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Angela Liang
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How to increase or decrease the temperature?Drakkith said:You can change the resistance of the circuit. Increasing or decreasing the temperature of part of the circuit is one way to do this.
How to increase or decrease the temperature?Drakkith said:You can change the resistance of the circuit. Increasing or decreasing the temperature of part of the circuit is one way to do this.
You could increase the temperature of a light bulb filament by shining a high power light source on it. You could decrease the temperature by placing the bulb in a blackened container that's held a a very low temperature - say in liquid nitrogen and that would reduce the surface temperature by about 200°C.Angela Liang said:How to increase or decrease the temperature?
Wow. Thanks :)sophiecentaur said:You could increase the temperature of a light bulb filament by shining a high power light source on it. You could decrease the temperature by placing the bulb in a blackened container that's held a a very low temperature - say in liquid nitrogen and that would reduce the surface temperature by about 200°C.
I'm not sure which element of the thread this has come from - Ohm's ratio, temperature dependence, power in a resistor? But whichever it is, perhaps we need to be clear about the details of the context, else statements like this are hard to evaluate.houlahound said:So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
That's what the standard formula relating length CSA and Resistivity to Resistance says. Resistivity is temperature dependent. Edit: The Power will not have a simple relationship with Resistance because of this because the temperature changes with Power and the R will change accordingly etc...houlahound said:So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
houlahound said:So power in a light is proportional to diameter of filament assuming constant length tungsten filament?
I hate to be pernickety, but CSA is proportional to D2 not D, so I don't think it is what the standard formula says.sophiecentaur said:That's what the standard formula relating length CSA and Resistivity to Resistance says. ...
The expansion affects the radius which affects CSA and the length. But it wouldn't affect the number of atoms / electrons involved. Gets harder and harder , dunnit?Merlin3189 said:I hate to be pernickety, but CSA is proportional to D2 not D, so I don't think it is what the standard formula says.
Possibly more interestingly your rider, that resistivity varies with temperature, provoked me to think that length and CSA would also vary with temperature. Then the thermal increase in length would be more than compensated by the thermal increase in CSA , leading to a lower resistance with increasing temperature. Unfortunately the effect of thermal expansion is about 1000x less than the increase in resistivity with temperature (for Tungsten), so I can see why no one ever mentions it.
houlahound said:may have missed it, CSA is ?
jim hardy said:Ohm derived his law by parallel thought to contemporary experiments with conduction of heat through metal. He was using galvanic cells and wire.
That's a fascinating thing.David Lewis said:Ohm used a thermocouple as his voltage source. (He realized source impedance of galvanic cells varied as a function of current and state of charge, which obscured the underlying principle he was trying to discover.)
I believe you are right and thanks for the correction .David Lewis said:Ohm used a thermocouple as his voltage source.