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chem conceptual questions

Discussion in 'MCAT Discussions' started by sendwich, Apr 27, 2004.

  1. sendwich

    sendwich you rock!
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    it's been a LOOOONG WHILE since i last went over this stuff and was wondering if anyone can help out de-cloud some of this beginning concentual stuff.

    1. force holding nucleus together: strong nuclear force. this force must oppose the repulsive electrostatic force (repelling the nucleus apart) since most nucleus have (+)charged protons packed together, strongly repelling one another. so, it's stronger than the electromagnetic (+) forces?
    so electromagnetic=electrostatic?

    2. B- decay and electron capture (both forms of Beta decay). both convert protons to neutrons except B- decay release e+ (positive electrons? how does this work?), versus EC which takes in e- (normal negative elctrons?

    thanks!
     
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  3. Nutmeg

    Nutmeg Morir es vivir... morir es vivir...
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    Yes, EM = ES, for the most part--electrostatic applies to stationary electromagnetic forces (in contrast to electrical circuits, which are electromagentic, but aren't static). Electrostatic is stationary electronic forces, hence static electricity is electricity that isn't conducted and evenly distributed. When Maxwell unified electricity and magnetism, the combined field became electromagnetism. In terms of forces, there is a single electromegnetic (EM) force, from which all aspects of magnetism and electricity can be derived. The strong nuclear force is stronger but has a shorter range than EM, just as gravity is weaker than either but has a greater range than either.

    e- is a normal electron, while e+ is called a positron. A positron in an antimatter particle, meaning it is the complete inverse of an electron. If a particle and an antiparticle of the same mass hit one another, they are both annihilated, and the lost kinetic energy is carried away by two photons. So a positron can hit an electron and the two will annihilate eachother. Now, since a proton can become a neutron either through absorption of an electron, or through loss of a positron. PET scans are positron emission tomography, where an unstable isotope decays to release positrons, and the source of decay is detected to map the area. Important lesson--matter and antimatter exist, and a positron is antimatter.
     
  4. Shrike

    Shrike Lanius examinatianus
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    Just a small thing to add: the strong force is, as you'd suspect, strong, stronger than electromagnetic. It is also NOT an inverse square law force (clear if you think about it: if it weren't, it would dominate eloctromagnetic and gravitational forces at macroscopic distances; as it is, we never have to worry about it in most calculations).
     
  5. SitraAchra

    SitraAchra Attending Anesthesiologist
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    sorry to be nit picky here, but don't you mean it IS an inverse square law? The closer the particles, the stronger the force? Sorry - if I'm wrong feel free to blast me.
     
  6. liverotcod

    liverotcod Lieutenant Crunch
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    No blasting, but it really doesn't work that way:

    "Unlike the other fundamental forces, the strong interaction also acts on the strong exchange particles themselves, since gluons carry color charge. This leads to a very limited range of the strong interaction (not much farther than the hadron's radius) even though the gluon does not have mass. It also has the strange effect that the force gets stronger as the distance between the quarks increases. This effect prevents free quarks from being observed. As the distance between two quarks increases, the amount of energy in the force between them increases. If the force becomes strong enough, there is enough energy to create new quarks. This is the reason that one only sees quarks in pairs or triplets and never individually. The textbook allegory is that of a rubber band. When the rubber band is stretched far enough, the band breaks and you have two new rubber bands. Similar with quarks: separate the quark pair far enough, and two new quarks will pop up."
    From here.
     
  7. Shrike

    Shrike Lanius examinatianus
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    Liverotcod's answer is a great explanation of the science behind my quick response. But more importantly, what does this mean for taking a test?

    Many, many MCAT physics problems are best solved just by looking at proportions; for example, here, does the strong force vary as the inverse of the distance, or the inverse of the square of the distance, or what? We know that, at some distances (big ones), electromagnetic force dominates strong. At another distance, strong force dominates electromagnetic. Any time an MCAT physics problem asks you how the magnitude of some thing (force, acceleration, whatever) changes, but doesn't give you exact numbers in the answer choices, you should think proportions. Here, if the forces were each inverse square laws they'd each equal some (different) constant divided by distance squared (you know that's how the electromagnetic force varies). But at least one constant would then have to change, for the forces to swap which one is strongest. No good -- constants are constant. So the strong force (the one we didn't know about for sure) must not be an inverse square force.

    That, by the way, is more than you will ever need about the strong force for the MCAT.

    Didn't mean to "blast" (your word), I just can't resist an opportunity to talk about how to do problems.
     

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