Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: sacred99 on 07/07/2021 13:29:37
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Hi,
I got a different answer from the book. Please confirm the answer, my answer for question 1 was 0.0417A but the book is 0.046A. I do not know how to do question 2. Thanks
Question 1 An energy saving bulb with 240 V power supply, produces 10 joules of light energy per second. Calculate the current flowing through the bulb.
Question 2 Calculate the heat energy loss of the bulb in one second
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Hi.
Thanks for declaring this to be a homework question. It's standard practice not to give out answers but instead, to try and help the student learn.
1. There seems to be some information missing here. Just before the question was there a diagram or general information about energy saving light bulbs?
For example, something that told you that the bulb is 90% efficient?
I can see how you obtained your answer of 0.0417 A and it looks like you have done well - it's just that there is a little bit more information we needed to consider.
2. I have some concern that the question has stated 240V. It seems that they want you to assume DC but they have quoted a voltage that was a common voltage in AC systems. Let's not worry about this too much to begin with.
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Thank you, Eternal Student. Unfortunately only a picture an energy saving bulb was shown. Here's the complete question as attached
The book gave (11 - 10) J = 1 J as the answer for question 2 which I do not understand how 11 was obtained or where did 10 come from? I assume that the watt of the bulb is 10 W (10 joules/second as per the question) so shouldn't it be 10 instead of 11? I'm lost, please enlighten me
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Hi again.
There is not enough information here. I'm 99.9% confident of that. If we assume the bulb is 100% effcicient and produces only light energy from the power supplied to it then there is 0 Joules of heat produced.
The information we need may be in Question 1) part a) or something that was mentioned elsewhere. Possibly the question is badly put together and the information we need was never printed. For example, if the diagram was bigger we could have read the information about the voltage and current drawn since it would be printed on the base of that bulb. My first guess is that the information is in earlier parts of question 1*.
* Actually I don't know if this is question 1. Only that in your picture it is section (b) of some question number.
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Yes, I fully agree with you that the question is badly phrased. Unfortunately, the previous part of the question is non-related (refer to attachment). It's really driving me nuts
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A single compact fluorescent tube converts about 10% of the electrical energy into light, so if you want 10 W (10 joules/second) of light output you need 100W of electrical input. The multi-folded lamp illustrated has a lot of self-absorption (the inner half of the tubes does not contribute much useful light outside the assembly) so you might be down to 6%.
To generate 10W of photon power you therefore need 166.67 W of electrical power, i.e. 0.694 amp at 240V DC or rms. The heat loss is 156.67 joule/sec.
Whoever set the question clearly has no understanding of the subject. A 150W compact fluorescent unit would probably have six long or eight short U-tubes, quite different from the domestic 4-tube unit shown
10 watts of light is a hell of a lot - you might encounter it in hydroponics or a photographic studio. A typical domestic CFL consumes 10 - 20W and outputs 500 - 1000 lumens, or about 0.5 - 1.2 joules of visible photons per second.
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Anita's problem is genetic - she seems to be related to the author of the question and therefore should not be attempting domestic wiring.
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Hi Chibi,
I hope you are well. Many would think that your post is an attempt to set things up ready to advertise something soon. Try the Just Chat section if you do want a more general discussion.