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Homework Help: ID a particle

  1. Mar 23, 2004 #1
    why u should not skip steps

    In a high-energy physics experiment, a subnuclear particle moves in a circular arc of 0.27-m radius perpendicular to a magnetic field of 2.7*10^-2 T. The kinetic energy of the particle is determined to be 4.1*10^16 J. Identify the particle from its mass. The masses of the electron, pion, and proton are 9.1*10^-31 kg, 2.5*10^-28 kg, and 1.67*10^-27 kg, respectively. Assume that the particle is known to have a positive charge equal to the magnitude of the electron charge.

    Okay, so I know r= 0.27, B= 2.7*10^-2, q= 1.6*10^-19 C, and KE= 4.1*10^-16 J. Now I need to find mass to id the particle.
    [tex] KE = \frac{mv^2}{2} [/tex], so [tex] v = \sqrt{2m * (KE)} [/tex].
    [tex] r = \frac{mv}{qB} [/tex], and then I subbed in the v from the kinetic energy equation.
    [tex] r = \frac{m\sqrt{2m * (KE)}}{qB} [/tex]
    [tex] \frac{rqB}{\sqrt{2KE}} = m^{3/2} [/tex]
    m = 1.49*10^-9.

    I really screwed this up didnt I? Was it a conceptual or a math error?
    Last edited: Mar 23, 2004
  2. jcsd
  3. Mar 23, 2004 #2

    So it was a dumb math error. Perfect example of why you shouldnt skip steps like I do.

    [tex] KE = \frac{mv^2}{2} \rightarrow 2KE = mv^2 \rightarrow \frac{2KE}{m} = v^2 \rightarrow v = \sqrt{\frac{2KE}{m}} [/tex].

    Remember guys, show all your work!
  4. Mar 23, 2004 #3
    By the way, the particle is a proton.
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