Funny C&G exam question

[richardindorset]

C&G seem to think an electric fire heats a room by conduction? Surely it only heats by convection and radiation? They ask for three ways an electric fire heats a room.

Electric underfloor heating would certainly use conduction but an electric fire?

Maybe there is conduction after the radiated heat has warmed up the floor but just seems an odd answer to me!

I was going to answer: Convection, radiation and physcologically (the anger you feel at the cost of electricity...)

 

[JohnW2]

C&G seem to think an electric fire heats a room by conduction? Surely it only heats by convection and radiation? They ask for three ways an electric fire heats a room. .... Electric underfloor heating would certainly use conduction but an electric fire? .... Maybe there is conduction after the radiated heat has warmed up the floor but just seems an odd answer to me!

I suppose it depends a little on what they mean by 'electric fire' - but, more generally, I suppose that the body/case/whatever of any sort of heating appliance (using any energy source) will warm up to at least some extent, and hence do a bit of heating of adjacent air by conduction. However, I agree that it's basically a silly question (since it's pure Physics, and nothing specifically related to anything electrical).

Do I take it that this was not an MCQ question (because, if it were, you would know what options for answers they had in mind) ?

 

[richardindorset]

I suppose it depends a little on what they mean by 'electric fire' - but, more generally, I suppose that the body/case/whatever of any sort of heating appliance (using any energy source) will warm up to at least some extent, and hence do a bit of heating of adjacent air by conduction. However, I agree that it's basically a silly question (since it's pure Physics, and nothing specifically related to anything electrical).

Do I take it that this was not an MCQ question (because, if it were, you would know what options for answers they had in mind) ?

Yeah this course has just one written exam for the whole course, no MCQ at all, sadly.

Even the course notes don't mention conduction... For good reason too IMHO.

 

[ericmark]

So the energy is conduction along the feed wire, then convection and radiation once it reaches the element? Seems someone has a lack of command of the English language? Like when I say to wife, "You don't want a cup of coffee." to which she is now wise to, and answers "I would love a cup of coffee" carefully avoiding using yes or no.

I had to do a health and safety exam, with tick the box answers, so if you find a drill with a damaged cord would you?
A) Repair it your self.
B) Return it to stores.
Etc

But I am the electrician, so for me A) is the right answer, but that's not the box I ticked.

I remember in collage being given quadratic equations to work out. 19 were easy, but the last one took me ages, seems one word was missed out of the question, it should have said find real answers and someone missed out the word real, can you imagine that?

 

[JohnW2]

Yeah this course has just one written exam for the whole course, no MCQ at all, sadly.

Designing 'good' exams (or questionnaires/surveys) is far from easy (over the years, I've had a fair bit of experiences of trying!) but, no matter how much thought and care goes into the design,the questions really ought to be 'field tested' before being used in anger.

In general, as a 'candidate, MCQs have always driven me mad (since so few things are 'black and white'), and I'm not sure whether they would help in this situation - if 'conduction" had appeared as one of the offered answers, it would not have been clear as to whether the examiners regarded 'yes' or 'no' as the 'correct' answer. I suspect that most people would probably answer 'no' (since conduction is, at most, going to be a pretty trivial part of the 'heating') - but that might not be the answer which the examiners regarded as 'correct'. In that situation, field testing would hopefully have indicated that most candidates were likely to answer 'no', regardless of the views of the examiners

 

[EFLImpudence]

Surely the parts of an electric fire are not heated by conduction - i.e. heat being conducted from the element to other parts.

Would these other parts not just be heated by convection and radiation as well?

 

[richardindorset]

Surely the parts of an electric fire are not heated by conduction - i.e. heat being conducted from the element to other parts.

Would these other parts not just be heated by convection and radiation as well?

I'd go so far as to say that any conduction of heat in an electric fire was a fault, leading to parts that could get hot enough to start a fire or burn people... but hey what do i know?

Maybe the resultant house fire, is the third method of heating the room...

 

[JohnW2]

Surely the parts of an electric fire are not heated by conduction - i.e. heat being conducted from the element to other parts. ... Would these other parts not just be heated by convection and radiation as well?

Sure, but if one looks at the heating appliance as a whole, after #parts of it' have been heated (as you say, almost entirely by convection and radiation), those heated parts can then (slightly) heat the adjacent air by conduction. However, as said, that is such a tiny part of the total 'heating' that it's a little silly to suggest that conduction is an appreciable part of the mechansim.

The above is far more dramatic with a wet CH radiator. The space heating is, of course, almost all by convection and radiation, but none of that could/would happen without the outer surface of the radiator first being heated (by conduction) from the water within the radiator.

 

[JohnW2]

I'd go so far as to say that any conduction of heat in an electric fire was a fault,

... I'm not sure about that. In the real world, some (but usually very little) conduction is inevitable, not the least down the electrical conductors feeding the appliance!

.... leading to parts that could get hot enough to start a fire or burn people... but hey what do i know?

The design would hopefully be such as to avoid that eventuality!.

As I've just written in relation to wet CH radiators, in the case of, say, an oil-filled electric heater, although the space heating would essentially be by convection and radiation, that could not happen if there was not first transfer of heat (by conduction) from the oil to the outer surface of the heater.

Maybe the resultant house fire, is the third method of heating the room...