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TBR Contradiction regarding Magnetic Force

Discussion in 'MCAT Study Question Q&A' started by Majik, Jul 27, 2011.

  1. Majik

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    I encountered two contradicting answers in TBR for two different passages. One questions pertains to a a mass spectrometer w/ different charged and uncharged atoms being passed through the field. This question asks:

    (Q47) In the apparatus of experiment 1, which particle will be most deflected?
    Choices A, C, and D all have charge and mass. However, one choice (B) is ruled out because it has no charge. They explain, "Choice B should be eliminated immediately, because Ne is not deflected, given that it has no net charge."

    Here is the next question which can be answered by basic knowledge on E&M. The question says: (Q60) A straight current-carrying conductor has no net charge on it and generates its own magnetic field. When immersed in an external magnetic field, it is subjected to: The answers consisted of: a force due to its own magnetic field, an external magnetic field, both or neither at all (since it has no net charge). The correct answer here was that it experiences a force due to an external magnetic field.

    For this question, they responded that magnetic forces depend on moving charges, not on net charges. The current in a conductor is moving charges, so the current experiences a force from the external magnetic field. I'm confused with both of these explanations. I tried to reason this out myself, but I would like some confirmation if possible, preferably from someone whose very knowledgeable with this topic:

    Neon, a single atom has no net charge so it doesn't experience a magnetic force due to the magnetic field. In the later situation, the wire consists of a "sea of electrons" and though the wire itself is electrically neutral, each individual electron does have a net charge. Because these electrons are moving (the question states theres a current), these electrons / the current, experiences a magnetic force due to some external magnetic field. Does this sound about right?

    Thanks in advance!
     
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  3. rpdnathan

    7+ Year Member

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    It is because the first question deals with an electric field where as the second question deals with magnetic field. I maybe wrong though.
     
  4. MD Odyssey

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    Let me address the first question about the force on a neutral particle, like neon, due to an electric field. As the passage points out, the force on the neon is zero since it is uncharged. We get this explicitly from Coulomb's law:

    [​IMG]

    where we can use the definition of the electric field to show that the force on a charge q is just F = qE.

    This is somewhat misleading because in reality, the electron cloud of the neon atom is pulled in one direction while the proton is pulled in the other direction, which produces a small electric dipole. But, the net motion of the particle remains unchanged because there is no net force on the atom.

    The same argument would hold for magnetic fields as well,

    [​IMG]

    therefore the electron cloud and the nucleus both experience a force. However, since the two forces are equal and opposite, there is again no net force on the atom.

    Now, let's look at a current-carrying wire and, to be more precise, let's focus upon the actual charge carriers, which are the electrons (current is properly defined as the motion of positive charges). A piece of uncharged copper wire has a uniform electron density throughout it. Since these electrons have some amount of thermal energy, they are moving in all random directions, so if we place it in the presence of a magnetic field, the net force on the wire will be zero, since the force on all the randomly moving particles ultimately sums to zero. This is because there is no preferred direction of motion for the charge carriers.

    But, if we put a voltage across it, this applies an electric field down the wire and causes the entire electron sea to drift down the wire. This rate is very slow and is referred to as the drift velocity. Now, we have a huge quantity of charges which have a preferred direction for their motion and it is this velocity which is acted upon by the magnetic field. Only electrons are able to move in this fashion, since the copper atoms are bound in place, therefore the net force on the wire is proportional to the direction of the electron drift velocity.

    That help?
     
  5. Majik

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    I see what you're saying. So basically what this simplifies down to is that the electrons on the neon atom are physically bound to the atom, while the electrons bound to a metal such as copper are free to move along the wire. Because there's a net movement of copper electrons relative to the wire, these electrons would experience a magnetic force in the presence of an external field, while for neon atoms, these electrons aren't free to migrate off the neon atom but instead remain bound to the atom itself. I hope I have that right. Thanks for your thorough explanation.
     
  6. Majik

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    Both questions dealt with a magnetic field. In mass spec, anything that gets deflected is due to the the magnetic force rotating its direction. The neon atom wasn't deflected because it didn't experience a magnetic force though, while atoms that were charged did.
     
  7. MD Odyssey

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    Yup, that's pretty much it. Sounds like you have a pretty good handle on it.
     

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