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Burn Out:
Why does a light bulb burn out?
A tungsten filament is drawn to a very uniform diameter when
it is manufactured. As a result, when a light bulb is first
turned on the filament emits light relatively evenly along the
length of the filament. Explore the failure of a filament using
the following movie which shows an accelerated view of a filament
failing. Notice how the light emitted from the filament changes
intensity and location with time.
Why does the filament get bright at one point
before it fails ?
Standard electrical outlets in the United States provide 110
volt (V) electricity. For an incandescent light bulb, the electric
current (i) used to heat the filament is determined by the electrical
resistance (R) of the filament according to Ohm’s Law:
V = iR
Electric power (P) is the rate of conversion of electrical
energy to another form, such as heat. For a resistor, such as
a tungsten light bulb filament, the power may be expressed as:
P = i2R = V2/R.
The voltage drop across the filament is essentially constant.
As a result, when R varies, so does i. In particular, R can
vary locally with the cross-sectional area of the filament:
R =
(l/s),
where is the
specific resistance of tungsten (ohms), l is the length of a
filament region (cm) and s is the cross-sectional area of the
filament region locally (cm2).
If the cross-sectional area of the filament changes with time
to vary along its length, the current passing through each part
of the filament will remain constant. Although if the overall
resistance varies, so will i. The resistance of each section
will be inversely proportional to s.
Suppose a filament has three regions (1, 2, and 3) with different
values of s and, consequently, R. Then V = iR1 + iR2 + iR3 and
the total power dissipated as a function of filament region
is given by:
P = i2 R1 + i2 R2 + i2 R3 = i2 1(l1/s1)
+ i2 2(l2/s2)
+ i2 3(l3/s3)
Explore the effect of filament thickness on the three regions
of the filament using the Three Filament Thickness Tester.
Now take a short quiz on how the filament thickness
affects failure.
A Closer
Look:
Tungsten is obtained as mining ore powder, which is sintered
and shaped into feedstock to manufacture the filaments. The
tungsten is drawn through diamond extruding molds at a high
temperature to yield very long, thin filament wire. The wire
is then wound into spirals and double spirals to allow the filament
to more efficiently maintain the high temperatures needed. The
spiral shape minimizes the convective cooling of the filament
by the inert gas in the bulb.
Figure - 1
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Figure - 2
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Figure - 3
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The above Scanning Electron Microscope (SEM) pictures
show a new filament, its highly uniform thickness, and the characteristic
axial marks left from the extrusion process.
Investigate:
Although tungsten is the most temperature resistant filament
material known, it is highly reactive when hot. Light bulbs
are filled with an inert gas such as nitrogen or argon to avoid
the filament reacting with air. Exposing the hot filament to
even the smallest amount of air causes the tungsten to oxidize
to tungsten trioxide (WO3):
2W + 3O2(g) ------------> 2WO3
The oxide forms a gas which solidifies as smoke particles in
the atmosphere in the light bulb when the filament is white
hot. The Scanning Electron Microscope pictures below show a
filament which failed after a slow air leak.
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Figure - 1
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Figure - 2
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Figure - 3
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Figure - 4
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The first two pictures show the filament away
from where it failed. Note the white WO3 smoke particles that
have settled back on the smooth surface. The extrusion marks
have been etched away by oxygen. The next two pictures show
the failed region of the filament and the failure surface, respectively.
Why is this region more sensitive to failure? What can you
say about the region where it failed? Now take a short quiz
on the likely process of filament failure. Use the pictures
which will appear below to answer the quiz.
Even when the light bulb is perfectly sealed from air it still
“burns out,” but not as fast. Tungsten directly vaporizes from
the filament surface while at the extremely high temperatures
tungsten filaments operate at. The pressure of tungsten vapor
over a filament is 10-4 Torr at 2,757oC.
W(solid) + heat ------------> W(gas)
As the trace of tungsten vapor leaves the surface by sublimation,
the cool argon/nitrogen gas around the filament causes the vapor
to solidify as smoke, which slowly settles on the glass bulb
giving it that irregular gray tint as it ages. Filaments fail
by brittle fracture when they become too weak from thinning.
Atomic View:
The goal of this section is to introduce students to the packing
of the individual atoms in the tungsten filament and how their
different packing arrangements affect how the filament burns
out. The scanning electron microscopy image below shows how
an old filament has aged.
Its surface has been etched away by sublimation revealing beautiful
tungsten single crystal shapes and a smoother region with distinct
lines etched in the surface. These type of crystal features
will be used to interactively introduce students to how the
atoms are packed in the single crystals exposed as the filament
ages. Grain boundary features will be used to illustrate how
grain boundaries (poorly packed regions between well packed
single crystal regions) are weaker, more reactive areas, where
filaments often fail.
The next image shows how the shape of the exposed filament
crystals can be related to the atomic-level packing of their
tungsten atoms. Students will use an interactive tutorial to
learn to identify the different crystal planes and directions
associated with describing the cubic tungsten crystal structure.
The atomic packing arrangements of the basic planes seen in
the images of the filament crystals are shown.
Students will be able to rotate various atom packing arrangements
to match them with the shapes they see in the filament crystal
images.
After students have become familiar with the atom packing of
body centered cubic materials like the tungsten crystals in
the filaments, they will be introduced to grain boundaries such
as those seen in the following two scanning electron microscope
images. These images show filaments that have not yet been extensively
eroded. The grain boundaries between the single crystal regions
in the filament appear as depressed lines on the surface in
the images. The atoms across these boundaries are more weakly
bound together. Hence, these areas erode more quickly, as seen
in these depressed surface regions. They are also physically
much weaker and fracture more easily, as seen by the grain boundary
fracture in the right image.
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Figure - 1
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Figure - 2
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Such images together with atomic modeling tutorials, using
models of grain boundaries, will be used to interactively introduce
students to how the atomic arrangement of atoms can affect the
strength and corrosion resistance of materials, like light bulb
filaments. These tutorials will interactively demonstrate how
filament grain boundaries erode faster, becoming thinner and
hotter, which in-turn accelerates the thinning process. The
tutorials will also demonstrate how a thinning grain boundary
region becomes too weak to support the filament, eventually
fracturing at the grain boundary.
Once students have completed this section they will be able
to do a real-time remote scanning probe microscope investigation
of their own filaments using the remote scanning probe microscopes
available in IN-VSEE.
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