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By Calculation:
20W is parallel with 40W so their total is 60W if they are fed 240V.
But they have another 60W in series so the total power from 240V is cut in half to only 30W.
In Reality:
The incandescent light bulbs are cooler at half power so their resistnce decreases which increases their actual power.
An incandescent light bulb draws 10 times or more than its normal power when it is cold.
Making assumptions that are not specified is dangerous.
Only possible answer is taking the stated info as is. Total power is 120 watts.
This would be the case if they are all 120v bulbs and the power numbers are bulb wattage rating, but I would take the power numbers stated as actual circuit consumption, not making any assumption to bulb wattage or voltage rating, since they are not given.
Yes if the bulbs are getting the correct voltage then the total is 120 watts.
If they only get 1/2 the required voltage (240v units run on 120v) then their power is 1/4 of that shown because
the power varies as the square of the voltage, so the total would be 60 watts.
In other words, if they are 120v bulbs then the power is 120 watts,
but if they are 240v bulbs then the total power is 60 watts.
If instead the upper case "W" stands for "Ohms" (which it does sometimes) then the total power would be 785.45 watts.
Hello
By observation of the diagram, a 240V rms voltage supplies a circuit consisting of 3 lamps. The lamps are dissipating 60 watts, 20 watts and 40 watts. The 20 and 40 watt lamps are connected in parallel, the pair is in turn connected in series with the 60 watt lamp. This configuration resolves to:
60 watts in series with (40+20w) = 60w in series with 60w. We have a 1:1 voltage divider, therefore the voltage across the (20+40w) pair is half of the supply, ie 120 Vac. Thus V(60w) = V(20+40w).
Since P = EIcos(phi); and phi = 1 (resistive circuit elements) then P = EI or P = VI. Now Is is the circuit current flowing through the 60w and the series connected (20 + 40w). Therefore the total power is Pt = E x (P60/V)
Pt = 240 x (60/120)
Pt = 240 x 0.5 = 120watts.
Since no ratings (ie lamp voltages) are given one must deal with the problem as presented. From the rms values being given for E and I it seems evident to me that it is a simple ac power calculation. I would expect that values for the branch currents were expected also.
Yeah it is tricky if you look into it, but if you take it at face value it's either that the bulbs are indicating what the true wattage is (so just add them up) or it is showing what the resistance is (W taken as upper case Omega). If the bulbs are not really operating at true voltage then yes it is harder to calculate because the resistance is not linear with voltage like a pure resistance is. Often these problems assume linear resistance though, so again we're stuck with guessing until the OP comes back and clarifies.
This kind of thing happens a lot really. Draw a resistor and ask "what is the resistance of this resistor?"
Maybe they are 12V light bulbs for cars? The 60W bulb is a headlight and the other two bulbs are at the rear.
Then their total power dissipation will be tremendous when the 240V is turned on.
Perhaps a bulb symbol has been used. But i suppose we have to accept them as loads of 60,40 and 20 watts.
Leaving other assumptions, I feel, the AudioGuru's reply at post#2 is relevant.
Hello
By observation of the diagram, a 240V rms voltage supplies a circuit consisting of 3 lamps. The lamps are dissipating 60 watts, 20 watts and 40 watts.
...
I believe that was Colin's point too. But there is no evidence the lamps are "dissipating 60W" etc.
Correct labeling in a schematic is to label the COMPONENT, not it's dissipation. The labeling indicates it is a "60W lamp" not an unknown lamp that is dissipating exactly 60W.
Look at 100 components on schematics, how many of the 100 does the label show the component type? How many of those 100 labels is it showing component dissipation?
Maybe if the examiner was more competent he could have used a dissipation symbol.
Hello Mr RB and Sallman92
My background is electrical and this type of problem was often presented in quizzes and exams, during my training, usually worth about 5 marks from a total of 100 marks. Rather than a schematic, I see a diagram to illustrate a "problem". I see a steady-state condition, otherwise we would be looking at more sophisticated and or complex description of the loads and the supply: we would be asked for a more complex analysis, step response or such. It looks like a typical elementary "electrical trades course" problem to me. It would be presented in the context of the subject matter being studied...single phase ac power probably.
While probability can be considered, if any definitive replay is given a student takes it for granted and if the examiner has some other answer in his key to the paper set, the poor student just looses the marks as the valuation is robotic.
Perhaps it is time that the question setter must make the conditions of the question dam clear for no further ambiguity. Let us see this question it self.
NO one, ever, connects 240V loads in series. The only exception is a older version of Fan regulators where series resistor is connected to control the fan speed. IMHO, while not intending to hurt any one, there is need for updating knowledge base, to the teachers as well. They are teaching to students who are expected to live in society of technological revolution.
At times we watch questions with multiple answers and the key is modified on public demand or that question is removed from valuation all together.
Hello again
Is this an end of year wind-up?...a carry over from april maybe...and I've been sucked in? I was tempted to stay away from further responses...but then.
The examiner in this instance is Sallman92: he has posed the question to the forum has he not?
By the look of the diagram accompanying the query, I get the impression of a 1940s...1950s text book or magazine article; the choice of lamps and the rendering of the illustration of them. I think that originally the problem has been set in a certain context; valid at the time but which doesn't prevail here.
Is there a minimum level of complexity required of postings to these forums (fora for Romans and the pedants) that the participants feel it necessary to seek it out from "the simplicity" rather than address "the simplicity" first.
I hardly think it's fair to blame teachers and examiners for who are they but us. Don't we all learn from simple ideal principles, advancing to apply a deeper analysis of circuits and components to address their intrinsic properties.
Sorry Phil,
Never intended to pull you in .
Neither it was my intention to blame or even point out teachers. I am myself son of a teacher. In fact I never knew that sallmann29 is the examiner.
I was only critical of how questions need to be framed so that there would not be a possibility for multiple replies. I was discussing the need of the times in general and perhaps applicable even to my nation. ONly to indicate that questiion framing is to be meticulous to have one solution or reply as the student's state of mind can be imagined while taking a 30 mt paper of this nature. Once he /she encounter two or more bits that confuse them, gone is that paper.
However, i passed thro those stages well before 1964 and now am a retired person having served in the telecom scenario.
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