Home
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. 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)
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.
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)
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.
No comments:
Post a Comment