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Reply 1
equili3rium
please help its important!! thanks


when its on?
Reply 2
ShOcKzZ
when its on?

lol
Reply 3
Filament of a 100W bulb goes to 3000K when ON. Don't know how much gets through to the outside though
equili3rium
please help its important!! thanks


Depends how long it is turned on. i.e.) The more current that flows through the filament of the lamp, then the more heat this generates and hence the greater the effect this has on increasing the temperature of the surface of the lamp. This is obviously done via the radiation of thermal energy which then makes contact with the lamp's surface i.e.) the glass.
Reply 5
Well 100W implies it loses 100J of energy per second. Filament bulbs are about 4% efficient, so 96J of heat per second goes to the glass. You'd need to know rates of conduction from the glass to the surroundings to find after x seconds the glass reaches thermal equilibrium, and at what temperature.
Reply 6
what is the maximum temp?, can it boil water if its put in?
Reply 7
Well if you think, the higher the temperature of the glass the faster the conduction process is, so there is a negative feedback and so the temperature cannot keep increasing indefinately. If you put water in, the maximum temperature will probably be lower because the water has a higher specific heat capacity than air because of its higher denisty. You can either chose to get very complicated with things like how much water do you put it in, of course the water will begin to heat up on average if it's a small container relatively quickly, like the glass does, and then you'll get the water reaching thermal equilibrium where the heat from the glass in matches the heat from conduction out. Then there's evapouration as well to consider... It all depends on unknown contants (well unknown to me), such as the rate of transfer of energy from the bulb to water, which depend on the surface area, the temperature of the bulb etc.
mik1a
Well 100W implies it loses 100J of energy per second. Filament bulbs are about 4% efficient, so 96J of heat per second goes to the glass. You'd need to know rates of conduction from the glass to the surroundings to find after x seconds the glass reaches thermal equilibrium, and at what temperature.


That was my point, it depends on how long the filament lamp is turned on i.e.) The duration for which the electrical current flows through the lamp. As the duration of current flowing through the filament increases, the greater the amount of thermal energy generated and the greater the temperature of both the filament and the glass.
Reply 9
Depends on the mass of the bulb, the specific heat capacity of glass, the efficiency of the lamp, the time the lamp is turned on and the rate of heat loss to the surroundings.
Reply 10
bono
That was my point, it depends on how long the filament lamp is turned on i.e.) The duration for which the electrical current flows through the lamp. As the duration of current flowing through the filament increases, the greater the amount of thermal energy generated and the greater the temperature of both the filament and the glass.

Well, actually, the temperature will level off after a certain amount of time because the temperature gradient increases as the temperature increases, so at a certain point there is as much heat lost by the bulb as is produced by it.
JamesF
Well, actually, the temperature will level off after a certain amount of time because the temperature gradient increases as the temperature increases, so at a certain point there is as much heat lost by the bulb as is produced by it.


Yep, although I did say that it depends on the duration of current flow, I just didn't explain what would happen after a certain point! :tongue:
The temperature will level off because the heat loss will be given by k(T-t), where k is a constant, T is the temperature of the bulb and t is the temperature of the surroundings. So as T increases, T-t increases and so the rate of loss of heat increases.
Ralfskini
The temperature will level off because the heat loss will be given by k(T-t), where k is a constant, T is the temperature of the bulb and t is the temperature of the surroundings. So as T increases, T-t increases and so the rate of loss of heat increases.


I understand. :smile:
Reply 15
does anyone actually know how you could measure the surface temperature of the glass envelope? thanks
Reply 16
adamg
does anyone actually know how you could measure the surface temperature of the glass envelope? thanks


Lol, OCR Physics A2 Practicle by any chance? I'm doing it too. I would suggest a digital thermomter with a surface probe(s). Or a more simple method would be to surround the bulb in some oil, in a well insulated container and read the temp with a normal probe...
Reply 17
lol yeah got it in one. i did a draft and just said use a button probe and computer. but i thought that might be a bit of a cop-out. I thought they might want something more complicated like a thermocouple, but i have no idea what. thanks alot
Reply 18
adamg
lol yeah got it in one. i did a draft and just said use a button probe and computer. but i thought that might be a bit of a cop-out. I thought they might want something more complicated like a thermocouple, but i have no idea what. thanks alot


Ah yes, thermocouple thermometer is what i meant btw, for most of them there are a selection of probes to attatch to them, but the most suitable seems to be the surface probe.
Reply 19
can you give anymore info on what it is? like how it works etc? thanks