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Context for this case:
Prerequisites:
Cognitive Apprenticeship Features:
Supporting References:
Relevant CA Content Standards |
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(A) Role Recognition (Deprecated).
Although you can certainly use the text of Exercise 5 for more practice in recognizing the features of a description and their roles, no student version with scaffolded feature prompts is provided (and, hence, no annotated teacher version either). My experience is that by the time they reach Exercise 5, all students need the greater activity level that text reconstruction (below) offers.
(B) Text Reconstruction (Background).
Reconstructing Exercise 5 from its scrambled sentence-length pieces involves the same practical, work-relevant attention to text features and linguistic clues as did Exercise 3 (which introduces this activity) and Exercise 4. Note that the pieces here are (a) more fine grained, calling for more student attention to their details, and (b) more numerous (making twice as large a reconstruction project as the previous exercises).
(C) Text Reconstruction (Process).
Below I provide a "segmented" version of the student fluorescent-lamp description. It has no scaffolding, but marks (---) divide it into 30 sentence-sized chunks. The descriptive chunks omit all high-level headings, which appear in a separate list for you to use as the project outline. Although the text chunks are short, each contains signals or rhetorical clues (including lowest-level or "run-in" headings and figure callouts) about each chunk's intended role in and contribution to the overall description.
- Print out
the segmented version of the fluorescent-lamp description and cut it into pieces (of paper) along the marks (---) indicated.- Enlarge
each text chunk on a photocopy machine for easier in-class reading and sharing. Everyone can participate more easily if the description parts (that you assemble as puzzle pieces on the wall) have print big enough to read from across the room. Alternatively, use less enlargement and reassemble the description on a big table.- Scramble
the text pieces so that their original order is hidden.- Distribute
the (enlarged) text chunks randomly, one to each student (or perhaps to each pair of students). GROUPS VERSION: Teachers who prefer that students work in small groups can easily adapt Exercise 5 for small-group practice. Make enough complete sets of the description text chunks so that each group of students can have one whole set. Scramble the text pieces within each set and let the students of each group cooperatively reassemble the description from their set of pieces as best they can. Then have one group post their reconstructed description on the wall for you (or them) to read, adapt, and evaluate (as below).- Read each piece aloud
and try to find its best place. Use its internal rhetorical features (as mentioned on the guidelines) as clues and the list of headings and the figures as an organizing "target" framework, a broad outline of the intended result. Student discussion may perform most of this work in some classes; you will need to provide considerable leadership and encouragement in others since this is valuable but unfamiliar territory for many students. In some cases, even reading their text chunk aloud for classmates to consider may challenge the student who holds it.- Post
each text chunk on a wall or blackboard (with little pieces of tape or Post-it notes) as students decide on its preliminary role and place. But don't tape the paper sheets to each other unless you can easily undo them, because changing the order and grouping is a natural and appropriate part of reconstructing the whole description from its parts.- Adapt
the growing description as new pieces of the puzzle are read and reviewed. As in real life, first guesses may need to be revised to accommodate later arriving chunks of text that clarify the overall structure of the description that you are (re)building.- Review
the emerging whole as the last pieces fall into place, as you would with any puzzle. This approach enables students, cooperatively, to "write" a long, complex technical description using important, real-world design principles (the guidelines), without having to compose each separate piece of prose. It shows "actively" how the pieces of a good description have features that knit together to form a coherent pattern intended to help readers use the text well. Since the students must focus on those same text features to rebuild the description, they come to see why writers bother to deploy them.
Description Case 5: Fluorescent Lamp
Overview
Structure
Size
Contents
Labels
Operation
Gas Discharge
STARTING.
EMITTING.
VISIBILITY.
Wavelength Conversion
Efficiency
HEAT/LIGHT RATIO.
BULB LONGEVITY.
Student version [segmented, no scaffolding or headings]:
Description Case 5: Fluorescent Lamp
---
A fluorescent lamp is a long
straight glass tube that glows
when a current passing through
low-pressure gas within the tube
causes a coating on the glass to
emit white light.
---
Fluorescent lamps were first
introduced commercially in 1938.
---
A standard fluorescent lamp is a
cylindrical glass tube 1.5 inches
in diameter and 48 inches long
(other sizes are available).
---
A 2-pin metal base or cap seals
each end of the tube (see Fig. 1).
---
Inside each end cap, attached to
the pins, is a filament or
electrode, a thin thread of wire
from which electrons boil when it
is heated by an electric current.
---
Sealed within the tube by the
caps is a drop of mercury and a
very low-pressure inert gas
(usually argon).
---
A light-emitting chemical (see
the Operation section) called a
phosphor coats the entire inside
surface of the glass.
---
Fluorescent lamps carry
standardized labels outside that
identify their internal physical
and electrical properties.
---
For example, a lamp with the
black characters
F40-T12
stenciled on one end is a
fluorescent (F) tube (T) that
uses 40 watts of power and has
a diameter of 12 eighths of an
inch (12/8 = 3/2 = 1.5 inch).
---
STARTING. When the lamp is off,
the mixture of mercury and gas
inside does not conduct
electricity.
---
So every fluorescent lamp is
attached to a starting device
called a ballast, which combines
* a "transformer" to produce an
initial, high-voltage burst,
and
* an "inductor" to limit
current flow while the lamp
is on.
---
EMITTING. When power is first
applied, a 250- to 400-volt burst
of electricity vaporizes the
mercury (see Fig. 1, left).
---
Electrons in the mercury atoms
absorb energy and jump to
"higher," more energetic orbits
(Fig. 1, middle).
---
They then fall back to less
energetic orbits (Fig.1, right).
---
This repeating process, called
gas discharge, continuously
emits the absorbed energy as
light.
---
Once started, only about 175 volts
are needed to maintain this
discharge in a 40-watt lamp.
---
VISIBILITY. When an applied
voltage causes discharge in some
low-pressure, inert gases, they
emit visible light.
---
Ionized neon gas emits red
light, for example, seen
directly in a glowing neon bulb.
---
But in a fluorescent lamp, the
discharge comes almost entirely
from the mercury vapor, even
though it is only 1 percent of
the enclosed gas.
---
And almost all of the mercury
discharge is ultraviolet (UV)
light, whose wavelength is too
short for human eyes to see.
---
The phosphor that coats the
inside of the lamp tube converts
the UV mercury discharge into
useful light that people can see.
---
The phosphor absorbs the
invisible, short-wave UV
emissions from the excited
mercury atoms (Fig. 1, right).
---
It then emits other light with
a longer wavelength, almost all
of which is visible.
---
The chemical composition of the
phosphor lining the tube controls
the color of the visible light
emitted, which may be
* "cool white" (partly blue), or
* "warm white" (partly pink), or
* other visible colors.
---
HEAT/LIGHT RATIO. All lamps
convert current into visible
light and heat.
---
Fluorescent lamps are about 2 to
4 times more efficient than
incandescent (glowing filament)
lamps.
---
For the same power, they produce
2 to 4 times more light and less
heat.
---
BULB LONGEVITY. Fluorescent
lamps also have longer lifetimes.
A typical incandescent bulb lasts
1000 hours before the filament
fails.
---
But a typical fluorescent lamp
lasts 10,000 to 20,000 hours,
depending on how often it is
started.
---
Annotated version:
Description Case 5: Fluorescent Lamp
Overview
A fluorescent lamp is a long
straight glass tube that glows
when a current passing through
low-pressure gas within the tube
causes a coating on the glass to
emit white light.
Fluorescent lamps were first
introduced commercially in 1938.
Structure
Size
A standard fluorescent lamp is a
cylindrical glass tube 1.5 inches
in diameter and 48 inches long
(other sizes are available).
A 2-pin metal base or cap seals
each end of the tube (see Fig. 1).
Contents
Inside each end cap, attached to
the pins, is a filament or
electrode, a thin thread of wire
from which electrons boil when it
is heated by an electric current.
Sealed within the tube by the
caps is a drop of mercury and a
very low-pressure inert gas
(usually argon).
A light-emitting chemical (see
the Operation section) called a
phosphor coats the entire inside
surface of the glass.
Labels
Fluorescent lamps carry
standardized labels outside that
identify their internal physical
and electrical properties.
For example, a lamp with the
black characters
F40-T12
stenciled on one end is a
fluorescent (F) tube (T) that
uses 40 watts of power and has
a diameter of 12 eighths of an
inch (12/8 = 3/2 = 1.5 inch).
Operation
Gas Discharge
STARTING. When the lamp is off,
the mixture of mercury and gas
inside does not conduct
electricity.
So every fluorescent lamp is
attached to a starting device
called a ballast, which combines
* a "transformer" to produce an
initial, high-voltage burst,
and
* an "inductor" to limit
current flow while the lamp
is on.
EMITTING. When power is first
applied, a 250- to 400-volt burst
of electricity vaporizes the
mercury (see Fig. 1, left).
Electrons in the mercury atoms
absorb energy and jump to
"higher," more energetic orbits
(Fig. 1, middle).
They then fall back to less
energetic orbits (Fig.1, right).
This repeating process, called
gas discharge, continuously
emits the absorbed energy as
light.
Once started, only about 175 volts
are needed to maintain this
discharge in a 40-watt lamp.
VISIBILITY. When an applied
voltage causes discharge in some
low-pressure, inert gases, they
emit visible light.
Ionized neon gas emits red
light, for example, seen
directly in a glowing neon bulb.
But in a fluorescent lamp, the
discharge comes almost entirely
from the mercury vapor, even
though it is only 1 percent of
the enclosed gas.
And almost all of the mercury
discharge is ultraviolet (UV)
light, whose wavelength is too
short for human eyes to see.
Wavelength Conversion
The phosphor that coats the
inside of the lamp tube converts
the UV mercury discharge into
useful light that people can see.
The phosphor absorbs the
invisible, short-wave UV
emissions from the excited
mercury atoms (Fig. 1, right).
It then emits other light with
a longer wavelength, almost all
of which is visible.
The chemical composition of the
phosphor lining the tube controls
the color of the visible light
emitted, which may be
* "cool white" (partly blue), or
* "warm white" (partly pink), or
* other visible colors.
Efficiency
HEAT/LIGHT RATIO. All lamps
convert current into visible
light and heat.
Fluorescent lamps are about 2 to
4 times more efficient than
incandescent (glowing filament)
lamps.
For the same power, they produce
2 to 4 times more light and less
heat.
BULB LONGEVITY. Fluorescent
lamps also have longer lifetimes.
A typical incandescent bulb lasts
1000 hours before the filament
fails.
But a typical fluorescent lamp
lasts 10,000 to 20,000 hours,
depending on how often it is
started.
REWRITING, REVISING.
For every paragraph in a technical description, alternative versions exist that could have been used. Some are clearly better or worse that the original, while others involve tradeoffs that improve the description in one way but weaken it in another. (For background on the real-life relevance of text revision to laboratory science, see the teacher notes for Exercise 9.)You can introduce students to this important idea that good writing demands revising, and revising requires carefully comparing the relative merits of alternative chunks of text. I have used paragraph 5 (on the electrode, first within the "Contents" subsection) as the focus for such extended activity. You can present and discuss alternative versions of this paragraph (below) most easily in large-print format or by projecting them to large size on the wall.
- Version 1.
The original version of the electrode paragraph (reproduced here) is very concise. But it involves a single long, complex (though clear) sentence, potentially difficult for nonnative English readers to understand. Have students count the total words used in this version.Inside each end cap, attached to the pins, is a filament or electrode, a thin thread of wire total = 30 words from which electrons boil when it is heated by an electric current.- Version 2.
You can ask able students to rewrite Version 1 as many short sentences that make explicit each assertion embedded within it (or show them Version 2 below). Version 2 spells out every implicit claim made in Version 1: the result is four very easy sentences, but sentences that give each claim an equal linguistic emphasis, for a repetitious, annoyingly flat tone. Have students count the total words here too. At 38 words, this is not only much more awkward than Version 1 but it is 25% longer as well.A filament attaches to the pins inside each end cap. A filament is an electrode. A filament is a thin thread of wire. total = 38 words Electrons boil from a filament when a filament is heated by an electric current.- Version 3A, B.
You can ask students to rewrite Version 2 with the goal of keeping the simplicity but improving the focus and emphasis (or show them Version 3A, below). Version 3A makes the same claims as Version 2 but introduces some linguistic signals and verbal combinations to save text and make a more readable result:A filament or electrode (3A) attaches to the pins inside each end cap. This is a thin thread of wire. From it electrons boil when heated by an electric current.Version 3A, however, introduces two different (unintended, but typical) mistakes or verbal flaws that will mislead readers.
(1) The second sentence begins with the wrong pronoun. "It is a thin thread..." would refer back to the beginning of the first sentence (whose subject is "a filament"). "This is a thin thread..." refers to the end of the first sentence, to the noun immediately before the pronoun (here, to "end cap"). Using a pronoun to bridge the two sentences is a good strategy, but this is an inappropriate, misleading choice. Version 3B (below) shows where "this" takes the topic of the description (which is not the original intention here).A filament or electrode (3B) attaches to the pins inside each end cap. This [cap] is made of aluminum.(2) The third sentence in Version 3A contains the ambiguous phrase "when heated." Is the electrode heated or rather the electrons that boil off? Adding another pronoun here can eliminate the danger of misreading this sentence (as in Version 4, below).- Version 4.
This version keeps the strengths of Version 3A but removes its weaknesses, and it eliminates the unwanted detour of meaning in Version 3B:A filament or electrode attaches to the pins inside each end cap. It is a thin thread of wire. From it electrons boil when it is heated by an electric current.Version 4 preserves all the claims made in Version 1, offers (three) simplier sentences to the reader, and yet maintains the proper thread of continuity too. Version 4 is virtually the same length as Version 1 (31 words); this suggests that improving on the conciseness of Version 1 is quite hard, but one can (perhaps) improve it somewhat in other, more subtle ways. Able students can compare the benefits of Version 1 and Version 4 as serious text alternatives.
Contact: T. R. Girill trgirill@acm.org