Heat and heat transfer
Temperature (T)
-Measures how hot something is
-Measurement of average (translational) kinetic
energy
-NOT total kinetic energy (2 different size volumes of
water)
o
o
o
degrees Celsius or degrees Centigrade or °C
Fahrenheit scale
Absolute Freezing
Kelvin Scale
Boiling of
zero
of water water
-273 °C
0 °C
100 °C
-460 °F 32 °F
212 °F
0 °K
273 °K 373 °K
Heat (Q)
-Objects DO NOT have heat
- Heat is a transfer of thermal energy
- Heat is NOT temperature! They are not interchangeable!
- Heat is like work…
-Work is the transfer/changing of mechanical energy
-Heat flows from hot objects to cold objects
- Just like how water flows, it goes from high to low
Thermal Equilibrium
When two objects are in thermal contact, they will exchange
heat from the hot object to the cold object until they reach
thermal equilibrium.
- Until they are the same temperature
A thermometer
reaches thermal
equilibrium with the
material it is in.
Thermal Expansion
Materials expand when heated and shrink when cooled
- there are a few exceptions: Water
Transfer of heat
When heat is transferred, it is pulled from hotter objects into
colder objects. NEVER THE OTHER WAY AROUND! Alton
Brown’s Explanation of the transfer of heat and thermoses
-Conduction
-Convection
-Radiation
Insulators Vs. Conductors
Which of the following are good conductors:
Aluminium
Glass
Copper
Iron
Polythene
Cardboard
Nickel
Paper
Chocolate
Steel.
•
•
•
•
•
Good
Aluminium
Copper
Iron
Nickel
Steel
Bad
•
•
•
•
•
Glass
Polythene
Cardboard
Paper
Chocolate
• Insulator is the special posh sciencey
name for a poor conductor.
• For the following objects, state whether
you would want them to be made out of a
good conductor, or a good insulator:
•
•
•
•
The bottom of a saucepan
The handle of a saucepan
A duvet/quilt
An ice cream tub
• Good conductors
• Good Insulators
• Bottom of a saucepan
• Saucepan handle
• Duvet/quilt
• Ice cream tub
Conduction
Conduction
• Stir your hot soup with a metal spoon
• Pretty soon you need a pot holder because the end
of the spoon you are holding gets hot
• This is heat transfer by conduction
• Energy travels up the spoon from the end in the hot
soup to the end in your hand

Heat transfers along an object

metals have high conduction

Heating through touch/ contact
Conduction
Metals have some electrons that are very
loosely bound to the atoms in the material
These electrons can move easily and can
rapidly pick up additional kinetic energy
Metals are good conductors
Wood and plastic don’t have loosely
bound electrons, so they are poor
conductors
So…
When you eat a popsicle, why does the stick feel warmer
than the popsicle part if they were both in the freezer
together?
Convection
•Hot air rises because…
•It is less dense than cold air
•Why?
•When something is heated the particles
move around more and spread out.
•Why does cold gas and liquid fall?
•When it cools the particles move around less,
move closer together, and therefore become
more dense.
Convection
• A phenomenon in fluids
• Instead of having energy moved by
successive collisions of electrons,
atoms and molecules, the fluid itself is
set into motion called a current
• These moving fluid currents are
convection
Convection
• When the radiator heats the air, it becomes
less dense and rises
• Cool air moves in to replace the air that
rose
• This generates the air flow
• So radiators don’t need a fan to stir the air
and to distribute heat throughout a room
• The rising air cools until its density matches
that of the surrounding air
Convection
• We take advantage of the cooling that
occurs during an expansion
• We make refrigerators and air
conditioners operate by forcing gas
under pressure through a small hole
and expanding it into an empty space
Convection
• Explains why breezes come from the
ocean in the day and from the land at
night
Radiation
Radiation
• Energy carried by electromagnetic
waves
• Light, microwaves, radio waves, x-rays
• Wavelength is related to vibration
frequency
Radiation
• Every object is emitting electromagnetic
waves regardless of temperature
• Things we can see from their own
radiation are very hot to have energy
emitted in the visible region of the
spectrum
• Most things emit primarily in the infrared
• Night vision goggles, etc.
Radiation
• Interior of a car on a sunny day
• Sunlight comes in as visible light
• Seats and interior are much cooler so
they radiate in the infrared instead of
visible
• Glass in the windows blocks infrared so
energy can’t get out
• Car interior heats up!
Radiation
• A good absorber reflects very little
energy
• Think about dark pavement
• A poor absorber reflects a lot of energy
• Think about snow that doesn’t melt in
sunshine even though 1400
watts/meter2 are hitting it
The diagram below represents a saucepan on a hot plate.
The saucepan is partially filled with water and the hot plate is
turned on. After some time, the air at point X above the pan
becomes hot.
a. Explain how heat is transferred from the hot plate to the base
of the saucepan.
b. Explain how heat is transferred from the base of the saucepan
to all of the water in the saucepan.
c. Explain how heat is transferred from the hot water to the air at
point X.
The diagram below represents a home heating system.
a. Give an example of where heat convection occurs in this system.
b. Give an example of where heat radiation occurs in this system.
c. What purpose do the ceiling fans serve in helping to heat the home?
d. Explain why pipes carrying hot water run along the floors of the rooms
instead of the ceilings.
Change of Phase
When heat is added to, or taken away from a substance, the
temperature of the substance will either increase or decrease.
However, when a substance changes phase (like when water
boils or freezes), heat is need to change the properties of the
substance.
When this happens, the temperature of the substance will not
change, but the substance will change phase.
Change of Phase
This is known as the latent heat
80 cal/g
540 cal/g
Change of Phase
•Evaporation
from liquid to gas at a surface
requires energy (heat) to vaporize a liquid
Absorbs heat
•Condensation
from gas to liquid
gives off heat
fog and clouds are condensed water vapor
•Boiling is a special case of evaporation
takes place below the surface
100 °C for water at atmospheric pressure
temperature of solid
and liquid are the same
540 cal/g to evaporated water
Called Heat of vaporization

Change of Phase
•Freezing
from liquid to solid
losses energy (heat) to freeze a liquid
•Melting
–From solid to liquid
–Requires heat to melt a solid
–Absorbs heat
temperature remains constant
80 cal/g of heat lost to freeze water
Called Heat of fusion

Change of Phase
•Sublimation
from solid to gas
snow can turn to gas directly in winter
Think of dry ice

•Deposition
from gas to solid
Water vapor can turn into snow flakes
through deposition

Latent Heat
Latent Heat: The amount of heat required to change the
phase of a material.
•Dictated by either the Heat of vaporization or the Heat of
Fusion
•Heat of vaporizations- the heat required to change
between liquid and gas (in either direction)
•Heat of fusion- the heat required to change between
solid and liquid (in either direction)
The temperature of an object doesn’t change when it is
changing phase, that heat energy actually goes to changing
the phase.