Does size matter?

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  #1  
Old 02-24-07, 09:13 AM
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Does size matter?

My 9-year old son is doing a science project in which he is to prove or disprove that the size of a light bulb affects energy use. This is a bit challenging for us to prove. We are considering using a 9-volt battery to light two different size light bulbs, each 15 watts, theorizing that they will burn out at the same time. My question is, would a 9-volt battery provide enough energy to light the bulb in the first place? And, if not...what would you suggest we use to play out this hypothesis? Thanks.

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Old 02-24-07, 10:10 AM
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when you say "burn out" do you intend to mean quit producing light due to the battery being drained?

I would doubt you would have enough current flow to cause the lamps to light using a 9 volt battery and a 120 volt lamp.

You might consider an automotive 12 volt lamp with a 9 volt battery. Just don;t know how long mom (or dad) is going to have to stay up tonight to watch the thing and time it. also may have a hard time finding same wattage lamps with different size envelopes.

rather than trying to time it (although that would show a lot of patience and determination) you might look for a couple of reletively inexpensive in-line ammeters to actually measure the current. Obviously at the same voltage, a lower current would indicate less power consumption.

gonna have to think a bit to come up with any real aid or advice.
I'll be back. Hopefully a few of the other more creative folks will pop in with some ideas.
 
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Old 02-24-07, 10:33 AM
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if both bulbs are 15 watts, what is the size difference?
 
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Old 02-24-07, 10:53 AM
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I took it as a difference in the size of the envelope.

Maybe OP will clarify.
 
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Old 02-24-07, 11:03 AM
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If there were enough current to flow through a 120 v bulb with a 9 v battery, the results would be skewed as the lamp would not get hot enough for it to reflect it's normal operating conditions.

a cold filimant has zero resistance. it only becomes a resistor when it gets hot.

I would look to buy a cheep multi meter from a auto store or radio shack.

set the two bulbs in different sockets to run off of 120, and use the multimeter , set on amps, to complete the circuit. The difference in amperage will be the result.

Of coarse you cannot let the child do the testing since the 120 volt would be dangerous.
 
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Old 02-25-07, 04:44 AM
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Size of "envelope" is very unlikely to have any effect. I think you need to clarify exactly what the question is. Burning out at the same time if that is what you mean wouldn't be relevant to power consumption. If you mean battery going dead yes to some extent but the batteries would be an unknown variable so it wouldn't be accurate. There would be no way to know if each battery had exactly the same charge or same exact physical characteristics. While the older square 9V batteries (not the small ones now used in electronics) might provide enough for a bulb the more traditional battery set up for experimenters is a 6v lantern battery or a series hook-up of one or more "doorbell" batteries.

Now you can find 120v ac bulbs with the same wattage say a 40 watt candle bulb verses a standard Edison based but while they have different size envelopes they obviously draw the same amperage. Of course that could mean different physical characteristics of the filament to compensate for envelope size but you couldn't determine that. All and all the question seems to be poorly asked. If this is in a workbook or teachers assignment you need to ask for clarification.
 

Last edited by ray2047; 02-25-07 at 05:09 AM. Reason: To add more thoughts/opinions
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Old 02-26-07, 10:18 AM
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Question

P=EI or I=E/P (120ac over 40watts would be about 1/3amp).. so, same wattage bulb, same voltage, should be same amperage.. and same life (give or take mans ability to build clone bulbs)... ? but, like you say, resistance incresases as the heat increases.. so, a smaller glass shield should create more heat interior, .. so I figure they adjust the filaments according to the glass enclosures to make up for the resistance/heat in order to maintain their wattage values... ?
 
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Old 02-26-07, 10:35 AM
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Since this a 9 year old I would not be using 120 volts for this experiment. Use 12 volts. Measure the current with $20 digital meter. Use something like a low voltage garden light transformer. Use different sizes of 12 volt garden lights and auto brake lights etc. Incandescent light bulbs don't care if the votage is AC or DC.

You could even something like a kill-a-watt meter.
 
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Old 02-26-07, 10:37 AM
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Another thought that would be safe.
Use the kill-a-watt meter and different types/sizes of 120 volts bulbs in a table lamp. Turn the lamp on off with different bulbs and see how many watts they use.
 
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Old 02-26-07, 11:11 AM
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The variations between individual batteries and individual bulbs will far overshadow the difference between the bulb size. Furthermore, for the results to be of any practical use, you should measure the light output too, since efficiency can only be stated as how many units of light you are getting per unit of energy. You may find that one 15-watt bulb uses more energy than another, but it may also produce more light. A 9-volt battery will not cause a 15-watt 120-volt bulb to produce enough light to be seen by the human eye. The suggestion to use a automobile bulb is a good one.
 
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Old 02-26-07, 11:27 AM
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Using kill-a-watt and all 60 watt bulbs....

GE standard bulb (new).............58 watts

GE standard bulb (used)............56 watts

GE standard bulb (used)............55 watts

GE clear pointed bulb................55 watts

Value Bright (elcheapo).............54 watts

Don't think that proved anything.
 
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Old 02-26-07, 11:30 AM
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yep.. it proved " you don't get what you pay for most of the time"... ha...
 
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