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Homework Help: Rather simple question for not a physics major

  1. Mar 15, 2004 #1
    Ok, for some this might seem like a question for the k - 12, but in fact I am taking a college level class. My teacher, Whom I dislike and consider an ass (sorry for the language) does not use a book and anal on us learning things with tiny little information packets. Its rather annoying but doesn't explain much. (more work than information).

    Can anyone explain to me if a solid and a liquid can have the same mass and the particles of both can have the same mass, can the substances have the same number of particles or anything about the volumes of either substance compare?

    Sig
     
  2. jcsd
  3. Mar 15, 2004 #2
    In general, solids are more dense than liquids, so if a solid and liquid have the same mass, the liquid will usually occupy more volume. That being said, there are some very notable exceptions, such as [itex]\textrm{H}_2\textrm{O}[/itex], which has a solid state that occupies more volume than its liquid state.

    It is by no means a rule that solids have more mass than liquids.

    cookiemonster
     
  4. Mar 15, 2004 #3
    so solids substances are going to have more particles than liquids or liquid substances are going to have more particles than solids?



    (i'm sorry for this, but Physics is not an easy subject for me)
     
  5. Mar 15, 2004 #4
    Ah, well, you have to specify that question more.

    A solid can have more or fewer molecules. For example, you compare the number of molecules in a grain of salt to the number of molecules in the ocean. Or compare the number of molecules in a cup of water to the number of molecules of the ice in Antarctica.

    Are you asking given the same mass? Given the same volume? What's given?

    cookiemonster
     
  6. Mar 15, 2004 #5
    Actually, nothing is given. (like I said, my teacher is an ass from my view point anyways) The question starts off with: We have a liquid sample of substance A and a solid sample of substance B. There is the same mass in each sample and the particles of both substances have the same mass.
     
  7. Mar 15, 2004 #6
    Ah, okay. We're assuming the same mass and the same atomic mass, then.

    In that case, the number of particles is the same.

    [tex]\frac{\textrm{given mass}}{1}\cdot\frac{\textrm{\# of molecules}}{\textrm{amount of mass}} = \textrm{\# of molecules}[/tex]

    You'll notice that in both substances A and B, the given mass and the # of molecules/mass are identical, so the # of molecules in each is identical.

    cookiemonster
     
  8. Mar 15, 2004 #7
    So the samples in both substances have the same number of particles then?
     
  9. Mar 15, 2004 #8
    To add to my above post, this does not say anything about the number of atoms. If substance A requires 5 atoms in each molecule to get to its molecular mass while substance B requires only 2 atoms in a molecule to add up to the same mass as A's 5-atom molecules, then substance A will have more atoms. The number of molecules in both substance A and B will be the same, but the number of atoms will not be.

    cookiemonster
     
  10. Mar 15, 2004 #9
    are atoms and particles the same thing?

    ok, so just because the particles of the substances have the same mass, it doesn't mean they necessarily have the same number of particles then? (I think I get what you mean, but I just want to confirm it)
     
  11. Mar 15, 2004 #10
    All right, I'm starting to clutter this up too much. Let me step back a bit just to get this clear.

    I don't think you're looking for the word "particle." A particle is usually considered to be a proton, neutron, or electron. So let's start working with atoms (i.e. a nucleus made up of protons and neutrons that is surrounded by electrons) and with molecules (something made from atoms that are attached by intramolecular bonds) and with substances (something made from molecules that are attached by intermolecular bonds).

    If you're looking for molecules, then the equation I used is the one you're looking for.

    If you're looking for atoms, then we have to expand it a bit, in particular to:

    [tex]\frac{\textrm{given mass}}{1}\cdot\frac{\textrm{\# of molecules}}{\textrm{amount of mass}}\cdot\frac{\textrm{\# of atoms}}{1\textrm{ molecule}} = \textrm{\# of atoms}[/tex]

    The first two fractions in that equation are the same for both. The problem doesn't tell us about the third fraction. As such, we don't know about the number of atoms.

    So all we can say is that the number of molecules is identical.

    Does that clear it up?

    cookiemonster
     
  12. Mar 15, 2004 #11
    I think it clears it up, but i'm not a physic's major so I don't really understand this stuff.
     
  13. Mar 15, 2004 #12
    If it's still giving you trouble, I can explain it in more (and more, and more...) detail. Which part's still giving you trouble?

    cookiemonster
     
  14. Mar 15, 2004 #13
    The entire part of the question is this:

    We have a liquid sample of substance A and a solid sample of substance B. There is the same mass in each sample and the particles of both substances have the same mass.

    a.) Do these two samples have the same number of particles? explain fully.
    b.) Do we know anything about how the volumes of these two samples compare? explain fully.

    The reason I left A and B out is because I was trying to understand this without giving the full question, but apparently I am missing something because everything seems the same now. Particles are not atoms, equations I've never seen (but make sense to me). Its all very confusing to me.
     
  15. Mar 15, 2004 #14
    Grr.. I don't think I like your professor either!

    All right, I think he's using the word "particle" and meaning to use "molecule."

    Equations are supposed to make sense! It's more important that an equation make sense than it be seen a lot.

    For (b), you can't say a whole lot. The density (the amount of mass in a certain amount of volume) is what's important there, and the question doesn't specify density.

    cookiemonster
     
  16. Mar 15, 2004 #15
    So instead of using "particle" I should use "Molecule"? So both substances can have the same mass and the "Molecules" of both can have the same mass but that doesn't mean they have the exact same number of "molecules"?

    The Volume is dependant on density?
     
  17. Mar 15, 2004 #16
    You made a little typo for the last word. It does mean they have the same number of molecules, but not the same number of atoms (remember that molecules are made of atoms).

    I would use "molecule" instead of "particle." All "particle" is doing is causing confusion (as I'm sure you've noticed).

    The volume is dependent on density. Note:

    D = m/V, so V = m/D. The mass for both is the same, but we're not sure about the density. Because we don't know the densities of each, we can't say anything about the volumes.

    cookiemonster
     
  18. Mar 15, 2004 #17
    alright, I think I have it. They can both have the same mass, the molecules can have the same mass, so that means they both have the same mass?
     
  19. Mar 15, 2004 #18
    Heh, you just ran in a little circle there (mass = mass = mass), so I think you're going to need to clarify what you mean before I comment on it.

    cookiemonster
     
  20. Mar 15, 2004 #19
    It made sense when I wrote it. But I think what I mean was that the two substances could have the same mass and the molecules of both substances could have the same mass, so that the two substances would have the same number molecules? (I hope that clarify's it because I have class in 8 hours :(
     
  21. Mar 16, 2004 #20
    There we go. That's what I thought you meant, but I just wanted to make sure.

    Looks like you got it down.

    cookiemonster
     
  22. Mar 16, 2004 #21
    alright, awsome. Thanks, I appreciate the help. I'll be back on later to bug more people about this stuff :)

    -Sig
     
  23. Apr 7, 2004 #22
    help involving photoelectric cells and voltage stopage

    I hope you don't hate me even tho this is a grade 12 physic 30 question.
    here is my question :

    In a photoelectric cell, 1.5 V is needed to reduce the current folow to zero. When the voltage is turned off, the maximum speed of the photoelectrons is _____

    i have all the formula's need however everytime i try the question i ether get a number larger then the speed of light or a number so low it isn't fesable

    formulas i have used Ekmax= q*Vstop

    getting the answer from that i put it into Ek=1/2mv^2

    i am not sure where i am going wrong so somone plz help
     
  24. Apr 8, 2004 #23
    EDIT: oops.. i didnt see a second page :eek: lol, oh well

    Well.. I think what cookiemonster is getting at is that particle is generally used to identify something sub atomic (that is, something which makes up an atom). Personally, I remember highschool teachers using the word 'particle' when really they should have been reffering to 'molecules'.

    Just by nature of the question it is obvious that your prof is intending to say molecule. But lets not get caught up on symantics (because often in education, what is right is only what the prof thinks is right, the truth isn't important ;)).

    If both samples were of the SAME substance, we could say that it is very likely that the liquid is going to take more space (have a larger volume)... except ofcourse if the substance was water.

    However, we have DIFFERENT samples. So, to visualize this, imagine a kilogram (or pound) of lead, and a kilogram of water. In that case you might think the volume can be inferred by the mass. But now consider a kilo of feathers and a kilo of water. Now things have reversed themselves. The actual material is needed to make any conclusions as to the volume.
     
    Last edited: Apr 8, 2004
  25. Apr 9, 2004 #24

    turin

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    Homework Helper

    SigInt,
    I think that you should answer the question as:
    a) Not necessarily. It depends on what you mean by "particle," you jackass. There could be ten particles of sand floating in 1 lb of water sitting next to a 1 lb block of sandstone, for instance.
     
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