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I'm taking my first physics exam in university this Saturday, and I'm currently working on a practice exam.

I got the answer 3.1MeV for a alpha-decay question, but the answer was 3100keV. I understand that they represent the same thing, but why is my way of answering discouraged? Are there actual reasons? Might I lose points on my exam for answering like I did here?

EDIT: 0.31MeV is not equal to 3100keV...

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closed as off-topic by ZeroTheHero, Kyle Kanos, Chemomechanics, John Rennie, Jon Custer Jan 18 at 14:18

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – ZeroTheHero, Chemomechanics, John Rennie
If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ I've added the homework-and-exercises tag. In the future, please use this tag on this type of question. $\endgroup$ – Ben Crowell Jan 17 at 15:54
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    $\begingroup$ "I understand that they represent the same thing," No, they are a factor of 10 different. You would certainly lose marks for making that mistake! $\endgroup$ – alephzero Jan 17 at 16:00
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    $\begingroup$ Then what the question is now? $\endgroup$ – Alchimista Jan 17 at 16:18
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    $\begingroup$ Fundamentally we can't answer questions about why your paper was marked as it was. In the absence of explicit instructions to answer in $\mathrm{keV}$ I would have accepted $3.1\,\mathrm{MeV}$. $\endgroup$ – dmckee Jan 18 at 2:31
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Within reason if they are both correct values either answer should be acceptable.
The idea of using prefixes is to avoid the use of very small or very large numerical values.
For example it is probably best not to quote a value as 3,100,000 eV or 0.0000031 TeV.

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In 3100 keV the number of significant figures is ambiguous; you could be claiming 2, 3 or 4. But 3.1 MeV is unambiguous: 2 sig figs! If you were claiming 4 sig figs, you'd write 3.100 MeV. That's why I prefer the larger unit!

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  • $\begingroup$ Someone didn't like this answer. It would be good if they explained the reason for their dislike; we might learn something! $\endgroup$ – Philip Wood Jan 17 at 18:42
  • $\begingroup$ My experience is that you will not get a reply. I have noticed that sometimes all answers are downvoted for no obvious (to me) reason. Personally I do not often down vote but when I do I like to give a reason. $\endgroup$ – Farcher Jan 17 at 19:32
  • $\begingroup$ You're right! Even an agreed code (e.g. B = contains bad Physics, A = doesn't answer question, H = unnecessarily hard to follow, Q = question shouldn't be answered) would be better than nothing. $\endgroup$ – Philip Wood Jan 17 at 19:46
  • $\begingroup$ @PhilipWood I didn't downvote, but "significant figures" are not a very serious method of showing accuracy, except in school work. "Using a large unit" doesn't help if you want to express things like $3100 \pm 60$ keV or $3100$ keV $\pm 10$%. $\endgroup$ – alephzero Jan 17 at 20:40
  • $\begingroup$ I agree that sig figs are a wholly inadequate way of showing uncertainty, but surely they have a place as a useful informal guide. $\endgroup$ – Philip Wood Jan 17 at 22:24

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