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# Edexcel further mechanics question watch

1. I have a question that I would like to ask, its from the 'loads of unit 4 questions' document on scribd

"The average wind speed in the UK is 5.8 m s–1, which results in an actual average power output of 100W. Discuss whether it would be better for the environment to replace some filament light bulbs with low energy bulbs than to use this turbine. Assume each filament light bulb is rated at 100W and each low energy bulb is rated at 11 W."

This is what the mark scheme says:

"Recognises that 100 W is produced over 24 hours (1) Estimates if this would fulfil lighting needs for a day (1) Estimates energy used by low energy bulbs in day (1) Conclusion (2) The answer must be clear and be organised in a logical sequence Example:The 100 W is an average over the whole day. Most households would use light bulbs for 6 hours a day in no more than 4 rooms, so this would mean no other energy was needed for lighting.4 low energy bulbs would be 44 W for 6 each hours so would require energy from the National grid.[Accept an argument based on more light bulbs/longer hours that leads to the opposite conclusion]"

I have never come across a question as involved as this - should I be worried?

Also, how is power over 24 hours? If Watts is in joules per second, and both the bulb and turbine use/generate 100 joules per second, surely they would cancel each other out?

Any help would be much appreciated mates !
2. You're not expected to reproduce the markscheme on your exam script - it's a shorthand to remind the marker what to give you marks for.

fairly obviously 24 hours is the time taken for complete a day-night cycle - which is important because household lights are mostly used at night.
3. (Original post by ThatWasHard!)
I have a question that I would like to ask, its from the 'loads of unit 4 questions' document on scribd

"The average wind speed in the UK is 5.8 m s–1, which results in an actual average power output of 100W. Discuss whether it would be better for the environment to replace some filament light bulbs with low energy bulbs than to use this turbine. Assume each filament light bulb is rated at 100W and each low energy bulb is rated at 11 W."

This is what the mark scheme says:

"Recognises that 100 W is produced over 24 hours (1) Estimates if this would fulfil lighting needs for a day (1) Estimates energy used by low energy bulbs in day (1) Conclusion (2) The answer must be clear and be organised in a logical sequence Example:The 100 W is an average over the whole day. Most households would use light bulbs for 6 hours a day in no more than 4 rooms, so this would mean no other energy was needed for lighting.4 low energy bulbs would be 44 W for 6 each hours so would require energy from the National grid.[Accept an argument based on more light bulbs/longer hours that leads to the opposite conclusion]"

I have never come across a question as involved as this - should I be worried?

Also, how is power over 24 hours? If Watts is in joules per second, and both the bulb and turbine use/generate 100 joules per second, surely they would cancel each other out?

Any help would be much appreciated mates !
The question is really about energy supply and energy demand (consumption) and the difference between average power and instantaneous power demand.

Think of the generator in total energy generation terms: wind is not constant and wind speed varies over the whole day. At any given instant, the generator output can both far exceed 100W (could be many kW or MW for periods during windy days) or none at all with no wind or even too much wind (during storms) causing the generator to be stopped and tethered for safety reasons. Thus wind power is somewhat unreliable compared with other forms of electricity generation.

Hence the generator output is averaged over a 24hr period to be 100W (100J/S) for the full 24hrs. It does not mean the generator produces 100W continuously.

On the other hand, consumers can and do demand instant power at any time wind or no wind. But demand is not continuous either and fluctuates dramatically throughout the day and is also very seasonal. (Winter vs summer, Christmas day vs summer holidays abroad etc.)

For example: It's a now quite well known fact that TV audience numbers are monitored by monitoring the electric power demanded by consumers during commercial breaks - millions of people turn on their electric kettles and visit the lavatory at that time. The electricity companies keep a close track on television programme times so they can predict demand and bring it on-line at the right times!

The variable instantaneous power demanded by consumers, cannot be guaranteed provision by wind power generation on it's own. Hence a mix of generation technology is required to meet that instantaneous demand.

In other words, wind power can supplement traditional power generation but cannot totally replace it. Changing to 11W light bulbs aggregated across the country, will reduce the total energy a country needs. That would allow some fossil-fuel power stations to be closed down and replaced with renewable sources to reduce greenhouse gas emissions accordingly. But there is a limit because it will not eliminate instantaneous demand - only reduce it to a certain extent.

There is no right or wrong answer in questions of this type. But you need to make a logical argument based on the information presented to you.

4. (Original post by ThatWasHard!)

x
Are you now OK with your understanding of what the question wants and the explanations have provided?
5. (Original post by uberteknik)
Are you now OK with your understanding of what the question wants and the explanations have provided?
I understand it now - so average wattage is over a 24h period in this case? Just how was I supposed to deduce that, there was no information to say anything about that (it could be power output per hour couldn't it?)
6. (Original post by ThatWasHard!)
I understand it now - so average wattage is over a 24h period in this case? Just how was I supposed to deduce that, there was no information to say anything about that (it could be power output per hour couldn't it?)
If it was per hour it would state so. When talking about these kind of topics, when it states average power, it's really referring to an average of a cycle, day - night here, which is obvious for something such as a wind, which will vary over the whole 24 hour period. You couldn't average it per 2 hours, because each of the 12 averages per day would be different.

An average is mostly always taken over the whole range of values for something, hence the average is taken from every value from the range of values for power output. These values will change continuously in a 24 hour period, and hence why the average is per 24 hour.

However Uberteknik's post really nails this.
7. (Original post by ThatWasHard!)
I understand it now - so average wattage is over a 24h period in this case? Just how was I supposed to deduce that, there was no information to say anything about that (it could be power output per hour couldn't it?)
Phichi replied with a good answer.

I guess the limitation of being a teacher is both time to cover everything that's needed and also the teachers own knowledge over and above that required by the syllabus. The latter is something that needs passion and a love of the subject matter.

With all A-level subjects, the syllabus and official text books will only go so far.

The exams are moving towards more stretching questions which require the student to acquire knowledge about the subject over and above the syllabus.

This has been done for many years with subjects like English Literature where the student is encouraged to read as much as they can handle to hone their comprehension and writing skills. With subjects like music and art, the students motivation is clearly obvious.

It's the same for science but that will show up as the difference between students who learn Physics as a means to an end, or those that learn it to satiate a curiosity and because they have a passion for the subject.

The next stage is undergraduate where the student is expected to learn a significant part of the course under their own steam. This is one of the biggest shocks when moving from A-levels to university and probably the biggest reason for students dropping out in the first year.
8. (Original post by ThatWasHard!)
I understand it now - so average wattage is over a 24h period in this case? Just how was I supposed to deduce that, there was no information to say anything about that (it could be power output per hour couldn't it?)
I think you need to assume it's a *long term average* of at least 24h but IMO you shouldn't be punished for calculations based on longer periods e.g. 1 week or 1 year

don't get hung up on reproducing the mark-scheme in your answer, I think the marks here come from understanding what average means and how this is likely to affect the usefulness of wind turbines at supplying different load levels. any logical use of the numbers would appear to get you the marks.
9. (Original post by Joinedup)
I think you need to assume it's a *long term average* of at least 24h but IMO you shouldn't be punished for calculations based on longer periods e.g. 1 week or 1 year

By intuition, the average should be taken over the whole interval of which a variable is changing, but should really be the smallest possible set. If you took an average of two weeks, you could quite easily have made fourteen 24 hour averages which would be better. A week average will give you a more general value which is more susceptible to errors (i.e the wind may have stopped for a day). Having it per day would narrow it down. This all of course relies on how it's calculated though. Taking an interval larger than the cycle in question however would be slightly silly.
10. (Original post by Phichi)

By intuition, the average should be taken over the whole interval of which a variable is changing, but should really be the smallest possible set. If you took an average of two weeks, you could quite easily have made fourteen 24 hour averages which would be better. A week average will give you a more general value which is more susceptible to errors (i.e the wind may have stopped for a day). Having it per day would narrow it down. This all of course relies on how it's calculated though. Taking an interval larger than the cycle in question however would be slightly silly.
Not sure I follow that - if the wind stops for a day and your wind speed meter says it stopped for a day that's not an error, it's still data and should still be included in an average.

afaik what you need to know about averages for this question is that it's the amount you'd expect to get over a long period but that it's made of individual observations that could be different from the average (by a lot)
11. (Original post by Joinedup)
Not sure I follow that - if the wind stops for a day and your wind speed meter says it stopped for a day that's not an error, it's still data and should still be included in an average.

afaik what you need to know about averages for this question is that it's the amount you'd expect to get over a long period but that it's made of individual observations that could be different from the average (by a lot)
It all depends on how long you take recordings for. If you took recordings per day for a year, and found the average per day, it'd be a lot better than taking them per week and finding the weekly average. Also it was mentioned above that random errors, such as generator failures could take place. The idea of the wind stopping, isn't a great example, however, I think you misunderstood what I said. If you took a week average, it may well not include such an event, however a daily one, would.

The idea of averages for this question is realising that the average power output is almost guaranteed to be over a period of 24 hours for obvious reasons, not any long period. There is no intuition in assuming 100 W must be over a year.
12. (Original post by Phichi)
It all depends on how long you take recordings for. If you took recordings per day for a year, and found the average per day, it'd be a lot better than taking them per week and finding the weekly average. Also it was mentioned above that random errors, such as generator failures could take place. The idea of the wind stopping, isn't a great example, however, I think you misunderstood what I said. If you took a week average, it may well not include such an event, however a daily one, would.

The idea of averages for this question is realising that the average power output is almost guaranteed to be over a period of 24 hours for obvious reasons, not any long period. There is no intuition in assuming 100 W must be over a year.
tbh I'd very much expect a single quoted figure of 'average wind speed' to be the unweighted average of many (several per day) instantaneous windspeed readings taken over a period of the year - because the earth has a clear 12 month seasonal cycle.

this isn't an average windspeed per year because like power it's based on instantaneous measurements of a rate at which something is changing over time.

I'm still not really sure what you're saying, the sampling frequency is a separate thing to the interval over which you take the average.
13. Oh thanks everyone I get it now. Cheers for the help!

(Original post by uberteknik)
Phichi replied with a good answer.

I guess the limitation of being a teacher is both time to cover everything that's needed and also the teachers own knowledge over and above that required by the syllabus. The latter is something that needs passion and a love of the subject matter.
P.s. I don' think its fair on the teacher uberteknik because there isn't one haha! I'm teaching myself the course like a term ahead of the school!
14. (Original post by ThatWasHard!)
Oh thanks everyone I get it now. Cheers for the help!

P.s. I don' think its fair on the teacher uberteknik because there isn't one haha! I'm teaching myself the course like a term ahead of the school!
Haha. Good for you.

You ain't doing so bad then is ya! lol.
15. So is it better for the environment to replace filament light bulbs with low energy bulbs than to use this turbine?
16. (Original post by mystreet091234)
So is it better for the environment to replace filament light bulbs with low energy bulbs than to use this turbine?
From a power consumption viewpoint, of course it is.

The less energy we consume, the fewer power stations of any description we need to build. Which means other benefits like lower energy and resources consumed in the building of the power stations and other infrastructure etc.

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