Download Heat Energy and Transfer - Cleveden Secondary School

HEAT ENERGY AND TRANSFER Level E/F: Teacher’s Science Notes
Heat Energy and Transfer
Teacher’s Science Notes
Heat and Temperature
Heat is the total amount of thermal energy available to a substance and is measured in Joules, as is all energy. Temperature
is a measure of the hotness or coldness of an object and is measured in degrees Celsius (°C) .On a microscopic scale the
heat energy is related to the kinetic energy of the molecules of a substance. The greater a material’s temperature, the more
kinetic energy the molecules have and the heat energy is therefore greater. Also, 3 litres of a liquid at 35°C will hold 3 times
as much heat energy as I litre at 35°C, since, although the liquids are both at the same temperature there are three times as
many molecules in 3 litres. Another nice illustration is to compare the temperature and amount of heat energy in a kettle
and a bath. The kettle will have about 3 litres of water at 100°C whereas the bath will have about 60 litres of water at about
45°C. The bath has more heat energy even though the kettle is at a higher temperature.
Conduction of heat
HOT
(lots of vibration)
COLD
(not much vibration)
Conduction is the transfer of heat energy
through a solid material either by the
movement of free electrons or by the
heat travels along the rod
collision of the molecules of which the
material is made.
heat travels from
hand to ice cream
making the hand
feel cold
Heat flows from the hot end of a solid
substance to the colder end e.g. if a metal
spoon is placed in ice cream then heat
energy flows from the hand holding the
spoon towards the ice cream and the hand
starts to feel cold. Although most pupils will
have experienced this many will think that
it is the cold that flows along the spoon and
some careful explanation may be required.
If the spoon is then placed in a hot substance then heat flows by conduction from the substance to the hand making the
hand feel hotter.
Solid materials, which are good conductors of heat, are also often good conductors of electricity also.
These tend to be metals such a copper, aluminium, silver, and iron. Solids, which do not conduct heat well, are said to be
insulators of heat. Some examples of these are wood, plastic and polystyrene.
Although they are given the name insulator it is very important to realise that all solids conduct heat to some degree due to
the way their atoms are arranged.
Glasgow 5-14 Science Programme Education Improvement Service 24
HEAT ENERGY AND TRANSFER Level E/F: Teacher’s Science Notes
Water, air and almost all other liquids and gases are poor conductors of heat and are
therefore known as insulators. Trapped air in particular is an excellent insulator and
this is used in clothing where pockets of trapped air between the fibres prevent body
heat from escaping. For many years, natural materials such as wool, fur and feathers
made the warmest clothes but now man-made fibres such as polyester are used in
survival equipment as they are light and the thousands of fibres trap lots of air between
them. Birds fluff up their feathers on a cold day to trap air between them and thus provide
more insulation. Many pupils will have seen this and this is also why pillows and duvets
are shaken before use, to increase the amount of trapped air between the fibres.
Convection of heat
Convection is the transfer of heat by means of the
movement of a locally heated fluid substance (gas or liquid)
the most common of which are air and water. As the fluid is
heated it expands which decreases its density. The less
dense warm fluid begins to rise, and is replaced by cooler,
denser fluid from below. This sets up convection currents
which result in a continuous flow of heat upward from the
source.
There are many examples of convection currents in
everyday life. A single electric convector heater set up in a room warms the air at one place in the room and the resulting
convection current transports heat around the room. Many pupils will have observed that paper or posters placed above
a heat source in a room move because of the flow of air created by the convection current.
Domestic hot water systems depend on convection currents to transfer heat from the immersion heater to the rest of the
water in the hot tank. This is also how water is heated in a kettle and why smoke rises from a fire.
Convection currents also explain why firemen enter smoke filled rooms by crawling, there is less smoke and heat there due
to the convection currents. Wider uses of convection currents in nature explain Monsoon winds and the flight of gliders
using thermals.
One way of demonstrating convection currents in the classroom is to take a square of
paper and make a paper pot from it. Place the paper pot on a tripod and half fill it
with water. Heat the paper pot with the blue flame of the Bunsen burner. Take care to
heat the bottom and not the sides of the pot, as the sides will catch fire. Convection
currents will heat the water in the paper pot without burning the paper. Water can be
made to boil by this method. Due to the safety implications this should be carried out
by teacher demonstration and it is a good idea to practice beforehand.
Glasgow 5-14 Science Programme Education Improvement Service 25
HEAT ENERGY AND TRANSFER Level E/F: Teacher’s Science Notes
Radiation of heat
Radiation is the transfer of heat without the presence of
a material medium. For example the suns rays reach us
short
wavelength
through space by radiation. This type of heat radiation
Gamma rays
is known as Infrared radiation, a method of energy
transfer that is part of the electromagnetic spectrum. In
fields, and the energy present in these electromagnetic
fields is transmitted in wave form which does not require
the presence of a material medium.
The electromagnetic spectrum is made up of a ‘family
‘of waves which all travel at the speed of light
(300,000,000 metres/second) through air and space.
Although all travel at the speed of light each has a
different wavelength (the distance between wave crests)
and frequency (the number of waves passing a point in
X-rays
Ultraviolet (UV)
wavelength
formation of rapidly vibrating electrical and magnetic
Electromagnetic spectrum
this the vibration of electrons in materials causes the
Visible light
green
yellow
orange
Infra-red (IR)
red
Microwaves
very high frequency (vhf)
1 second). The members of this ‘family’ in order of
increasing wavelength are gamma rays, X-rays, Ultra-
short wave
Radio
waves
violet rays, visible light, infrared, microwaves, radio and
television waves.
medium wave (mw)
Infrared radiation has a wavelength of between 1
–3
violet
indigo
blue
–6
millimetre (10 m) and I micrometre (10 m).
long
wavelength
long wave (lw)
Heat rays come out of anything which is hotter than its
surroundings. The hotter the object, the more radiation given off. We can feel these rays coming from a fire or the sun. The
whole of the earth is heated by radiation from the sun. Heat radiation travels in straight lines and this explains why cloudy
days are normally cool days, the heat can’t bend round the cloud to reach the earths surface. Heat radiation has many
applications in everyday life. Food is cooked under a grill by radiation, the grill radiates heat downward to cook the food.
Many pupils will think this happens due to convection currents in the air. However this cannot be the case as hot air rises
upwards.
Some burglar alarms detect heat radiation coming from a person’s body and the police and armed forces also use this
method to locate people. Soldiers have infrared sights on guns to locate targets in the dark and the police attach infrared
cameras to helicopters to look for criminals who are hiding or to search for people lost in mountains or rough terrain. Pupils
may have seen these uses in television programmes such as Police, Camera, Action or 999 about the emergency services or
even the news when infrared cameras are used to look for survivors after earthquakes or other disasters.
Glasgow 5-14 Science Programme Education Improvement Service 26
HEAT ENERGY AND TRANSFER Level E/F: Teacher’s Science Notes
Infrared radiation is also used extensively in the field of medicine where a body scan such as a thermogram can be used to
detect the presence of cancer. A thermogram uses infrared radiation to take a heat picture of a person’s body or face. Hot
areas show up white while colder areas show up blue. The colours yellow, orange, red, purple and green correspond to
temperatures in between. Cancerous cells can be detected since they are slightly hotter than the surrounding tissue. It has
recently been suggested that thermograms could be used in lie detectors since hot blood rushes to the eyes when a person
is telling lies and this will show up as a lighter colour on the thermogram.
Physiotherapists also use infrared radiation in the form of heat lamps to heal some sporting injuries more quickly.
Absorption of radiation
Some surfaces absorb heat better than others do. Black surfaces
absorb heat better than white or silvered surfaces. Black surfaces
also radiate (give out) their heat energy better than white or
silvered surfaces. This can be demonstrated in the classroom using
a Leslie cube which is a cube whose sides are made up of different
surfaces (most physics departments will have one of these). Hot
water is placed inside the cube and the infrared radiation coming
from each surface can be detected using an infrared detector.
In hot countries houses are white washed and people often wear
white clothing to reflect heat and prevent them from becoming
too warm. Equally car radiators are often painted black so that heat can radiate from it quicker thus preventing the car from
overheating.
An interesting point to discuss with the pupils is why then are radiators in houses often painted white?
Black coloured radiators would give out more heat and indeed silver foil is often placed behind radiators to reflect the heat
rays out into the room. The answer is simply that, rather than heating the room by radiation, they heat by convection! The
hot ‘radiators’ warm air and it is the air which travels round the room in convection currents (modern radiators have vents
between the outside and inside panels to aid this convection). Also, it is often more pleasing to the eye for radiators to be
painted a light colour.
Glasgow 5-14 Science Programme Education Improvement Service 27