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Charge to mass

  1. Mar 15, 2007 #1
    Well, i'm a little unsure about where to post this thread... but. my problem is that i've been told to figure out J.J. Thomson's experiment on the charge to mass ratio of an electron. I've been specifically told i should find a simple explanation so that i can later try to describe and explain the experiment to the class. All i've found is websites for a level of understanding that i definitely haven't reached. I was wondering if anybody knew some good, simple explanations for this experiment, or could explain it themselves.



    Relevant informations may be found here



    from what i understand, based on the force of the magnetic field, the kinetic energy provided to the cathode and the bend in the ray, one can determine mass of the electrons. What i don't understand is how this works if we don't really know the number of electrons and what the different equations do, exactly...

    thanks!
    BTGS
     
  2. jcsd
  3. Mar 15, 2007 #2
    Ok. What happens is, that we accelerate the electron through an electric potential to reach a certain kinetic energy.

    Because of this, it attains a certain velocity that we can calculate. Lets assume that the velocity that the electron gains is 'v'.

    The force applied by the magnetic field depends on this value of 'v' which can be split up in to the horizontal and vertical directions (speed in the horizontal and speed in the vertical direction).

    Another property of the magnetic force is that it is always perpendicular (or at right angles) to the direction of velocity. Therefore, it only deflects the electron in the vertical direction (either up or down depending on the direction of the magnetic field).

    When the electron finally leaves this setup, it hits the screen and we can calculate the deflection it underwent while in the magnetic field.

    Depending on the magnitude of the deflection, we can calculate the charge to mass ratio of the electron.

    A simple analogy is when you throw a ball in the horizontal direction, it falls some distance away. The ball experiences a gravitational force in the gravitational field of the earth. In a similar way, the electron experiences a deflection in the magnetic field.
     
  4. Mar 16, 2007 #3
    Got it!!! thank you very much! :-)
     
    Last edited: Mar 16, 2007
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