Engineering Exam Question Missing Information...

danza

danza

danza

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Guten evening all!

Had a materials and engineering systems exam today, and one question was seemingly missing information required in order to be able to answer it. This meant nobody could answer it without making a complete guess as to the missing value, and therefore coming up with likely totally different answers to one another.

This really threw me as I thought it was something I missed during revision, and instead spent the 30 mins I should have used to answer that question double checking other answers.

It turns out that I did know how to answer the question as it was something I had revised and knew well. Other people have confirmed that the info was missing and basically guessed.

Has anyone had a similar experience, and if so, what was the outcome?

I can't believe this managed to get through and into the exam room without someone noticing. :/
 
danza said:
Guten evening all!

Had a materials and engineering systems exam today, and one question was seemingly missing information required in order to be able to answer it. This meant nobody could answer it without making a complete guess as to the missing value, and therefore coming up with likely totally different answers to one another.

This really threw me as I thought it was something I missed during revision, and instead spent the 30 mins I should have used to answer that question double checking other answers.

It turns out that I did know how to answer the question as it was something I had revised and knew well. Other people have confirmed that the info was missing and basically guessed.

Has anyone had a similar experience, and if so, what was the outcome?

I can't believe this managed to get through and into the exam room without someone noticing. :/
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Possibly the purpose of the question was to see how you might cope with solving the problem without having full data?

I can certainly remember mid term exam questions back in my Uni days that provided questions/data that resulted in "insane" answers. I suspected then that they were "Character" tests rather than "Arithmetic" ones

EG A hydro power plants feed-water pond freezes in the winter.

The Power Station over comes this by using some of the electrical output to melt the ice so that the power plant can continue to operate during the winter.

How high does the head of water need to be to break even.

IIRC the answer was about 65,000m :eek:
 
Sometimes you are expected to guestimate roughly intelligent values for unknown parameters, otherwise you can leave things and a function of the unknown parameter.
In physics this is often the case.
 
I don't think that was meant to be the case. Nothing we were taught or told indicated that the questions would be anything other than 'straightforward', with all information provided. This was confirmed by about a dozen tutorial questions and 4 previous year's exam papers.
 
Its possible the guy writing the exam made a mistake, i've had a couple like that.

Otherwise, it could be a test of general knowledge, for example with materials it could be something you should be able to provide a reasonable guess at.

The uni systems are weird, if its an en-masse change in answers caused by a question mistake caused by the exam they'll compensate, otherwise the final mark will be final. I've had a couple of issues including losing out an entire questions worth of marks for drawing a 3-view from a different orientation (they didnt specify what orientation and i found it produced clearer representation of the part), the german lecturer with a phd in design agreed me but it stuck cos it was final.

Or there was losing 26% for handing in a peice of paper with 10 numbers on it (arbritary ones too) to the school office and getting a receipt rather than sticking it under the lecturers door.
 
danza said:
I don't think that was meant to be the case. Nothing we were taught or told indicated that the questions would be anything other than 'straightforward', with all information provided. This was confirmed by about a dozen tutorial questions and 4 previous year's exam papers.
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well it is possible for mistakes in exams to be made. I've seen a couple myself.

It isn't fun to work through a problem that is unanswerable but at the end of the day the marks for that questions will be ignored, giving you a distinct advantage if you kept your cool and didn't waste too much time on it. It is also possible that the grade distribution will be checked against previous years and grades bumped as necessary.
 
How long would it have taken to calculate a "set" of answers that took into account the missing data as a variable number somewhere in the region of the "sensible".
With the resulting multiple answers, you could then extrapolate a graph to show possible outcomes of other data you hadn't input as the missing variable.

Hope that makes sense :D
 
D.P. said:
Sometimes you are expected to guestimate roughly intelligent values for unknown parameters, otherwise you can leave things and a function of the unknown parameter.
In physics this is often the case.
Click to expand...

probably the latter I'd guess
 
If they have made a mistake, you'll be fine :) it sounds like they did to be honest; I never say a single question in any of my mech eng years that required you to remember or estimate a value.

As someone else has suggested, could you just have retained it as a constant?
 
Gaseous propane (C3H8) is burned with 50% excess air in a combustion chamber. The propane enters at 25oC (298.1 K) and the air at 7oC (280.15 K). The mass flow rate of the propane is 0.1 kg/min. The products of combustion leave the combustion chamber at 1400K. The enthalpy of reaction for gaseous propane is -2214.52 MJ/kmol.

Calculate:

the mass flow rate of the air
the rate of heat transfer from the combustion chamber
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The question is worded in a way that could suggest that there is only one answer by the bold text above, i.e. not 'a mass flow rate of the air' etc.

The exam question was basically exactly as above, but the quantity of excess air was not given as it had been in all previous exam/tutorial questions (either that or there's been a way to calculate it from the information given). Simply, 'XX%' was missing. Also, the question was worth 18% of the exam and was one I was pretty confident in answering. :/

If I've missed a trick here, I'll gladly wear the forum's dunce hat for a day.
 
Just use the real world method (rather than the exam method) of wild-***-guessing whatever information is missing ...

(or possibly talk to your lecturer/tutor about it; either they might be able to tell you if you were missing a trick on how to solve the problem or arrange for the issue to be investigated and potentially the marking scheme changed).
 
memyselfandi said:
Just use the real world method (rather than the exam method) of wild-***-guessing whatever information is missing ...

(or possibly talk to your lecturer/tutor about it; either they might be able to tell you if you were missing a trick on how to solve the problem or arrange for the issue to be investigated and potentially the marking scheme changed).
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Tutor has been emailed. :)

And of course, real world engineering is often far removed (but not so far) from the theoretical in order for the product to function, whilst at the same time being physically possible (we get this a lot at work).
 
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I'd have calculated based on the air required for combustion with no excess. Then if there was a difference between that and the 1400k exit temperature i'd assume that any cooling was caused by excess cool air and work out the excess from the temp difference.

Eg if no excess air gives 2500k heat coming out, then 1100k is being lost to mixing with excess cool air.

If that sort of makes sense, the main assumption would be that heat isn't being lost through the chamber walls.
 
I don't think you can work out the excess air, because the flame temperature would depend on the heat transfer from the combustion chamber, which depends on the quantity of the reactants.

Best I can think of is state that you are assuming a typical value of excess air, say 50%, or alternatively assume that they meant enough air for stoichiometric combustion, and a negligible amount of excess air. Either way I don't think you could do anything apart from making a best guess and briefly explaining the assumption.
 
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If i recall my a level chemisty its 10 parts oxygen to 1 part propane, and the atmosphere is 21% oxygen. So you'll need at least 47 times as much air as propane for complete combustion

So there's the starting amount, so you can do the energy calculations to work out if you burn that much together what the temperature is if you burned with no excess air.

Then it'd just be a matter of comparing the temperature of that with their 1400k exit temperature. It'd be tricky though if they wanted you to calculate the heat lost through the chamber walls with that temperature difference.

I guess you'll know when you hear back from the lecturer, we can but (badly) speculate. Depending how the second part went about the heat loss it sounds like it could be a mistake
 
During one of my exams, some crucial information was put in a text box at the bottom of the page. What many people didn't realise was that the last column had been cutoff and was printed on the reverse side of the sheet and there was no indication that this had happened (For anyone that doesn't know question don't spread across multiple pages in the exception of when there are diagrams and graphs). A quite a few people spent a lot of time (10 - 20 minutes of a 2 hour exam) trying to derive that information.

The only reason I knew it was on the reverse side was because when the exam started I went through the whole exam paper deciding which questions i was going to do and happened to spot it.
 
Chuk_Chuk said:
The only reason I knew it was on the reverse side was because when the exam started I went through the whole exam paper deciding which questions i was going to do and happened to spot it.
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I remember always being told to do this by my teachers at school/college. It's meant to help you get in the right mindset or something. They even recommended we do the exam back to front, spending as much time as we could on the essay parts of exams to score maximum points.
 
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