# Why does 4 watt bulb light first?

Thread Starter

#### Troy4

Joined Jan 30, 2015
4
A simple grade school project of a bicycle wheel turning a DC motor to light three light bulbs - 60 watt, 25 watt, and 4 watt wired in parallel. When turning pedals very slowly (low voltage created), 4 watt filament glows faintly but not the others. Why? They're receiving the same voltage.

#### #12

Joined Nov 30, 2010
18,223
They have more resistance and larger mass to the filament which takes more energy to get it to glow.

#### LDC3

Joined Apr 27, 2013
924
They have more resistance and larger mass to the filament which takes more energy to get it to glow.
The 4 watt bulb has the largest resistance. So how does (a lot) less current get the bulb to glow?

Thread Starter

#### Troy4

Joined Jan 30, 2015
4
They have more resistance and larger mass to the filament which takes more energy to get it to glow.
Yes, but I mean the 4 watt filament glows faintly and the 25 and 60 watt filaments don't glow at all. They're all getting the same low voltage as the bicycle turns slowly. And with a higher resistance in the 4 watt filament, the 4 way bulb is also getting less amps than the other two. Yet the 4 watt filament is the first to glow as you pedal slowly.

#### absf

Joined Dec 29, 2010
1,949
Did you put a volt-meter across the 3 lamps? I bet you're not getting the rated voltage for the Lamps on them. But putting a big enough battery across them would be a different story...

Allen

 You are not using 110V bulbs, are you?

Last edited:

#### crutschow

Joined Mar 14, 2008
27,725
It could be that the smaller, thin filament of the 4W bulb radiates and conducts heat away proportional more slowly than the larger filaments, so heats up faster and starts to glow first.
Just a guess.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,078
The resistance of the lamps, assuming tungsten incandescents are 10-15x lower in resistance when cold. At low voltages (2 V) they are not say 50 W lamps if they are 50 W lamps at 12V.

#### #12

Joined Nov 30, 2010
18,223
The 4 watt bulb has the largest resistance. So how does (a lot) less current get the bulb to glow?
I guess I was wrong this time.

#### MrChips

Joined Oct 2, 2009
24,190
There is no mention on the voltage rating of each lamp. What are they?

#### GopherT

Joined Nov 23, 2012
8,012
If all bulbs have a filament of roughly the same length and the lower wattage bulb just has a thinner cross-section, then it would heat up faster than the others (shorter time constant). The smaller bulb also has a smaller surface area for black-body (gray body) radiant emission of its energy so it will release its energy slower.

These two items both correspond to the physical test you made.

Equations for power and emission are here...

http://faculty.trinityvalleyschool.org/hoseltom/labs/Lab-28-(Light Bulbs).pdf

#### wmodavis

Joined Oct 23, 2010
739
Part of the school project should be to measure and record the resistance of each bulb. It might be helpful to also measure and record the current at which each bulb begins to glow and the voltage at which that occurs. Perhaps a bit advanced for grade school project but me thinks that might depend on the student you are coaching.

Thread Starter

#### Troy4

Joined Jan 30, 2015
4
Did you put a volt-meter across the 3 lamps? I bet you're not getting the rated voltage for the Lamps on them. But putting a big enough battery across them would be a different story...

Allen

 You are not using 110V bulbs, are you?
Yes, these are all 120v bulbs.

Thread Starter

#### Troy4

Joined Jan 30, 2015
4
There is no mention on the voltage rating of each lamp. What are they?
They are all 120v incandscent bulbs.

#### wayneh

Joined Sep 9, 2010
17,153
I speculate that there is actually more current going through the larger filaments, since they should have a lower resistance when operating and certainly more when cooler, but the current is not enough to heat them to glowing. The smaller filament cannot radiate its heat away until it gets hotter and glows. It must achieve a higher temperature to reach steady state. The larger filaments can shed their heat over a larger area.

#### AnalogKid

Joined Aug 1, 2013
9,353
The 4 W bulb comes on before the 60 W bulb because a 4 W bulb takes 1/15th the power of a 60 W bulb. As you increase the power, of course the smaller bulbs will come on first.

ak

#### wayneh

Joined Sep 9, 2010
17,153
AK, you've restated the observation from #1 and said, "of course". That's not an explanation. But I think you you're saying what I did in #14 - the small filament will glow at low wattage but the larger ones will not glow. Even if they are dissipating more wattage than the 4W bulb, that is still not enough to make them glow.

#### MrChips

Joined Oct 2, 2009
24,190
I performed some simple tests comparing a 4W and 40W/120V incandescent bulbs.
They both start to glow at the same voltage, 14V, the 4W bulb slightly earlier than the 40W.

4W bulb glows at 14V @ 9mA =0.126W = 3% of 4W
cold resistance = 340Ω
glow resistance = 1500Ω

40W bulb glows at 14V @ 120mA = 1.68W = 4% of 40W
cold resistance = 28Ω
glow resistance = 116Ω

Filament resistance increased 4 times from cold to glow.
Filament resistance can increase 10 times from cold to full wattage.

My conclusion is it depends on the physical construction of the filament. The 40W filament has more thermal mass.

#### John P

Joined Oct 14, 2008
1,909
"Thermal mass" doesn't really explain it though, because that would just suggest that it takes longer to warm up. You have to consider how well the configuration of the filament might radiate/conduct heat, and also the construction of the bulb. Incandescent bulbs are filled with argon at low pressure, and the pressure is a compromise that lets the gas take heat away from the filament (allowing for some heat being lost via the base too) at a rate that leaves the filament hot enough to glow but not to burn out. Would a lower-wattage bulb contain gas at a lower or higher pressure? I don't know that. But it's pretty certain that operating the bulb at a very low voltage will cause it to run in a different way from the rated voltage.

#### wayneh

Joined Sep 9, 2010
17,153
They both start to glow at the same voltage, 14V, the 4W bulb slightly earlier than the 40W.
That doesn't seem to replicate the TS's observation that the 4W comes on before a 25W. Unless it's in the "slightly earlier".

A great demonstration though, that the bigger bulb is using most of the power while the little bulb starts to glow, as predicted.

#### MrChips

Joined Oct 2, 2009
24,190
Bear in mind that "glow" is highly subjective.
Yes, I believe that the TS's observation is correct. The small bulb glows before the larger bulb. How much "slightly earlier" is also subjective.