Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 26/09/2007 08:07:15
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do two 50w light bulbs give as much light as one 100w bulb?
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Where I used to live, I had 3 lights in my lounge. 2 were 60w and the 3rd was 100w. With the 2 60s on it didn't seem as bright as the single 100w, but that may have been due to positioning (1x60 at each end of the lounge & the 100 in the middle).
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In theory yes but lower wattage tungsten filament bulbs tend to be slightly less efficient. Tungsten filament bulbs also dim with age to two old "60w" bulbs are quite likely to be less bright than a new "100w" one. The reason for this is that the filament slowly evaporates and deposits on the inside of the bulb making it less transparent and the filament becomes thinner and therefore higher resistance and cooler so the actual wattage and the luminous efficiency of the bulb goes down with age.
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The light output of Tungsten type lamps goes up as the fourth power of the applied voltage so when making a comparison you would have to check that the specified voltage was the same.
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Ian - if the resistance increases, wouldn't that cause a corresponding increase in temperature?
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Ian - if the resistance increases, wouldn't that cause a corresponding increase in temperature?
It all depends of course how the power is supplied to the circuit if it is from a constant voltage source P=V^2/R but if it is from a constant current source P=I^2 * R.
The former would of course apply to lamps on a domestic circuit.
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Ian - if the resistance increases, wouldn't that cause a corresponding increase in temperature?
It all depends of course how the power is supplied to the circuit if it is from a constant voltage source P=V^2/R but if it is from a constant current source P=I^2 * R.
The former would of course apply to lamps on a domestic circuit.
Is that a yes or a no? [???]
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I thought the formulae provided made that clear, but to reiterate.
If the current is held constant an increase in Resistance will lead to an increase in power dissipated hence an increase in brightness.
This is not what happens when the lamp is supplied with a constant voltage as is the normal case in a domestic installation, in this case an increase in Resistance results in a decrease in current and a consequent drop in power and brightness
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do two 50w light bulbs give as much light as one 100w bulb?
You have to check the light efficience, that is how many lumen/watt that lamp gives. Infact what you add are luminous fluxes, that is lumens, not watts.
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Ian - if the resistance increases, wouldn't that cause a corresponding increase in temperature?
If the currente were the same, yes; in this case what remains the same is not current but voltage. The electric power dissipated is:
W = Ri2 = V2/R
and in this case you use the second equation; so the power dissipated decreases if R increases.
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Ian - if the resistance increases, wouldn't that cause a corresponding increase in temperature?
There's more to it than just the resistance of the filament.
Filaments are made of the same stuff, always - tungsten (because it has such a high melting point).
You can make a filament of a given resistance with various lengths - a long, thick one may have the same resistance as a shorter, thinner, one. They will both transfer the same amount of energy BUT-
The 'brightness' of a filament will depend upon, not only the power supplied to it , but the surface area. A smaller filament will have a smaller surface area, so it will get hotter before it dissipates the same amount of power as a larger filament. The light from it will appear whiter / bluer than the redder light from the cooler, larger filament.
In addition to light wavelengths filament radiates infra red - a continuous spectrum, in fact. You get better 'value' if it operates at a high temperature because there is more visible and less IR. Problem is that the filament won't last. It's a compromise between lifetime and light output.
Mains voltage bulbs need to be a high resistance - e.g. for a 100W bulb, operating at 230V, the resistance must be about 500ohms.
The filament needs to be quite 'flimsy' (as you can see, when you look at the coiled coil in a modern bulb) if you want the filament to glow at a temperature that we find acceptable. They gradually lose metal from their surface and this causes them to die, eventually.
Low voltage bulbs (say car headlamp bulbs) are a much lower resistance (1.4 ohms for a 100W bulb). The filament is shorter and chunkier and can be operated at a higher temperature. So you get more useful light out of it and it will last longer.
Quartz Halogen bulbs can be operated at a higher temperature because the chemistry inside reduces the rate at which the filament disintegrates. For a QH bulb, at any given design voltage will have a smaller filament (same resistance though) than the equivalent normal bulb. Both may 'consume' the same power but the QH bulb produces more visible light.
The luminous flux will be a bigger proportion of the input power..
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Syphrun, Lightarrow & Sophiecentaur - thank you. I think I understand it now.
P.S. I'm total crap when it comes to equations.