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I need help understanding conceptual question for particle tracks in a chamber

  1. Oct 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Figure 21.13a shows the bubble-chamber tracks resulting from an event that begins at point A.
    At this point a gamma ray (emitted by certain radioactive substances), traveling in from the left,
    spontaneously transforms into two charged particles. There is no track from the gamma ray
    itself. These particles move away from point A, producing the two spiral tracks. A third charged
    particle is knocked out of a hydrogen atom and moves forward, producing the long track with
    the slight upward curvature. Each of the three particles has the same mass and carries a charge
    of the same magnitude. A uniform magnetic field is directed out of the paper toward you. What
    is the sign ( +or - ) of the charge carried by each particle?
    According to the book the answer is B but i don't understand why,and how to apply the hand rule
    2. Relevant equations
    no equations

    3. The attempt at a solution
    i know i have to use the first right hand rule,and i know the thumb represents the velocity,the rest of the fingers the magnetic field and the force is perpendicular to the palm

    Attached Files:

  2. jcsd
  3. Oct 11, 2014 #2


    Staff: Mentor

    The spirals for particles 1 and 3 are both counterclockwise, while the spiral for particle 2 is clockwise. These facts suggest that the charges are particles 1 and 3 would have the same sign, and the charge on particle 2 would have the opposite sign. This rules out choices a and c.

    What does the right-hand rule say about things? That would help you figure out why b is the correct answer and d is not.
  4. Oct 11, 2014 #3
    putting it that way makes sense now thanks,but i still don't understand how we can use the hand rule to get to that answer,couldn't we have determined that b was the answer with just that reasoning that particle 2 is the only one going clockwise so that is why is negative
  5. Oct 12, 2014 #4


    User Avatar

    Staff: Mentor

    I don't think the right-hand rule involves "velocity" specifically.

    Remember, an electron travelling thataway equates to a current heading thisaway. ;)
    Last edited: Oct 12, 2014
  6. Oct 12, 2014 #5


    User Avatar
    Homework Helper

    Do you know the expression of he Lorentz force, the force exerted by a magnetic field on a moving particle (F=q vxB)?
    You know the direction of the initial velocity of all particles. The magnetic field is perpendicular to the paper and directed toward you. What is the direction of the force if the particle is positive, and what is the direction if the particle is negative?
  7. Oct 20, 2014 #6
    thanks guys it makes total sense now
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