Download The innards of an incandescent light bulb

Light Bulb Lab
Advanced Physics / Physics
The innards of an incandescent light bulb
The investigation for this part is to try to discover the
anatomy of a light bulb. Some of the parts of a light
bulb are observable; other connections, though, are
hidden. Let’s see if we can deduce the innards of a
light bulb.
The basic parts of the light bulb are shown in the
picture on the left. Most of you probably are familiar
with these basic parts.
The question though is how does the energized
current from the battery make the connections to
energize the tungsten filament? Let’s see if we can
figure this out…
For this part you will need the following:
One D battery without the battery holder
One #14 bulb (round bulb)
1 connecting wire (with alligator clips)
Use these items only and find some arrangement to
light the light bulb. You might find several arrangements that work and some that don’t. Think
through this and discuss with your partner (s).
Sketch several different arrangements in which the bulb will light.
Here is a pic showing some of your arrangements…Would the four different arrangements pictured
here light the bulb?
Light bulb lab, p. 1
Again, the goal is to figure out how the light bulb is wired to make it work. We can’t see all of the
wiring in the light bulb because the threads are hiding some of the wire. The arrangements on the
previous page should give you some clues.
Another clue comes from a light bulb socket (see pictures).
Take one of the sockets and screw in the light bulb. Do you see how the light bulb makes the two
connections with the metal strips? One connection is touches the metal tip at the bottom of the bulb
and the other connection encircles the metal base.
Can you make any deductions now about
additional wiring hiding behind this metal screw
base?
Draw your connections. They should look like the
following pics on the right
Remove the bulb from the socket and again,
set up the single wire, battery, and bulb so
the bulb lights. Then somewhere between
the battery and the bulb, stick in a variety of
material objects available in the room, such
as paper, coins, rubber bands, fingers,
pencils, keys, etc.
Reflect on which types of materials allow
the battery to light the bulb and which types don’t. Since it seems that
something flows from the battery to the bulb, we refer to materials that
allow this flow as CONDUCTORS and those that don’t as NONCONDUCTORS or INSULATORS.
List some materials that allow the bulb to light: ___________________________________________
List some materials that prevent the bulb from lighting: _____________________________________
What general category of materials are conductors? _________________________________________
Light bulb lab, p. 2
(Note: Pure metals are usually good conductors with low resistance (in the order of 10-8 to10-6 ohms
per meter and are used for electric wires. Insulators such as porcelain and mica are mostly non-metals
and have a very high resistance (in the order of 108 to 1016 ohms per meter. They are used to insulate
electrical conductors. Between the two extremes lie the very important group of semi-conductors like
germanium and silicon, which is used extensively in the micro-electronic industry. They have a
resistivity of about 1 to 2000 ohms per meter.)
Part 3: Resistance of the Round Light Bulb (#14)
Let’s investigate the resistance of the light bulb. We’ll find out that its characteristics are a bit
different than that of a resistor.
Put the round light bulb in the socket and attach the
multimeter. Move the dial to the 200 Ω range and get the
resistance. This represents the resistance of the “cold”
light bulb (filament is cold). I get a value of around 6.8 Ω.
What did you get? Yours may be a bit different.
_______________ Ω (cold resistance of a round light bulb)
Now let’s make a circuit like we did in
the first part (page 1) but we will
substitute the light bulb and socket for
the resistor.
Set up your ammeter with the 500 mA
scale in series as you did before and
your voltmeter in parallel. Start with
one battery and take current/voltage
data. Work yourself up from one
battery. You probably will find that
the light bulb will ‘blow’ after 4 or 5
batteries hooked up. If your light bulb
blows that’s okay; don’t throw it
away right yet. We will look at it
later.
Part 3: Ohm’s Law using a #14 Round Light Bulb DATA TABLE
No. of batteries
1
2
3
4
5
Voltage (V)
Current (mA)
Graph with Logger Pro (see directions, next page)
Light bulb lab, p. 3
Current (A)
Again, graph current (in A) on the x data
and the Voltage on the y axis. Make
sure to change the scaling of your axes
in Logger Pro and remove the
connecting lines (see Graph options
under the options menu).
Because we couldn’t use as many
batteries, we don’t have as many data
points. When I look at my data, it
doesn’t really look linear—it doesn’t
appear to want to go through the origin
and a linear relationship doesn’t really
fit the data points very well.
When I do a PROPORTIONAL curve
fit to force the trend line through the origin, my data points don’t really follow the line.
When I try a different curve fit, the trend
looks a bit better but the fit is still not
perfect.
From our investigation on the first part, we
said that the resistance is the slope of the
trend line when we graphed current on the x
axis and voltage on the y axis.
Do we appear to get ONE slope here?
________________________
Make tangent lines with your hand (tangent
lines can show you how the slope changes
on a “curved” trend line.
Can you make any statements about the light bulb’s resistance as you increase the number of batteries?
_____________________________________________________________________________
In the Ohm's Law lab, we saw that the 25 Ω resistor gets hot. We said that electrical energy is changed
into thermal energy. What types of energy transfer do we see with the light bulb? Does it get hot, too?
_____________________________________________________________________________
Since we don’t see a nice linear relationship here with the light bulb, does a light bulb follow Ohm’s
Law like a resistor?
_____________________________________________________________________________
Light bulb lab, p. 4
In fact, a light bulb IS NOT OHMIC over this range of data. However, you will find that people will
talk about light bulbs following Ohm’s Law. This is generally an ok over a small range of data.
For example, if I take my last two data points, I get a perfect correlated fit. My slope is 31.11 V/Amps
(V/Amps is the same as Ω).
This graph tells me that this
resistance for these two data points
is around 31 Ω.
Can you see somthing similar for
your graph?
_____________
If so, what is your resistance?
_____________
Another thing we need to consider
is the fact that resistance of a
material often changes with the
temperature of the material.
On page 3, you calculated the
resistance of the cold light bulb
using the multimeter.
What did you get for the resistance of the cold light bulb (see page 3)? ________________________
The resistances we got with the graph represents a hot light bulb. How did the resistance change for
the hot light bulb vs. the cold light bulb?
_______________________________________________________________________________
So, how does temperature affect the resistance of the light bulb?
_______________________________________________________________________________
Why would higher temperatures for a material lead to greater resistances for that material? Discuss
this with your partner(s) and offer a hypothesis.
_______________________________________________________________________________
To check why a hotter substance has more resistance, you might want to look at several websites:
http://regentsprep.org/Regents/physics/phys03/bresist/default.htm (look at the temperature part)
http://en.wikipedia.org/wiki/Electrical_resistance
http://230nsc1.phy-astr.gsu.edu/hbase/electric/restmp.html#c1
Light bulb lab, p. 5
Part 4: A ‘Blown’ Light Bulb
Since we have ‘blown’ the light bulb, why don’t you compare a fresh round bulb to the one you
‘blew’. Use a magnifying lens.
What happened to the filament of the blown light bulb? ________________________________
Draw a quick sketch of both bulbs:
Here is something else to consider. Let’s say we have our a series
circuit with four light bulbs (we made in a previous activity)
Let’s say that one of our bulbs blew, what happens to the brightness of
the other bulbs?
What would happen to the reading on the ammeter?
(My comments: if you blow a light bulb in a series circuit, the whole
circuit fails. All the light bulbs will go out as you have destroyed the
pathway for the electricity to flow. The ammeter will now read zero as
no current can flow.)
Can you think of why a house or Christmas lights, for that matter,
should NOT be wired in series?
---------------------------------------------------------------------------------------------------------Light bulb lab, p. 6
Part 5: Electric Power is the product of applied voltage and current [Power (Watts) = V (Volts) x I
(Amps)]. Take the data from your light bulb (p. 3) and re-write the Voltage and Current data. Then
find the power (in Watts) for each number of batteries. Then multiply by time to get the energy. This
would be the energy supplied to/by the light bulb every minute (60 seconds).
Part 3: Ohm’s Law using a #14 Round Light Bulb DATA TABLE including Power information
No. of
batteries
Voltage (V)
1
2
3
4
5
Current (mA)
This is
milliamps
Change
Current from
millamps to
amps (divide
by 1000)
Power (Watts)
= Voltage x
Current (make
sure current is
in AMPS)
Time (sec)
Energy
(Joules)
E = P(t)
Power x
time
60
60
60
60
60
Complete the data table
Light bulbs are usually rated in terms of power and voltage. A
generalized relationship is that the more power delivered to the
light bulb, the brighter the light bulb will be. This is a pretty
complicated relationship because much of the energy delivered to
the light bulb goes into heat. And also, much of the light waves
generated are in the infrared region of the electromagnetic
spectrum and consequently, our eyes cannot see these photons. So, if you double, the power to the
light bulb, you may not see a doubling of brightness.
Go back to your data table on the previous page. You found that the #14 light bulb “blew” after you
added to many batteries. Using the information, look up at the data table here to tell me the maximum
“wattage” (i.e. power) for this type of light bulb.
________________________________________________________________________
How much energy would be delivered to the light bulb in one minute (60 sec) at this maximum power?
________________________________________________________________________
How high would you have to lift a one kilogram mass to give it the same gravitational potential energy
(Eg = mgh) as the amount of electrical energy found in the previous question? Solve it for h!
________________________________________________________________________
Let’s say we converted this energy to Ek instead. How fast could you throw this 1 kg mass? (Ek = ½
mv2) Solve for v!
________________________________________________________________________
Light bulb lab, p. 7
One source I found mentions this: “Power consumption in light bulbs is approximately proportional to
V1.6” (http://en.wikipedia.org/wiki/Incandescent_light_bulb) This is called a power relationship as
there is a variable to some power (in this case, 1.6).
The statement implies the following relationship: Power = [Some Constant Number]*Voltage1.6.
Using Logger Pro, look at the
data table on the previous page
and graph Voltage (x axis) and
Power (y axis). Select the data
and choose curve fit. Select a
power curve fit to see if this
statement is true for our little
round light bulb.
At the right is my nice-lookin
graph and I get the following
relationship:
Power = 0.1596(Voltage1.566)
Interesting. I get a power of
1.566 which approximates to
1.6 (see Wikipedia above).
Write down YOUR
mathematical relationship here:
_______________________________________________________________________
-------------------------------------------------------------------------------------------------------------------------Earlier in the lab we discussed the filament of the light bulb. We saw that if this filament is
broken, the light bulb doesn’t work. But what material is the best material for filaments? We
want something that is long-lasting and can “take” the heat without quickly breaking. It turns
out that the element tungsten (element symbol W) is a very popular filament substance. Why?
Read on to learn more about filaments and why people use tungsten in light bulb filaments.
Tungsten Filaments in Incandescent Bulbs
http://members.misty.com/don/bulb1.html:
http://invsee.asu.edu/Modules/lightbulb/meathist4.htm
It is widely regarded that Thomas Alva Edison invented the first reasonably practical incandescent lamp, using a carbon
filament in a bulb containing a vacuum. Edison's first successful test occurred in 1879. There were earlier incandescent
lamps, such as one by Heinrich Goebel made with a carbon filament in 1854. This incandescent lamp had a carbonized
bamboo filament and was mentioned as lasting up to 400 hours. At least some sources regard Goebel as the inventor of the
incandescent lamp.
Light bulb lab, p. 8
Joseph Wilson Swan began trying to make carbon-based incandescent lamps in 1850 and made one in 1860 that was
workable except for excessively short life due to poor vacuum. He made more successful incandescent lamps after better
vacuum pumps became available in the mid 1870's.
Since that time, the incandescent lamp has been improved by using tantalum and later tungsten filaments, which evaporate
more slowly than carbon. Nowadays, incandescent lamps are still made with tungsten filaments.
Tungsten is a great metal to use as a filament because it has a high melting point (3,410 deg
C or 6,170 deg F), evaporates slowly at high temperatures, and has a very strong tensile
strength. Because of its ductility, it can easily be formed into filament coils. At its high
operating temperature (3000 deg C), a tungsten filament glows white-hot providing good
brightness.
The filament of an incandescent lamp is simply a resistor. If electrical power is applied, it is
converted to heat in the filament. The filament's temperature rises until it gets rid of heat at
the same rate that heat is being generated in the filament. Ideally, the filament gets rid of heat
only by radiating it away, although a small amount of heat energy is also removed from the
filament by thermal conduction.
Above: An scanning electron microscope image (75x) of a 60 W line voltage light bulb
filament. In order to increase the filament length while keeping its physical size small, the
filament takes the form of a coiled coil. By comparison, low voltage lamp filaments usually take the form of a single coil.
(http://en.wikipedia.org/wiki/Incandescent_light_bulb)
The tungsten filament of a vacuum incandescent lamp is heated to temperatures where visible light is emitted by resistance
heating. The filament acts as an electrical resistor, which dissipates power proportional to the voltage applied, times the
current through the filament. When that power level is sufficient to raise the temperature to above 1000 degrees Kelvin,
visible light is produced. As the power dissipated is increased, the amount of light increases and the peak wavelength of the
light shifts to the blue. Typical vacuum lamps may have filament temperatures ranging from 1800 to 2700 degrees Kelvin.
The light from the low temperature lamps appears reddish yellow while the high temperature lamps have a ‘whiter’
appearance.
Write down three reasons (discussed above) why tungsten is used as filaments in light bulbs:
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Light bulb lab, p. 9