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 NON-
CONDUCTORS 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 10
8 to 10
16 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 Voltage (V) Current (mA) Current (A)
1
2
3
4
5
Graph with Logger Pro (see directions, next page)
Light bulb lab, p. 3

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

1
2
3
4
5
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) 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
(E g
= mgh) as the amount of electrical energy found in the previous question? Solve it for h!
________________________________________________________________________
Let’s say we converted this energy to E k
instead. How fast could you throw this 1 kg mass? (E k
= ½ mv
2
) Solve for v!
________________________________________________________________________
Light bulb lab, p. 7

One source I found mentions this: “ Power consumption in light bulbs is approximately proportional to
V
1.6
” ( 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]*Voltage
1.6
.
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(Voltage
1.566
)
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