An interesting GCSE question: Why are electric lamps connected in parallel in house wiring?

[TensorCalculus]

Homework Statement

Why are electric lamps in a house lighting circuit normally connected in parallel?
A. The current in every circuit must be the same
B. The lamps are always switched on and off at the same time
C. The voltage across each lamp must be the mains voltage
D. When one of the lamps blows, all the others go out

Relevant Equations

N/A

TL;DR Summary: Asking about opinions on a GCSE question which has an odd answer

So, back a couple months ago, our class in school were taking an end-of-topic physics test, after doing a term or so on electricity.

Now, we are a class of Year 9s (American Grade 8s, 13-14 year-olds). All of the questions in our paper were taken from past GCSE papers, so there was no debating what the correct answer was - definitely not on multiple choice questions.

However one question on the test, almost everyone in the class got wrong. Including all of us "nerds".

The question, word for word, goes like this:

Why are electric lamps in a house lighting circuit normally connected in parallel?
A. The current in every circuit must be the same
B. The lamps are always switched on and off at the same time
C. The voltage across each lamp must be the mains voltage
D. When one of the lamps blows, all the others go out

As you can imagine the majority of the class put C. The correct answer, according to the GCSE markscheme (I'm not sure which paper or exam board this question was from) was D. Our teacher gave the reasoning that it's because if the lamps were in series this would happen, so putting the circuit in parallel would prevent D. It was only one mark, on a test that did not really matter, so we all should have forgotten about it, but our class still debates about this question today. Especially since this question was on a paper for a national standardised exam.

Is it just that we have not been exposed to enough GCSE style questions yet? I like to think I am pretty good at physics for a 13 year old but I am still a beginner either way. Maybe we have just not seen enough physics yet to understand. Or is this really just an abhorrently worded question (and if so, it's interesting that they let this be on the actual GCSE)? I'm pretty interested to see people's opinions on this...

 

[berkeman]

Maybe I'm misreading the question, but as you say all the bulbs go out when one blows and they are connected in series. You wrote:

Why are electric lamps in a house lighting circuit normally connected in parallel?
<<snip>>
D. When one of the lamps blows, all the others go out

Are you sure you copied the question correctly? Maybe it really said:

Why are electric lamps in a house lighting circuit normally not connected in series?

 

[TensorCalculus]

Nope, that is the exact wording of the question. Which is why our entire class is all so confused by our teacher's reasoning:

 

[berkeman]

Hmm. Something is wrong there, IMO. Maybe the last answer should have said "If they were connected in series, then when one of the lamps..." Weird.

 

[TensorCalculus]

Hmm. Something is wrong there, IMO. Maybe the last answer should have said "If they were connected in series, then when one of the lamps..." Weird.

Agreed.
But then the question is: how was this on an actual GCSE paper?

 

[Baluncore]

The mains supply voltage is standardised, so you can use as much current as you need, to a point. The critical filament lamp design parameter is the operating voltage.

If lamps were wired in series, they would need to be matched in power, and the supply voltage would need to be variable.

For standard lamps in series, the voltages would be too high for domestic insulation to handle.

 

[TensorCalculus]

exactly!!! Our teacher said C. was wrong because lamps don't run on mains voltage which is OK fine, but it's the best option of the four because in series it is even worse...

 

[phinds]

exactly!!! Our teacher said C. was wrong because lamps don't run on mains voltage which is OK fine, but it's the best option of the four because in series it is even worse...

HUH? House lamps DO run on mains voltage, which in the USA is 120 volts. You can GET 240 for large appliances but that's not the normal mains voltage.

So for me, the question and answer C go together perfectly. "D" is just silly for parallel lamps.

 

[TensorCalculus]

Our teacher said that they put transformers in there specifically to make sure household lamps don't run on the mains voltage... what
And according to Google, household lamps here in the UK also run on mains voltage. HUH? indeed

What the people writing the GCSEs were thinking when writing this question, I have no idea I guess.

 

[phinds]

Our teacher said that they put transformers in there specifically to make sure household lamps don't run on the mains voltage... what

You need to find a new teacher.

 

[TensorCalculus]

Usually she is pretty great at teaching
School year just finished though so we have a new teacher next year I guess

 

[Steve4Physics]

exactly!!! Our teacher said C. was wrong because lamps don't run on mains voltage which is OK fine, but it's the best option of the four because in series it is even worse...

The lamps in my house all run on mains voltage!

With the question as stated, the only reasonable answer is C.

There are occasionally mistakes in mark-schemes. (Or even in the questions themselves.) Mistakes are quickly spotted during the marking process and all markers (I have been one) are quickly informed.

In the case of multiple choice questions, marking is typically automated so any errors which are discovered can be corrected relatively easily.

However, corrections may not always make it to the publicly viewable mark scheme. I suspect that’s what happened here. (Plus some added confusion due to your teacher having what sounds like an off-day!)

 

[TensorCalculus]

The lamps in my house all run on mains voltage!

With the question as stated, the only reasonable answer is C.

There are occasionally mistakes in mark-schemes. (Or even in the questions themselves.) Mistakes are quickly spotted during the marking process and all markers (I have been one) are quickly informed.

In the case of multiple choice questions, marking is typically automated so any errors which are discovered can be corrected relatively easily.

However, corrections may not always make it to the publicly viewable mark scheme. I suspect that’s what happened here. (Plus some added confusion due to your teacher having what sounds like an off-day!)

Oh that makes so much more sense, so the GCSE people (hopefully) aren't writing questions with answers like these after all. I would have assumed it was all checked enough times that mistakes like these don't happen, interesting that they do. (I also never knew that the multiple choice marking is automated!) Maybe our teacher was just trying to make some sense out of the answer on the Markscheme, I have no idea.

 

[kuruman]

Maybe our teacher was just trying to make something sense out of the answer on the Markscheme, I have no idea.

In physics, one is either provably right or provably wrong. It appears that your teacher saw (D) as the "official" correct answer and, trusting the source instead of her own judgment, tried to sell it to the class by adding the conditional "if they were connected in series ..." This creates a semblance of correctness to justify (D) as the correct answer.

Nevertheless, since the majority of the class put down (C), a good teacher must feel obliged to provide arguments why (C) is incorrect and set the thinking of the majority straight. I suspect she didn't do that and allowed the majority to "learn" that their thinking was incorrect not because of some physical impossibility or logical inconsistency but because she declared another answer correct. In my book, this incident indicates a teacher who does not adequately address her students' needs.

 

[TensorCalculus]

In physics, one is either provably right or provably wrong. It appears that your teacher saw (D) as the "official" correct answer and, trusting the source instead of her own judgment, tried to sell it to the class by adding the conditional "if they were connected in series ..." This creates a semblance of correctness to justify (D) as the correct answer.

Nevertheless, since the majority of the class put down (C), a good teacher must feel obliged to provide arguments why (C) is incorrect and set the thinking of the majority straight. I suspect she didn't do that and allowed the majority to "learn" that their thinking was incorrect not because of some physical impossibility or logical inconsistency but because she declared another answer correct. In my book, this incident indicates a teacher who does not adequately address her students' needs.

She did try, by saying that actually we would not want the bulbs to run on mains power... but then this was just wrong, as @phinds helpfully highlighted. Either way, our class was not convinced since we were still pretty sure that C made more sense than D. In the end, none of our class was having it, and for months we still debated whether we were right or not: because we weren't ready to just except the "correct" answer that was declared without some reason as to why it was correct. She teaches very well, and makes physics very understandable to our whole class... but I have to agree with you that on this occasion it was a case of her not doing what she should have - which is either change the answer to C or provide a satisfying (and correct!) reason as to why D is better than C (which, she did not do).