged nucleus.
away
electrons
BUT the negatively loose outer
charged outer electrons electron
are repelled away by the negatively charged inner electrons
Therefore, the outer electrons of a metal atom are only loosely attached.
positively charged
nucleus
negatively charged
The loose outer electrons can wander freely between atoms.
metal atoms
Conduction in metals
loose outer electrons
The loose outer electrons of metals are very small and fast moving.
If they get hotter they move faster ­ they gain kinetic energy.
The electrons move about and hit other electrons and also atoms making them hotter (vibrate more) too.
The rapid electrons can conduct heat through a metal very quickly
Non­metals do not have loose outer electrons. They can only conduct heat due to the vibration of big, slow atoms.
Convection
This is the second kind of heat flow. Convection works in fluids (substances that flow), ie, liquids and gases.
smouldering
wax taper
smoke
The waxy taper is lit, and then blown out.
This produces smoke.
How convection works
upthrust
less dense hot air
dense cool air
weight
dense
cool
air
dense
cool
air
on the ground
lift­off
A convection demonstration:
potassium permanganate
• Heat gently
convection current
• What do you observe?
How convection works
• cool water
• water molecules evenly spread out (on average)
hot water floats up
• hot water
• hot molecules vibrate more
• they are more spread out
• the hot water is less dense
cold water gets pushed down
This flow of fluid is called a convection current. The heat source provides the energy to keep the flow going
Any time that convection is occuring:
(1) Part of the fluid heats up
(2) It's particles gain heat energy, so they move apart.
The hot fluid is now less dense than the cool fluid, so it floats upwards
(3) The rising hot fluid pushes the cold fluid down
(4) The cool fluid gets heated up
(5) Goto (2)
Repeat until the heat source is gone
Examples of convection
Radiator:
ceiling
window
convection
current
floor
radiator
The convection current circulates heat around the room
Central heating system:
hot water tank
The water should flow by convection alone, but usually there is a pump to help with the circulation
convection
current
boiler
radiators
Winds:
Heat radiation from the sun passes straight through the air and warms the land.
The warm land heats up the air above it.
The warm air rises.
This creates a low pressure down at ground level which sucks in cool air to replace the air that has risen.
This movement of cool air forms a wind
1
Heat from the sun passes straight through the air
2 ...and warms the ground
hea
t e
gy
ner
1
3
warm
air
2
hot
ground
land
...which warms the air above the 3 ground causing it to rise
...which creates a low pressure at 4 ground level that sucks in cool air
4
sea
Heat Radiation
This is the third kind of energy transfer.
Heat radiation is properly known as infra red (IR) radiation.
Infra red is pure energy ­ it does not involve moving particles like conduction and convection.
Infra red can travel through anything including the vacuum of space.
Infra red is an electromagnetic radiation (much more about those later), and travels at 8
300000000 (or 3x10 )m/s (the speed of light).
Infra red is a wave. When it hits atoms it makes them shake ­ in other words ­ get hotter.
Sir Isaac Newton produces a spectrum
spectrum
t
gh
i
l
e
t
i
wh
glass prism
temperature rise detected
Visible light and infra­red
infra red
radiatio
Neither ultraviolet nor infrared can be detected by the human eye.
They are invisible
ult
rav
iol
et rad
ia
n
tio
n
Which will absorb the most heat?
Dull black
How will you know?
The water in the black can will be hotter
How is the test controlled (made fair?)
• same volume of water
• same distance from the heater
• identical cans
• same starting temperature
shiny
silver
dull
black
Heat (and light) can behave in a few ways:
(1) transmission
(3)
ref
lec
w
a
(2) absorption r
m
t
h
tion
material
(4) emission
Best reflector of heat
Shiny & silver/white
Best absorber of heat
Dull & black
Best emitter of heat
Dull & black
Worst reflector of heat
Dull & black
Worst absorber of heat
Shiny & silver/white
Worst emitter of heat
Shiny & silver/white
Example: Car Radiator
Example: Car Radiator
radiated
heat
radiator
The radiator pipes are black to help the heat to radiate away.
engine
Without good cooling, the engine overheats, then seizes (welds itself together) ­ get a new engine!
Example: Mediterranean Houses
These are often white to reflect the heat during the day
...and to keep heat in at night.
Similarly, space­suits are white or silver to reflect intense heat
Example: Desert Robes
reflection
su
sh n
ad
e
emission
An odd example: Radiators
Really, these should be dull and black for good emission of heat.
Instead, they are often white ­ a terrible emitter of heat!
On the other hand, if all radiators were black, it is possible that most of the heat would radiate out at the first radiator!
Testing emission from different materials
thermometer
material for testing
er arp
sh drop
hot water
Temperature
insulating lid
material A
poorer emitter
material B Time
better emitter
Independent variable:
Material
Dependent variable:
Temperature change
Insulation
Conductors allow heat to flow through them
Insulators reduce the flow of heat.
Technically, insulators don't exist because heat can pass through anything.
Technically, an insulator is just a very very poor conductor
Reducing heat flow
The thermos/vacuum flask
Silvered bottle:
(a) reflects heat back in
(b) is also a very poor heat emitter
Vacuum trapped in the hollow walls of the bottle:
No particles, so convection and conduction can't work
Reducing heat flow
Household insulation
Loft insulation:
The fibres trap warm air
Warm air is a fine insulator
warm
fibres
air
This insulation is cheap, and very very effective
Cavity wall insulation:
The fibres trap warm air
Warm air is a fine insulator
This insulation is cheap, and effective
Squirty insulation
Solid mats of insulation
Double glazing:
There is a partial vacuum between the panes. The lack of particles reduces heat flow due to conduction and convection.
Infrared (heat) radiation still gets through
double panes of glass
partial vacuum
dessicant
­absorbs moisture
­reduces condensation
Double glazing is very expensive but it is also:
• useful for sound proofing
• low maintenance
• looks good
• provides some burglar­proofing
Draught­proofing:
Draughts tend to carry away heat by convection. Draught excluders block this.
Payback Time
Installing insulation will save money.
Buying insulation costs money Eventually, the insulation will save you enough money to pay for itself.
The time this takes is called the payback time .
Loft insulation payback time
A house has a heating bill of £1000 per year.
25% of the heat is lost through the roof.
How much "money" is lost through the roof?
25% of £1000 = £250
Fitting loft insulation will halve the heat lost through the roof.
How much money does this save per year?
£250/2 = £125
The insulation cost £300 to install. What is the payback time?
£300/£125 = 2.4 years
Double Glazing payback time
A house has a heating bill of £1000 per year.
10% of the heat is lost through the windows.
How much "money" is lost through the windows?
10% of £1000 = £100
Fitting double glazing will reduce the heat lost through the windows by 25%.
How much money does this save per year?
25% of £100 = £25
The double glazing cost £10000 to install. What is the payback time?
£10000/£25 = 400 years!