VK2RK
Active member
Does not seem to be hard question to answer, however incandescent globes filament have a cold and hot resistance whose relationship is that the resistance increases as the filament heats up.
Since the question is - Which globe is brighter the question can be answered by performing calculations based on absolute maximum temperatures, this is when the individual globes are run at 200 Volts, then the resistance can be calculated, however it’s understood that at a lower voltage as is the case in the diagram shown the filament resistances will be less than the calculated value.
Since the relationship of hot to cold is linear, the calculated resistance values will serve to calculate which globe dissipates the most power.
Resistance of globes at 200V R = E^2/P
R60 = (200^2)/60 = 666.67 Ohm
R100 = (200^2)/100 = 400 Ohm
Circuit current I = Vs/Rt
I = 200/(666.67 + 400) = 187.5 mA
Power in each globe P = (I^2) x R
P60 = ((187.5/1000)^2) x 666.67 = 23.44 Watt
P100 = ((187.5/1000)^2) x 400 = 14.06 Watt
Thus the 60 Watt globe will be brighter
Then there is the intuitive approach, the 100 Watt globe will have a lower resistance, in a series circuit the current is the same in each globe
The voltage drop will be the greatest across the highest resistance globe thus the 60 watt globe will dissipate the greater power hence brightness.
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