I am a young student checking pupils homeworks. I've got a case - 2 pupils definetely cooperated when doing homework. Do you have tips to determine who actually solved it and who copied? Got some ideas myself (like maybe one who copied had a better handwriting) but it is better to ask.
Here is my anecdotal experience for dealing with copiers and plagiarizers. At the end of the day, everyone should be penalized. It's just a matter of sorting out who did what, and making sure they're aware that further offending can carry severe consequences.
If discussing problems in groups is okay and encouraged (it should be!), don't forget to reinforce this. Just stress that the solutions themselves must be individual work.
- The weaker piece of homework (i.e. less complete explanations and working, or missing parts) is likely to be written by the student who had no contribution (or a lesser contribution) to the solution.
- It's usually rather obvious that some copying has gone on, especially if they all make the same mistakes and lay out their working the same. Consider rounding up all the students and talking to them together to find out the full story. They've already been caught red-handed, so it's in their best interests to be honest with you!
- It's important to find out exactly what happened, because there are cases where someone has copied work without the other's knowledge. In this case, it's not fair to punish both parties.
- If the student doing the copying has accidentally written their friend's name or student number on the sheet rather than their own, that tends to be a dead give-away that it's a downright facsimile of other work (that has actually happened).
Although I do not teach physics I do have a solution that generally works quite well.
The main answer is: It does not matter who copied from whom - fail both.
If one student allows another to copy, then both fail. I enforce this quite strictly and there are some students (who do not pay attention to the warnings I give at the start of the semester) who do it, but they never make the same mistake twice.
As Moriarty said, it is possible that Student B copied from Student A without Student A knowing about it. I solve this problem but calling them both in front of me (private from everyone else) and tell them they have a choice:
Choice A: Both admit that the copying took place with consent of both, and both fail that homework/assignment/test/whatever is being assessed.
Choice B: Student A says that Student B stole Student A's work, and Student A gets off with a warning to be more careful (but no punishment) and Student B fails the module immediately without the opportunity to recover (Student B must re-take the module from the beginning).
I have dealt with many cases this way and only three cases where the students ended up in Choice B. In this case, the offending students admitted that they stole work.
You should never support the student who allows another student to copy from them. That behavior is simply unacceptable and that needs to be made quite clear to everyone.
I am surprised that no one mentioned this solution yet, but it shouldn't be too difficult to find out by interrogating them: question them both on how they solved the exercises. "So, tell me, which formula are you applying in this line? Why are the hypotheses satisfied? Show me the missing steps."
I have also got in that situation with a couple of students that I was lecturing. I decided to fail both and sent an email, to both of them, saying that they were going to be penalized even with the disciplinary committee of the University to expelled them both. In a couple of hours the person that was guilty confesses his participation in this situation. I only decided to fail that person from the exam and leave the other only with a disciplinary warning.
Long story short: Here you have only two ways, either you fail both of them which is simpler; or just look for the guilty person (which I usually do and I always discover the sinner")
First of all, you should remember that you are a teacher, and that you are responsible for your students' intellectual growth.
If you decide to randomly accuse one of the two students of "copying", and if you are wrong, then think about the effects that this accusation could have. Aside from the hurt feelings, that student could lose confidence, since your accusation shows that you think that this student is worse than the student who copied. There is no predicting how your actions could change the students' lives. Honestly, from your posts, it seems like you are almost trying to make this a bigger deal than it actually is.
The standard way to deal with this situation is to have a meeting with both students, and to put both students through the same disciplinary action, unless one student confesses that he copied from the other student, in which case the punishment level could be adjusted.
But the thing that shocked me the most from this post is the fact that you apparently seem comfortable with randomly accusing your own students. You should remember that you have HUGE effects on their lives, and make sure that you do not abuse it. Showing mistrust is one thing that you should never, never do to your students.