Transcript Heat Transfer
Heat Transfer
• • •
Heat Energy
Heat is the form of energy that flows from a
hot region to a cold
region Heat energy is also called
thermal energy
Heat energy flows by one or more of the processes of:
1.
2.
3.
4.
CONDUCTION CONVECTION RADIATION EVAPORATION
Thermal Energy
• Thermal energy (or heat) is measured in
joules
• A space shuttle can reach heats of
1500 °C
, because of the
friction
between molecules in the air and the craft (which is travelling at 23,612 mph) • The small beaker boils first, because it needs
less thermal energy
to reach 100 °Celsius
The Four laws of thermodynamics
• No heat flows between object at the same temperature • All machines are inefficient • Heat transfers to colder matter until both objects are the same temperature • At absolute zero (-273°C) no reactions occur
Questions on heat transfer
• How does heat move?
• It
flows
from
hot areas to cold
areas • What units is thermal energy measured in?
•
Joules
• Which has more thermal energy: a swimming pool with the water at 30 °C or a cup of coffee at a temperature of 80 °C?
• The swimming pool because it
takes longer to heat
the water to 30 °C
Conduction
• • All
metals are conductors Copper
is an excellent conductor so it is used to make pipes in the house
Conduction by Atoms
•
Atoms vibrate more when they are heated
•
Conduction is the only way for heat to travel through solids
•
Gases
are the
worst conductors
• A very poor conductor is called an
insulator
• A
vacuum does not allow conduction
Finding the best conductor
• All the rods have the
same length
• They are all burner reaches a
wax melts
stick falls off
heated equally
at one end with the bunsen • When the other end of a rod
certain temperature the paraffin
and the match • A match stick will
fall
off the best conductor first • This should be the rod made of
Copper
Water - a poor conductor
• A boiling tube of water is
heated near the top of the water
• Water
boils at the top
• The
bottom
of the tube
remains cool
enough to hold • This shows that
water (and glass) only conduct heat relatively slowly
• Microwaves make water molecules in food move quickly making them rub against food to heat it
Insulators
• • • • Most
non-metals are insulators Air
is an excellent insulator Lightening travels at 435,000 km/hr and heats the air to
28,000 °C Trapped air helps to keep heat in!
Polystyrene Coffee Cup Double Glazed Windows Cavity Wall Insulation
Questions on Conduction
• Place these substances in order starting with the best conductor:
Air Water Copper Glass
•
Copper Glass Water Air
• Why do string vests keep people warm even though they are full of holes?
• They are able to
trap air
Questions on Conduction
• Why are Coke cans made of metal, but coffee cups made of polystyrene?
• Coke can placed in a fridge will leave the
Coke cool. Polystyrene coffee cups have trapped air
to keep coffee
warm
• • Why do trapped mountaineers dig snow holes when it gets too windy?
To trap air and stay warm
Convection
• • This is how heat travels through
liquids and gases Hot air rises
because it is less dense •
Cold air sinks
(it is more dense) • This creates
convection currents
Convection Currents
• • Molecules move
more quickly and
occupy more space when they are
heated Hotter dense
fluids are therefore than colder fluids
less
• •
Hotter fluids rise
top of colder fluids up to float on • A
convection current
is the path taken by rising hot fluids and sinking cold fluids
Convection does not occur in solids
Hot air rises
• Heated air provides lift for a
hot air balloon
• Heated land causes rising air currents called
thermals
• Thermals can be used by
gliders
to provide lift • Many
birds
also use thermals for lift
Questions on Convection
• In which of the following can convection take place?
Solids Liquids Gases •
Liquids
and
gases
• How do fire fighters enter a burning room?
• They
stay low
because it is
cooler
and there is less smoke (it rises with the hot air)
• Questions:
Why does it take so long for exam halls to heat up?
Hot air rises and then circulates to warm the whole room Why would a fridge not work if the coils were at the bottom?
Cold air sinks, so no air would circulate
Thermal radiation & temperature
• The
hotter
an object the
more radiation
it gives off • Radiation can travel through
vacuums
• All
hot objects radiate heat
• The Earth is heated by the
Sun’s radiation
Radiation
• Hot objects beam (radiate) out heat in
all directions
• This travels through
vacuums
(this is how the Sun heats the Earth despite the Space in between) • • Shiny objects reflect heat
Dark objects absorb heat
Silvered surfaces
• A metal
kettle
, a
firefighter
and a
marathon runner
make use of silvered surfaces.
Dark Surfaces
This Lamorghini Diablo has it’s
radiators exits painted black
Solar panels are
painted black
to absorb more sunlight
Questions on Radiation
• Why is Pluto colder than the Earth?
• It is
further away from the Sun
less radiation so it gets • • Which form of heat transfer makes you stand back from a bonfire?
Radiation
Evaporation
• When we are hot
sweat
is produced by our skin • As this
water evaporates it cools our skin
The Thermos Flask
insulated cap silvered inner wall vacuum between glass walls hot coffee outer casing of plastic insulated support
Conserving Heat In The Home
• http://www.sgcscience.co.uk/physics/Simulations/EnergyHouse/appl et.html
• A house retains 20% of its heat
Questions on Conserving Heat
• Why is there a vacuum in the thermos flasks?
• To stop
conduction
and
convection
• How is radiation stopped in a thermos flask?
• • By the
silvered
inner wall • How would you prevent heat loss from a house?
Double glazed windows
,
Cavity wall insulation
,
loft insulation
,
draught excluders
, carpet,
curtains