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Homework Help: Electron or Proton gun (+/- charge) from magnetic field?

  1. May 23, 2012 #1
    1. The problem statement, all variables and given/known data

    The news just announced that aliens armed with either electron- or proton-guns have invaded the city. A quick glance reveals that there is indeed an alien on the street just below my top-floor, north-facing window. When the alien shoots his rifle vertically at the sky, the needle of the compass I keep on the window ledge points to the left. Which of the two kinds of gun is the alien using? (Explain your answer, if you like with the help of a sketch).

    2. Relevant equations

    3. The attempt at a solution

    I have this sketched on paper, but I'll just describe it because I can't attach.

    I have a square notated N E S W.
    In the middle, I have a circle with a dot to signify that the gun is pointing up.
    According to the right hand rule, my magnetic field is rotating counterclockwise.

    A compass always points in the direction of a magnetic field.
    And if I'm correct, doesn't positive electric charge produce a magnetic field which is orientated at a right angle to charges direction of motion.

    So it should be a protongun?
  2. jcsd
  3. May 23, 2012 #2


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    Homework Helper
    Gold Member

    Yes, everything is right so far, *assuming* that the charge moving up is positive. Keep in mind though, at this point, the charge polarity is just an assumption. But you have to start somewhere; you can correct that assumption later if necessary.
    Yes, it points in the same direction that the B field is pointing. :approve:
    Ummm, yes, that is technically correct. :uhh: But it's also correct for a negative charge too. You can use the right hand rule to determine the direction of a magnetic field caused by a moving, positive charge. You'll need to reverse the direction of the field if the charge is negative (all else being the same). But regardless of whether the charge is positive or negative, the B field is always perpendicular to the moving charge's direction of motion.
    You might want to think about this some more.

    Don't forget to draw the window ledge on your diagram. The window ledge is not at the same location as the center dot indicating the gun pointing up: It's somewhere below the dot (on the paper). Orient the figure such that the window ledge is between you and the center dot. You know that the compass needle points toward the left. Is your original positive charge assumption correct or does the assumption need to be reversed?
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