1.0
Introduction
Heat transfer is a common phenomenon
encountered in many areas in daily life. Therefore it is an important subject
in natural sciences and even more so in engineering and the field of
environmental physics. Often one is interested in the various possible ways to
cut down on energy use. To economize on home heating for example one will have
to optimize insulation, i.e. minimize heat transfer. Economizing on cooling on
the other hand implies maximizing heat transfer to some coolant. Other examples
of problems concerning heat transfer in engineering may be found in the
application of materials that cannot sustain extreme temperatures and the like.
This implies again that heat transport towards this material is to be
minimized, whereas the transport away from this material is to be maximized.
One is often dealing with minimization or maximization of heat transfer.
To be able to perform calculations
like these, one will have to be familiar with the concept of heat transfer and
the various mechanisms that result in heat transfer.
These mechanisms are:
I.
Conduction : on
a microscopic or atomic scale kinetic energy is transferred through collisions between the atoms so that on a
macroscopic scale
thermal energy is transferred
II.
Convection : if
the carrier of thermal energy is mobile the thermal energy may be transferred through mass transfer
III.
Radiation : thermal
energy may be transformed into electromagnetic energy, emitted and then absorbed so
that it is transformed into
thermal energy again.
Heat
transfer through radiation takes place in the form of electromagnetic waves,
mainly in the infrared region. Some body because of the thermal agitation of
its composing molecules emits the radiation. In a first approach, the radiation
is described for the case that the emitting body is a so-called 'black' body. A
black body is defined as a body that absorbs all radiation that falls on its
surface. Actual black bodies do not exist in nature though its characteristics
are approximated by the well-known hole in a box filled with highly absorptive
material. This heat transfer takes place only if the medium between the two
bodies is transparent for the relevant spectral region. Furthermore, the
radioactive transfer may be accompanied by other transfer mechanisms.
Particularly convection will be of importance and this mechanism is therefore
discussed in the following section.
Heat
transfer through convection arises when a moving fluid absorbs heat from some
surface and transports this heat to some other location such that the fluid
acts as a carrier. Two forms of convection are distinguished. In the first place,
convection may arise naturally. If for example a hot object at temperature T1
is in contact with a cooler fluid of temperature T2, heat is transported from
the object to the boundary layer through conduction. This leads to density
changes in the boundary layer and as a result, the fluid in the boundary layer
will rise and be replaced by cooler fluid that is heated again etc. This
phenomenon is called free convection.
The
second form of convection arises if the flow is brought about by for example a
pump or a fan. In principle, this form of convection, known as forced
convection, is generally more efficient since the period of contact between the
hot object and the fluid is shortened. This effectively comes down to an
increase of the temperature difference between the solid and the fluid and with
that, an increase of the heat transfer to the boundary layer through
conduction.
2.0
Engage
DrDAQ is being used here as a dual
temperature probe. In both KS3 and KS4, science students have to be aware of
the heat loss from objects due to radiation. Radiation is an electromagnetic
wave, which can travel through a vacuum. You cannot see it with the human eye
but the fire brigade use special cameras, which can see this type of radiation.
All hot objects will lose energy in this way. We wear white or light-coloured
clothes in summer because they are poor absorbers and good reflectors of heat.
This way they keep us cool. On the contrary, we prefer to wear dark-coloured
clothes in winters because they absorb most of the heat of sun and keep our body
warm. In hot countries, the people wear light coloured clothes and if you wear
black on a hot day, you seem to feel hotter. This experiment is examining
whether colour can affect how heat is lost. There are some questions regarding
this experiment.
Situation :
I can remember as a kid having one dull
black thermos bottle that came with my lunch box. One day my mother might put
grape juice in it and at lunch, I would have nice, cold grape juice. The next
day she would put hot soup in it and I would have hot soup for lunch. Moreover,
I can remember asking, "How does it know whether to keep stuff hot or
cold?" Where's the switch, in other words, similarly, "You heat
things up in an oven and cool them down in a refrigerator. How come this thing
can do both?"
Question :
How the thermos bottles keep the food
warm and cool?
How a thermos bottle prevents heat
transfer to the outside by using a vacuum between the walls of the bottle?
What is a link between the colour of
the flask and how quickly the heat was lost?
Why one coloured flask cooled more
quickly than another did?
What made up the wall of the flask?
3.0
Empower
An
experiment
Objective : To investigate how colour can affect
the heat lost through radiation.
Hypothesis : Black colour absorbed more heat than
light colour. The light colour reflect more heat.
Equipment
required
I.
DrDAQ
data logger connected to a PC
II.
Two
external temperature probes (DD100)
III.
One
250 ml conical flask painted black and one with silver foil around it
IV.
A
kettle or other resource to boil water
V.
Two
pieces of insulating material to place over the top of the flasks be (these
will have to be made with a small hole to let the temperature probe into the
flask)
VI.
Two
clamp stands
Experiment
Procedure
1.
Both
the conical flasks is filled with boiling water
2.
The
insulating covers is placed over the top of the flask
3.
The
temperature probes is placed in the flasks
4. The temperatures of the flasks is
started recording. (15 minutes should be long enough)
Figure 1: diagram showing the experiment set up
Safety
precaution : Safety is paramount
here. Take great care when transferring and handling boiling water. Please clamp the
flasks down to avoid the risk of spillage.
Result :
Questions :
1.
Which flask dropped in temperature the
quickest?
2.
Can you suggest a link between the colour of
the flask and how quickly the heat was lost?
3.
Use textbooks and/or the internet to find some
scientific reasons why one coloured flask cooled more quickly than another does.
Discussion of
results
All
hot objects lose energy by radiation. Radiation is an electromagnetic wave.
Radiation can travel through a vacuum, this is how the energy from the sun
reaches the earth. Other types of energy transfer are conduction, convection
and radiation. Based on the graph, we can see that, the dull black flask
(external 2) lost heat more quickly than the silver flask (external 1). The dull black surfaces lose energy
more quickly as they are better radiators. Bright polished surfaces act as good
reflectors of heat. Such surfaces absorb very little heat and reflect towards
us most of the heat radiations. These surfaces remain cool even after
continuous use of heat. This is because conduction, convection and radiation of
heat are minimum. Example, base of cooking utensils is made black. Such a black
surface absorbs more heat from the surroundings. The highly polished surfaces
of spacecraft reflect most of the heat radiated from the sun.
4.0 Enhance
In this part, we are going to improve the
product. It is important to ensure that heat lost due radiation is not too
much. As we need heat to cooking and heat food and drinks. On the other hand,
we also sometimes need to keep the food or drinks cool or warm. Hence, the
thermos bottle is very important in our daily routine especially for household.
Kids are like to bring lunch box to school so that they can enjoy their
homemade lunch. However, to keep the food or drinks in the same condition as it
prepared at home, thermos bottles or box is needed to keep the food warm or
cool. Funky and attractive the thermos bottles are suitable for kid. Hence, how
we can improve the thermos bottles into more interesting packaging without
alter it function. Can we replace the cover on the top of the flask with
colourful plastic material?
These questions below help us to find the solution how to improve this product
to be more attractive and funky.
·
Can you design an experiment to discover if different coloured flasks
full of water would heat up more quickly than others would? Show your teachers
your design
·
Why was an insulating cover placed over the top of the flask?
Conducting an experiment to investigate
if different coloured flasks full of water would heat up more quickly than other
would.
Planning a further experiment.
Objective :
To investigate if different coloured flasks full of water would heat up more quickly
than other would.
Hypothesis : The darker the object, the better its emission of heat
because, it is a better absorber of heat.
Procedure :
1.
Four
conical flasks are wrapped with same material (paper) but of different colours.
2.
The
conical flasks are filled with boiling water
3.
The
insulating covers is placed over the top of the flask
4.
The
temperature probes is placed in the flasks
5. The temperature of the flasks is
started recording. (15 minutes should be long enough)
Result
:
Question :
1. Is different coloured flasks full of
water would heat up more quickly than others would?
2. Why was an insulating cover placed
over the top of the flask?
Discussion :
This experiment works first time, based
on the graph every time we will see clearly that the rate of absorption and
radiation of heat energy for the black flask is greater than the other three. The
different coloured sheets absorb different parts of the light spectrum and emit
thermal radiation. The more light energy that is absorbed, the more thermal
energy is emitted. The second highest rate of absorption and radiation of heat
energy green flask, followed by blue flask and red flask. From this pattern, we
can see the darker colour absorb and radiate more heat than the lighter colour.
This because colour the colour of an object depends on the wavelengths of
colours reflected from the object.
A
red paper is red because red wavelengths in white light are reflected and other
wavelengths are absorbed. If a red paper were to be illuminated by light that
had no red wavelengths, the paper would appear almost black. When a black
object is illuminated by white light, all wavelengths are absorbed and none are
reflected. That is why the object appears black. When a black object absorbs
light, the energy carried by the light does not just disappear. Rather, it
raises the energy of the object doing the absorbing. The object, in turn,
releases the absorbed energy by emitting longer wavelength, lower energy
infrared (heat). This transformation of light into heat is the key to
understanding the process because it accounts for the law of conservation of
energy. Light just does not disappear when it strikes a black object, it is
transformed into another kind of radiation that is either radiated from or
retained within the black object.
Insulating
cover is placed on the top of the flask so that no heat is released or
absorbed. Hence it will not affect the result of this experiment.
Conclusion : The darker the object, the better its
emission of heat because, it is a better absorber of heat.
5.0 Extension
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