The 1.2kW bulb is only 1.2kW at the stated V and provided it can draw enough I (current). It's resistance varies from about zero ohms when cold and rises more or less linearly (I think) and therefore Ohm's law applies. Thus, increasing the V from 100 to 130, a 30% rise, will cause a 30% rise in it's resistance (assuming it can draw the current it 'wants'). If you add cable you add resistance which may or may not be significant. Calculate the R of the bulb at diffent I values and do the same for the cable then you can compare the ratio of R to see the effect. (eg if R of cable is 1/1000 of total R then effect is minimal but if it is only 1/10 of total R then you have great effect.).
The difference of opinion between those who say a 1.2kW bulb stays
1.2kW regardless and those who say it varies with current is probably due to the two sides focussing on one kind of power to the exclusion of the other kind. There is the stated or rated power, the 1.2kW; that's fixed even when it's still in it's box and sitting in a shop waiting to be sold. But then there's the power output or actual power which will vary according to the current flowing through it (which in turn depends on the voltage across it).
The greater the resistance the lower the current (Ohms law). The lower the current the lower the power. It's easier to grasp the basics of Ohms law using a resistor rather than a bulb because a resistor doesn't change it's value (theoretically) as the current increases but a bulb's does and so you have 'circular' references which become confusing until you understand the order of events, which in this case are: Switch on, zero R, surge of I (current), sudden heat, bulb blows or lights up, bulb R much higher now, current less, meanwhile V has been playing games too possibly, then there is heat in the cable, it's R changes and that affects total R.
The longer the cable the higher the R and therefore the lower the I. The lower the I the lower the P. There is P also being dissipated, used or 'drawn' if you prefer, by the cable (given off as heat). The actual figures involved are simply down to Ohm's law, E = IR and P = IE and from these two: P = I2R. Three simple but useful equations to make a note of.
BUT... if you have 500 ft of cable all coiled up and its AC not DC then the ball game changes to involve AC theory, induction for example
- don't ask!
With dimmers, there are the old sort, variable resistors , which Ohms law will answer your question about. And you have the modern type which are, I think so anyway, mostly thyristor or similar semiconductor devices and these are different in that they don't share the load but turn on and off at each cycle of the mains, 50 or
60 times per second. Thus only a part of each mains cycle is applied to the load and the dimmer itself is more or less no part of that load (unlike the previous type of dimmer). These semiconductor dimmers use negligible power regardless of the load.
That's my understanding but you should verify it yourselves rather than take my word for it. Tip: Get the 'order of events' clear first; split second by split second, what happens to I, P, R, and even E (or V if you prefer) and why. Knowing what affects what and in what order is 'the name of the game' here.
Paul E. Coughlin
When a tungsten filament is giivng off light, does its resistance vary significantly over a voltage range from 100V to 130V?
On our pro video list we've been having a discussion concerning current flow through a simple circuit containing a large wattage incandescent light and different cable lengths.
This is an example of the circuit:
One 1200 Watt tungsten filament (a light bulb) is plugged into a 120 Volt outlet. The cable used is 12/3 copper. If the cable length increases between the filament and the outlet from 2 feet to 502 feet, does the current in the circuit increase or decrease?
Others are maintaining that the 1200 Watt bulb remains 1200 Watts no matter how much resistance is added in series with it to the circuit. I maintain that the added resistance of the long cable increases the overall resistance in the circuit lowering the current and the power.
They are saying that the lightbulb's filament lowers its resistance to compensate for the added resistance in the cable thereby maintaining the lightbulb at 1200 Watts and the added resistance in the cable adds additional power draw on the circuit.
Obviously, the longer cable run adds resistance to the circuit, which along with the filament acts as a voltage divider. The filament is getting less voltage when in series with the long cable. We're back to my original question.
But which circuit draws more power, the one with the short cable or the one with the long cable.
Then there's a related question about resistance dimmers: Do they use the same amount of power when the lightbulb is dimmed out as when the lightbulb is at full brightness? I maintain that the bright bulb uses more power. I think I remember reading tat it takes something ike 5 to
6 times the resistance of the incandescing bulb in series with it to effectively quinch its glow.
Thanks for your input