Well, this was the question that I was asked in my interview with a famous power company:
"In a 100 W bulb, assuming that the changes in resistance due to temperature as being negligible, is Ohm's law still applicable? For a constant power (100 W) isn't voltage inversely proportional to current?"
And this was the answer I gave:
" The concept of assuming constant power in the bulb is misleading. 100 W is the RMS power and the instantaneous power actually varies with time in ac. Ohm's law is certainly applicable."
I later found that my answer had been rated as technically unsatisfactory.
(1) What will you answer for this question?
(2) What is wrong in my explabnation technically?
Thank you for answering.
You really should avoid the use of the term "RMS power". There is no such thing (except in the fevered imagination of certain sound engineers).
RMS applies to waveforms of Voltage and Current. Not Power.
I would have said that power does not come into play when considering if Ohm's law applies. The light bulb, because you're assuming temperature has no effect on the resistivity, is a constant resistance value. Therefore, V = IR does not change.
In reality, a light bulb does not observe this linear relationship, but it does when we ignore the effect of temperature on the bulb's resistivity.
The reason that voltage is inversely proportional to current for constant power is because P = VI. If voltage increases, in order to keep P constant, current must decrease; the opposite is true as well.
I'd say that your answer didn't really address either question directly. Nothing about it is technically 'wrong', since instantaneous power does change with time, but that's not what they asked.
I'd say you were close, but a bit off track. And just because you were interviewed by a supplier of AC power was no reason to bring in the unneccessary complication of an AC voltage source. I would have responded to each of the 2 questions separately, something like -
"1- Yes, Ohm's Law is fully applicable. For example, it may be used in this situation to derive the resistance of the filament, provided either the value of the voltage source or the current through the filament is known."
"2- Yes, for a constant power load, voltage and current would be inversely proportional, however, such is not the case in this example, because the 100W power rating assumes a fixed nominal voltage source. Were the voltage supplied to the bulb dropped to 1/2 it's nominal value, by Ohm's Law the current would also drop to 1/2 nominal, and power to 1/4, or 25W. So clearly a simple filament light bulb is not an example of a constant power device."
V(t) = I(t) times R
V is not inversely proportional to current, but directly proportional if resistance is held constant.
If power, on the other hand, is held constant, and current is varied, then resistance can't be constant, because P = I^2 * R.
P(t) = V(t) * I(t), so P(t) / V(t) = I(t)
Sounds like they are asking if P= V * I contradicts V = I * R.
I and E are proportional to W. If the power is held constant, and R doesn't change, then I and E must also be constant for Ohm's law to hold true (which it does.)
The business of RMS vs. instantaneous power is irrelevant. That's probably why they marked you down for it.
EDIT: what was the point you missed that they were looking for? That at constant power you don't get to tweak E and I by themselves without also tweaking R to keep the same power. This is the point about Ohm's law continuing to apply: you have to consider resistance too.
Or, W = I^2R = E^2/R.
If we assume that the filament does not change resistance as it heats up, then we are assuming the resistance is constant. Another name for a material with an unchanging resistance is an Ohmic material, and Ohm's law, by definition, applies to such materials.
My answer would be:
"If the bulb were constrained to output 100 W at all times, the voltage across it would indeed be inversely proportional to current going through it. However, no such constraint exists, and for a constant filament resistance, the power dissipated by the bulb will be equal to V^2/R, in accordance with Ohm's law. The 100 W labeling simply gives the bulb's nominal power output in a 120 VAC system."
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CONFUSED. Is Ohm's law still applicable?