Thickness of bulb filament affect brightness?

In summary, for a fixed voltage, the thicker filament will have a lower resistance and therefore dissipate less energy as heat, resulting in a dimmer filament. The thinner filament, with a higher resistance, will dissipate more energy as heat, resulting in a brighter filament. This is also why low wattage bulbs in cellars and cupboards last so long. However, the temperature of the filament also plays a role in its brightness, as the power irradiated per unit of surface goes to the fourth power of the temperature. Therefore, if the two filaments are at the same temperature and of the same length, the thicker one will be brighter because of its larger surface area. The reason tungsten filaments glow is due to their high
  • #1
kapellmeister
3
0
Hi,

does the thickness of filament affect the brightness of a bulb? So will a thinner filament be brighter than a thicker filament?
 
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  • #2
For a fixed voltage (ie. in a domestic light bulb) the thicker the filament the lower it's resistance and so less energy is converted into heat - the filament is dimmer.
Brighter filaments are thinner, have higher resistance and so heat up more -> brighter.
This is also why low wattage bulbs in cellars and cupboards last so long.
 
  • #3
If the two filaments are at the same temperature and of the same length, the thicker one will be brighter because is has larger surface. In turn, the temperature depends on the length, thichkness, and material, voltage... so the question is not clear.
 
  • #4
thanks for the replies!

ya, assuming the length, material of the filaments are the same and that they are connected to the same electrical source, then the one with a thinner filament will glow brighter as it offers more resistance. Am I right in saying so?
 
  • #5
mgb_phys said:
For a fixed voltage (ie. in a domestic light bulb) the thicker the filament the lower it's resistance and so less energy is converted into heat - the filament is dimmer.
Brighter filaments are thinner, have higher resistance and so heat up more -> brighter.
This is also why low wattage bulbs in cellars and cupboards last so long.

Uh :bugeye:

If the resistance is LOWER, and the voltage is fixed, then the dissipated power is higher:

P = V^2/R (or if you like, I = V/R and P = V I).
 
  • #6
kapellmeister said:
thanks for the replies!

ya, assuming the length, material of the filaments are the same and that they are connected to the same electrical source, then the one with a thinner filament will glow brighter as it offers more resistance. Am I right in saying so?

No, it is the other way around... the lower the resistance, the higher the dissipated power.

However, you have to take into account also the equilibrium temperature. The dissipated power (P = V.I = V^2/R) has to be irradiated away through the surface of the filament, and the power irradiated per unit of surface goes to the 4th power of the temperature. So the temperature of the filament will rise until there is as much power irradiated away as there is dissipated electrically inside.
For a cilindrical filament, if you double the radius, you do the section times 4 (pi radius^2), so the resistance lowers by a factor of 4, and hence the power dissipated increases by a factor of 4 (at constant voltage).

Its surface, however, grows only by a factor of 2 (2 pi radius). So per unit of filament surface, twice as much energy has to be irradiated away, and hence the temperature will have to be the fourth root of 2 larger (1.18 times the temperature).

So the thicker filament irradiates 4 times more power away, and does this at a slightly higher temperature.
 
  • #7
vanesch said:
Uh :bugeye:
If the resistance is LOWER, and the voltage is fixed, then the dissipated power is higher:
It was early, I'm still jet-lagged and I haven't had enough coffee!
 
  • #8
vanesch said:
the power irradiated per unit of surface goes to the 4th power of the temperature.

I am interested to know why that is ...

E.
 
  • #9
so a bulb with a thicker filament should glow brighter as more power can be dissipated due to a lower resistance (P= V^2/R).

Am I right in saying that the reason tungsten filament glow is due to its high resistance to electron flow, thereby creating heat. So wouldn't a thinner filament (offering higher resistance) create even more heat, thereby giving a brighter glow?

My question arose because I saw a video showing a bulb filament, moments before it melted. It showed the thinner region of the filament starting to glow brighter and subsequently melted. This led me to think whether filament thickness affects brightness of bulb.
 
Last edited:
  • #11
kapellmeister said:
so a bulb with a thicker filament should glow brighter as more power can be dissipated due to a lower resistance (P= V^2/R).

Am I right in saying that the reason tungsten filament glow is due to its high resistance to electron flow, thereby creating heat. So wouldn't a thinner filament (offering higher resistance) create even more heat, thereby giving a brighter glow?

My question arose because I saw a video showing a bulb filament, moments before it melted. It showed the thinner region of the filament starting to glow brighter and subsequently melted. This led me to think whether filament thickness affects brightness of bulb.

This is correct, but that is because you now have a series of "small resistors". Consider this: you have a small region of resistance r, and the rest, resistance R. The total resistance (series) is r+R.

Now, the current in your overall system is I = V/(r+R). The power, dissipated in the small region, is I^2 r.

So the power dissipated locally, is given by V^2 r/(r+R)^2.

The overall power dissipated, is V x I = V^2 / (r+R)

So we see that if the local region increases its resistance (r increases), with R constant, that:
The overall power diminishes slightly: V^2 / (r + R) decreases a bit if r increases a bit.
The local power dissipation INCREASES strongly:
V^2 r/(r+R)^2 increases.

Take V = 1V, R = 10 ohm, and r goes from 1 to 2 ohm:

overall power goes from 1/11 to 1/12, so it diminishes a bit.

local power goes from 1/121 to 2/144: it almost doubles.

On top of that, the AREA of the local piece has decreased, so locally, we dissipate more power, and we have a smaller area --> temperature increase.
 

1. How does the thickness of a bulb filament affect its brightness?

The thickness of a bulb filament directly affects its resistance. A thicker filament has a lower resistance, allowing more current to flow through and produce a brighter light.

2. Is a thicker filament always better for brightness?

Not necessarily. While a thicker filament may result in a brighter light, it also requires more energy to produce that light. So, a thicker filament may lead to higher energy consumption and higher electricity bills.

3. Can a thin filament also produce a bright light?

Yes, a thin filament can also produce a bright light if it is made with a material that has a high melting point and can withstand high temperatures without burning out quickly. However, it may not last as long as a thicker filament.

4. Does the thickness of a filament affect its durability?

Yes, thicker filaments tend to be more durable and have a longer lifespan compared to thinner ones. This is because they can handle higher temperatures and are less prone to breaking or burning out.

5. How do I know which thickness of filament is best for my needs?

The right thickness of filament depends on your specific needs and preferences. If you want a brighter light, a thicker filament may be suitable, but if energy efficiency is more important to you, a thinner filament may be a better choice. It's always best to consult with a professional or refer to the manufacturer's recommendations for the specific bulb you are using.

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