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Homework Help: Magnetic fields (multiple choice)

  1. Nov 12, 2008 #1
    1. The problem statement, all variables and given/known data
    Problem 1
    If all of the particles shown in the figure are electrons, what is the direction of the magnetic field that produced the indicated deflection?


    Up (toward the top of the page)
    Down (toward the bottom of the page)
    Out of the plane of the paper
    Into the plane of the paper

    Problem 2
    Which particle has the largest mass?

    2. Relevant equations

    3. The attempt at a solution
    Problem 1
    >_> I am really not good at this right hand rule for forces thing. Anyways...I had my hand in a position that pointed into the the computer, but because the question is about electrons I was thinking the answer was
    Out of the plane...

    Problem 2
    I was thinking...the bigger the mass, the bigger radius it would experience, and thus the answer was D. I just want to confirm my logic with this question.
  2. jcsd
  3. Nov 12, 2008 #2
    Here's my right hand rule "gun" trick for the Lorentz force:
    The equation is qv x B, with v and B as vectors or components of vectors, that are perpendicular to each other (resolve either v or B into components if they are not perpendicular).

    Take your right hand and form a gun with your index finger, third finger, and thumb (they should all be mutually perpendicular). What do you do when you fire a gun? You point, and trigger, and the result is that your hand will be thrown up and back via recoil.

    How does this apply to the equation? The first vector in the equation corresponds to the first thing you do... point your gun (index finger) along the vector for v. Second thing you do? Fire. Move your gun so that your trigger finger (third finger) pointing perpendicularly through the trigger hole of your gun, is along B. The result? Your recoil, the result for the direction of the force, would be along your thumb.

    If you are dealing with an electron you must do ONE of the following (but not both): Either take your results from your right hand gun and then switch it "negative" to the direction 180 opposite OR do the entire process with a left-hand gun instead of a right-hand gun.

    Without looking at the exact problems (but your reasoning in your attempts at solutions)... it looks like you are reasoning correctly. With the right hand rule you reverse the direction result is the particle is a negative charge.... and bigger particles moving with a given speed require more force to turn.
  4. Nov 12, 2008 #3
    You are correct in both cases :)

    B is pointing into the screen, electrons just follow the simple right hand rule, and will be deflected in the direction that your fingers curl. Flip this 'normal' alignment to deal with positive charges.

    Your logic on the second is just fine. The equation for the radius of a charge particle in a B field (which you can see by rearranging the equation you provided) is r=mv/qB, so R is proportional to m. Bigger m -> bigger r!

    *AHHH, scooped!! hehe
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