Download Thomas Sears

Physics Exemplars
AS 90181 (Physics 1.2) Process information to describe a use of
physics knowledge with direction (Version 3)
Level 1, 2 credits.
The following samples of student work exemplify the type of evidence required for
judgements at the different levels of achievement for this achievement standard.
The Achievement Criteria for the standard are:
Achievement
Achievement with Merit
Process information to
describe a use of physics
knowledge
Process information to
explain a use of physics
knowledge
Achievement with
Excellence
Process information to
discuss a use of physics
knowledge
The explanatory notes (EN) of the standard give guidance about typical evidence that
contributes to a particular grade:
EN4: Information for processing is to be selected from a range of sources that may be
provided, which may include secondary information, i.e. information that has been previously
collected and processed by another person. Sources of information are to be recorded in a
way that can be accessed by others.
EN5: Processing information could involve listing, sorting, collating, highlighting, or
summarising relevant information on science knowledge and its related use.
EN6: Terms:
• Describe means provide characteristics of, or an account of, the scientific knowledge
related to its use.
• Explain means provide reasons as to how or why the scientific knowledge applies to
the use.
• Discuss means link ideas to integrate relevant physics knowledge with its use, and
will involve elaborating, justifying, relating, evaluating, comparing and contrasting, or
analysing.
See also:
• Clarification of Achievement Standard 90181 www.nzqa.govt.nz/ncea/resources/physics/clarifications-level1.html
Student Grade
1
Not Achieved
Moderator Commentary
This student has given a general description of the historical
development of the technological application (steam iron), but has not
described the scientific knowledge related to its use, as required for
Achieved.
There is little evidence of processing: Much of the text has been copied
directly from the sources without any attempt to highlight or focus on
relevant science.
Only two sources were used. A minimum of three sources would be
expected. Sources of information are not recorded in a way that can be
accessed by others, as the listed web addresses do not specifically link
to relevant information.
To gain Achieved, the student would have to give examples of
scientific knowledge related to the steam iron. This material would
need to be processed (i.e. not be copied wholesale from its source, but
be sorted, listed, summarised, collated or highlighted in some way). A
range (at least three) sources should be used in gathering relevant
information.
Technological application -the Steam Iron
The electric iron was invented on June 6 1882 by Henry W. Seely of NYC. It used a carbon arc to create heat --a thoroughly unsafe technology. The Seely iron weighed 15 pounds and took a very long time to warm up.
In 1892, hand irons using much safer electrical resistance technology were introduced by Crompton and Co.
and the General Electric Company, and this has been the standard ever since. This iron had an electric heating
element in the base and a somewhat traditional hand-held chunk of metal on the top. Later irons would have the
heating elements built in to the hand-held tool. The advantage of electric irons is they kept your clothes clean. If
you heated your iron on a wood stove, it would easily get dirty and ruin the clothes that were being pressed.
We remember that both heat and moisture are required for proper ironing. Ironing was usually done the day
after washday. First the clothes had to be dampened by sprinkling with water, rolled and left to sit from one to
12 hours so that they would have just enough moisture for proper ironing. The needs of commerce led to the use
of steam in the ironing process to shorten the soaking period. In most cases live steam was fed through the
soleplate of the iron from an external boiler. This was expensive and also quite dangerous.
The inventor of the steam iron is believed to be Thomas Sears.
The technology for bringing steam to the home was not available until the 1930s. Among the very first home
steam irons to be successfully marketed was the Steemelectric. It was basically a kettle with a polished flat base.
In the 1950s, there was a period of intense competition that was known as the “Holey Wars”. Proctor began
with an advertising campaign that touted the efficiency of its 15 steam jets. Eventually some manufacturers
featured over 100 steam vents. The industry standard has evolved to be about 22 holes spaced evenly around the
perimeter of the sole plate.
One of the problems associated with early steam irons was weight. The self-contained steam iron had to have a
water tank which led to an increase in size and weight. Also, early steam irons could not be used for "dry"
ironing. Other problems revolved around filling the water tank and getting it high enough to assure flow to the
evaporation chamber. Many early steam irons used an external water reservoir that could be removed.
References:
http://en.wikipedia.org/
http://www.jitterbuzz.com/
2
Achieved
(low)
This student work is at the Not Achieved/Achieved boundary. Secondary
information has been taken from a range of sources. Some relevant physics
knowledge has been listed: Electrical resistance and heating, change of
state as water boils, metals as heat conductors. However there is very little
detail and some clearly relevant physics knowledge described in the source
material has not been mentioned. The student mentions the thermostat, but
does not attempt to describe the physics involved in its operation. The
property of metals as electrical conductors is wrongly related to their use in
the soleplate.
Processing of information is seen in the bullet-point summary of the
physics knowledge.
Sources of information are not recorded in a way that can be accessed by
others, as the listed web addresses do not specifically link to relevant
information. However, inadequate referencing does not in itself preclude
achievement, as referencing is not part of the achievement criteria outlined
in the standard.
Technological application -the Steam Iron
Introduction
The steam iron is a machine that uses a lot of physics to make it go. I will describe some of the ways that physics
knowledge is used in making a steam iron work.
What is a steam iron?
A steam iron is a small appliance used in ironing to remove wrinkles from fabric.
How does it work?
Ironing works by having a heated metal base – the soleplate. It also has a water tank that makes steam. The
steam comes out of holes in the soleplate onto the clothes that are being ironed.
How is the temperature controlled?
Irons have a thermostat ensuring maintenance of a constant temperature usually fitted with a temperature
control dial allowing the person using it to select the operating temperatures (usually marked with types of cloth
rather than temperatures: silk, wool, cotton, linen etc.)
What is the physics in a steam iron?
• There is a resistor in the soleplate that gets hot when the electricity is switched on.
• When it gets hot it also makes the water boil and turn to steam. When the steam cools down again it
condenses back to a liquid. This physics is called changes of state.
• The soleplate is made out of metal. It can be aluminium which is cheaper, or stainless steel which is easy to
keep shiny. Physics is important here because metals are good heat conductors and metals conduct electricity.
Conclusion
The steam iron is an important part of our lives, and without it our lives would be more difficult. There is a lot
of physics knowledge used in the steam iron to make it go, as you can see.
My Sources of information
Wikipedia.com
Jitterbuzz.com
My Mum
3
Achieved (high)
This exemplar is at the Achieved / Merit boundary. The student has
processed the referenced material, selecting, summarising and collating
relevant scientific information. The student has described several
instances of the use of physics knowledge in the steam iron (electrical
circuits, thermostatic switching, temperature and change of state, heat
and electrical conduction and insulation, etc.). However, there is no
explanation as to how or why the scientific knowledge applies to the use.
For example, there is no explanation as to why the handle is made of a
heat- insulating material but the soleplate is made of a heat-conductive
material.
Sources of information are recorded in sufficient detail for others to
access.
Technological application -the Steam Iron
The electric iron was invented in 1882. It uses an electric current flowing through a resistor to produce heat,
but originally there was no way to control its temperature, so it was not easy to use. In the 1920s irons with
thermostats to control their temperature were introduced. These could switch on and off the current at certain
temperatures so the iron stayed hot but not too hot.
The steam iron was invented in about 1930 by Thomas Sears.
A steam iron contains a small tank of water, which is heated by the same electric heater that makes the soleplate
of the iron hot. When the liquid water reaches 100 oC it begins to boil and turns into steam, which is water in a
gas state. When the steam hits the fabric which is less than 100 oC, it condenses back into liquid water.
The soleplate of the iron is made of metal, which is a good conductor of heat. The usual soleplate metal is
aluminium which is lightweight, or stainless steel which is easier to keep clean.
There is a dial that can be used to adjust the operating temperature of the iron. This is calibrated for different
fabrics such as cotton and linen (which need a hot iron), and fabrics like nylon and silk (which need cooler
temperatures).
The handle of the iron is usually made of plastic which is an electrical insulator as well as a heat insulator.
The power cord of the iron is sometimes covered in Teflon which is an electrical insulator.
Some irons have a special safety switch that turns off the iron if it is left lying flat for too long.
It is possible to get cordless irons. They plug into a charger unit to heat up, and then can be used for a while
until they cool down again. There are no battery-powered irons.
References:
http://www.jitterbuzz.com/ironing_history.html
http://en.wikipedia.org/wiki/Iron_(appliance)
http://home.howstuffworks.com/ironing-tips1.htm
4
Merit (low)
This exemplar is at the Achieved / Merit boundary. The student has identified three
key aspects of physics (energy transformation, heat transfer, change of state), and
described them, and provided reasons to explain why the scientific knowledge
applies to ironing. For example, energy transformation is related to the need to heat
the fabric in ironing. Heat transfer is applied to the need to move heat rapidly to the
soleplate and also to the need to prevent heating of the handle of the iron. The
change of state is explained as relevant because of the expansion of water on
vaporisation (penetrating the fabric) and because latent heat of vaporisation
increases the heat energy of the water.
Processing of information is shown in gathering and summarising the main physics
aspects, and then relating each one to the application.
Sources of information are recorded in sufficient detail for others to access.
Technological application -the Steam Iron
The steam iron is a small household appliance that involves the application of several areas of physics
knowledge. The main areas are:
• Energy transformation
• Heat transfer
• Change of state (phase change)
In my report I will look at how these areas of physics are involved to make the iron work.
1. Energy transformation
Ironing clothes takes out their wrinkles and makes them smooth. This takes heat, which is one type of energy. In
the iron, electrical energy is used, and it is transformed into heat energy. This happens when an electric current
passes through a resistor in the soleplate of the iron.
2. Heat transfer
Heat transfer means moving heat from one place to another. We want to move heat energy quickly to the
clothes, so they get smooth quickly. This means the soleplate should be a good heat conductor, like metal. That
is one reason why the soleplate is made of aluminium or stainless steel.
Sometimes you do not want the heat energy to travel quickly, e.g. in the handle of the iron. If the handle gets too
hot it is difficult to use. This means the handle should be made of plastic because plastic is an insulator and
does not transfer heat quickly.
3. Change of state
To make steam you have to heat up the water until it boils. Then it changes state to become a gas (steam). When
liquid turns into gas the particles move around faster and take up a lot more room. This is why a little bit of
water can turn into a lot of steam. The advantage of this is that the steam expands and comes shooting out of the
holes in the iron, and goes right into the clothes that you are ironing. When the steam hits the clothes it will
condense back into liquid water again.
It takes energy to change state. This is called latent heat of vaporization. When the steam condenses again the
heat energy comes out and helps to heat the clothes more.
Sources of Information
http://www.wisegeek.com/what-is-energy-transformation.htm
http://en.wikipedia.org/wiki/Heat_transfer
http://www.mikeblaber.org/oldwine/chm1045/notes/Forces/State/Forces04.htm
5
Merit (high)
This work is at the Merit/ Excellence boundary. The student has processed
the referenced material, and recorded sources of information.
This student has fully described the major areas of scientific knowledge
involved in the steam iron, and has also explained the use of this knowledge,
providing reasons how or why the scientific knowledge applies to the use
(e.g. that the volume expansion on vaporisation causes steam to come out of
the soleplate; that heating and pressure cause clothing to be smoothed; that
expansion of metals is used to switch current in the thermostat; that heat
capacity affects the time an iron remains hot; that latent heat energy heats the
fabric when the steam condenses).
This exemplar does not gain an Excellence grade as it does not proceed
beyond explaining how the physics knowledge applies to its use. For
Excellence, students must link ideas, and as explained in Explanatory Note
6, this will involve elaborating, justifying, relating, evaluating, comparing
and contrasting, or analysing. For example, in describing the handle’s
construction from an insulating material, no comparison with, or contrast to,
other materials was made. The concept of heat capacity of the soleplate was
used in explaining the operation of a cordless iron, but no justification or
evaluation or elaboration of soleplate material was given. The only place
where discussion involves comparison or contrasting is where the rapidheating metal soleplate is contrasted with “other materials”. However this is
not specific and is insufficient evidence for Excellence.
Technological application -the Steam Iron
The electric iron was invented in 1882. It uses an electric current flowing through a resistor. This causes
electrical energy to be transformed into heat energy. The resistor gets hot, and because it is embedded in the
metal of the soleplate, the heat is conducted into the soleplate. As the soleplate is made of aluminium or
stainless steel, it is a good conductor of heat: This means it heats up quicker than other materials would when
the iron is switched on.
A steam iron has a small tank of water which is also heated by the element that heats the soleplate. When the
liquid water reaches 100 oC it begins to boil and turn into steam, which is water in a gas state. Water expands
when it changes state from a liquid to a gas. This is because the molecules in a gas are much further apart
compared to those in a liquid. The steam comes out of the holes in the soleplate and into the fabric that is being
ironed. When the steam hits the fabric which is less than 100 oC, it condenses back into liquid water.
The way ironing works is to use high temperature and pressure to smooth out wrinkled clothing that has been
washed and dried. Ironing works by loosening the bonds between long chains of molecules in the fabric. When
the fabric cools down again the molecules stay in their new shape. Heat supplies energy to the molecules to
reshape them, and the weight of the iron applies pressure to the molecules, pushing them into new shapes.
The thermostat controls the temperature of the iron by switching on the current when the iron is cool, and
switching it off when the iron reaches a certain temperature. The physics of this is that metals expand when
heated, so a bimetal strip is used: This has two layers of different metals that expand by different amounts when
heated. The strip bends when one side expands more than the other, and this switches off the current. When the
strip cools down again the two sides contract, straightening the strip and then the current is switched on again.
Because the iron gets very hot, the handle must be made of an insulator. This keeps the handle from heating up
quickly which would make it dangerous to hold.
Cordless irons plug into a charger unit to heat up, and then can be used for a while until they cool down again.
The time they stay hot is limited by how much heat energy they can store. This depends on the heat capacity of
the soleplate of the iron. Heat capacity measures the amount of energy that an object stores for every degree
Celsius temperature rise. Steam irons take a lot of energy to run because it takes a lot of latent energy to turn
liquid water into steam at 100 oC. This latent heat energy is transferred to the fabric when the steam condenses,
heating it.
References:
http://www.jitterbuzz.com/ironing_history.html
http://en.wikipedia.org/wiki/Iron_(appliance)
http://home.howstuffworks.com/ironing-tips1.htm
6
Excellence
This student has processed the referenced material, and recorded sources so
that they can easily be followed.
The student has described various aspects of scientific knowledge that are
used in the steam iron (e.g. heat conduction, insulation, heat capacity,
change of state and latent heat), and has explained how the physics applies to
the use or functioning of the iron (e.g. how expansion of metals in a bimetal
strip in a thermostat is used to switch heating on and off). Furthermore, this
student has linked ideas integrating relevant physics knowledge with its use;
comparing and contrasting (e.g. comparing the advantages of using iron,
aluminium or copper for the soleplate), justifying (e.g. why plastic is better
than metal for the handle),elaborating (e.g. detail of polypropylene’s heat
conduction), relating and evaluating (e.g. the relative merits of aluminium
c.f. iron for the soleplate of a cordless iron).
Technological application -the Steam Iron
The invention of the electric iron, and later the steam iron followed the introduction of household
electricity supply. Before electricity was available to homes, ironing involved the use of heavy irons
that were heated in the fire. Even years later, poor people still used fire-heated irons:
“In the 1930's an iron was iron—a six- or seven-pound wedge of iron. The irons used in the Hill
Country had to be heated on the wood stove, and they would retain their heat for only a few minutes—
a man's shirt generally required two irons; a farm wife would own three or four of them, so that
several could be heating while one was working.” (Caro, R.A. (1983). The years of Lyndon Johnson:
The path to power. New York) (1)
Ironing smoothes out wrinkles and creases in clothing that has been washed and dried. The wrinkles
occur because long polymer molecules have bonds that link to other molecules (2) (3). These bonds
have to be broken using heat and pressure, and then the molecules will hold their new shape when
cooled again. Heating provides the energy needed to break these bonds, and the weight of the iron
presses the molecules into their new shape.
The electric iron is heated by letting a current flow through a resistor which makes up the heating
element. Resistors transform electrical energy to heat. This heat is conducted through the soleplate. In
order to maximise heat transfer, the soleplate should be a good heat conductor. Metals are better
conductors than non-metals. Copper is a very good conductor, but is expensive and easily scratched.
Aluminium is cheap, a good conductor, and has low density and so the iron can be lightweight and
easy to lift and use. One of the problems of the original irons was that they were heavy and tiring to
use. Some weight is still needed, as explained above, to press the molecules into position. Stainless
steel is not as good a conductor as aluminium (4), but it is harder and more resistant to corrosion
which can occur in the hot, wet conditions of a steam iron.
Steam irons have a water tank that is heated. At 100 oC the water boils and changes state from liquid
to gas. In turning into a gas latent heat energy is stored in the steam. (5) Water has a very high latent
heat, so it can store lots of energy compared to most other substances. This means steam has higher
energy than liquid water. Water and heat energy are both needed to break the bonds between
molecules, so steam is perfect for this. When the steam is cooled it condenses to liquid again,
releasing its latent heat energy. The spray of “steam” from the iron is really a mixture of liquid and
gas. Steam is invisible, but the spray from an iron can be seen. Liquid water is a better conductor
than steam, so when it hits the fabric, it heats it up faster.
A thermostat controls the temperature of the iron by switching on the current when the iron is cool,
and switching it off when the iron reaches a certain temperature. The physics of this is that different
metals expand differently when heated, so a bimetal strip made of layers of two different metals is
used, for example brass and iron. Brass expands more than iron (6), so when it is heated it bends
away from the brass as that side expands more. This bending is used to break contact in the circuit,
switching off the current. As it cools down, the metals shrink back again, straightening the strip, until
it makes contact again and switches the current on again. The thermostat can be adjusted by turning
a dial that moves the bimetal strip further away. This means it has to heat up hotter before the switch
contact breaks and stops the heating.
The handle and body of the iron are usually made of plastic because it is a heat insulator. For
example, polypropylene conducts heat about 800 times more slowly than aluminium (7). This stops
the handle from getting too hot to hold comfortably.
Cordless irons plug into a charger unit to heat up, and then can be used for a while until they cool
down again. The time they stay hot is limited by how much heat energy they can store. This depends
on the heat capacity of the soleplate.
Aluminium has a specific heat capacity that is double that of iron (8). This means if the soleplate is
made of iron it will store as much energy as an iron one, even if it only weighs half as much. This is a
good reason to make the soleplate from aluminium, as it can be lighter and easier to lift.
References:
1. http://www.popseed.com/h6h/h6h%20reading%20wk%2007%20-%20caro.doc
2. http://www.jitterbuzz.com/ironing_history.html
3. http://en.wikipedia.org/wiki/Iron_(appliance)
4. http://www.engineeringtoolbox.com/thermal-conductivity-metals-d_858.html
5. http://en.wikipedia.org/wiki/Latent_heat
6. http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion
7. http://en.wikipedia.org/wiki/Thermal_conductivity#Experimental_values
8. http://www.engineeringtoolbox.com/specific-heat-metals-d_152.html