The Thermos

1) How does a thermos work?
A thermos, also called a vacuum flask, is a storage
vessel which provides thermal insulation
thermal insulation = materials used to reduce the rate of heat transfer
The vacuum flask was invented by Scottish physicist
and chemist Sir James Dewar in 1892 and is
sometimes referred to as a Dewar flask after its
The first vacuum flasks for commercial use were
made in 1904 when a German company.
Vacuum flasks are used to maintain their contents
(often but not always liquid) at a temperature
higher or lower than room temperature,
Domestically and in the food industry, they are
often used to keep food and drink either cold or
A typical domestic vacuum flask will keep liquid cool for
about 24 hours, and warm for up to 8.
It is a bottle made of glass, metal, or plastic with an
inner and outer wall
the narrow region between the inner and outer
wall is evacuated of air (called a vacuum)
The bottle must have an opening for contents to be
added and removed.
A stopper made of insulating material must be used
to stop heat from exiting
 originally cork, later plastics.
A thermos provides thermal insulation, meaning it
reduces the transfer of heat between the thermos
and the external environment.
Heat is transferred in 3 ways:
 Convection
 Conduction
 Radiation
Convection is the transfer of heat energy in a gas
or liquid by movement of currents.
Example of a convection current:
Hot air is less dense (lighter) than cold air, so it rises
upwards in your home. As hot air starts to rise it has to
push colder air out of its way. So the cooler air near the
top of your house moves back toward the bottom to get
out of the way. This is why basements are always colder
and top floors are always warmer.
 is the transfer of heat energy through matter
from particle to particle (or atom to atom).
 Ex:
a spoon in a cup of hot soup becomes warmer
because the heat from the soup is conducted
along the spoon.
Heat transfer through radiation takes place in form
of electromagnetic waves mainly in the infrared
region of the spectrum. Radiation emitted by
matter is a consequence of thermal agitation of its
composing molecules.
The vacuum created by the inner and outer walls of
the thermos reduces the transfer of heat from the
contents of the bottle to the outside environment.
(prevents conduction)
The tight stopper stops air from entering or leaving
the flask (prevents convection)
The reflective lining of the inner chamber reflects
infrared radiation straight back in again (prevents
Inside a vacuum flask
Screw-on stopper.
Outer plastic or stainless steel case.
Outer layer of glass coated with reflective material (or
stainless steel in unbreakable flasks).
Inner layer of glass (or stainless steel in unbreakable
Supports between keep inner vacuum chamber in place.
Additional insulation reduces heat losses and cushions
flask against impacts.
Homework for tomorrow:
Individual research (3 good websites and some relevant
information from each), print by end of class, signed by me
Design plan: draw a model of your thermos which illustrates
it’s parts and the materials chosen (to be handed in)
Tomorrow: get together with group to plan materials
Friday: build
Monday: Test
Tuesday: Hand in report (individually)
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