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Magnetism and kinetic energy

  1. Oct 11, 2015 #1
    1. The problem statement, all variables and given/known data
    Suppose a subatomic particle of mass m kg has kinetic energy K.E. a nJ and is moving southward toward a vertical wall. When the particle is distance d cm from the wall a magnetic field is turned on: it has magnitude b T and points upward. The particle turns westward so it just skims along the wall. Find q, the charge (magnitude and sign) of the particle, in C.

    2. Relevant equations
    3. The attempt at a solution
    I attempt to use 1/2 mv^2 = K.E. to find initial velocity first, but the direction of velocity is changed due to magnetic field. the magnitude of the velocity will change also? and I do not know how to find out the force acted on particle due to magnitude field either. Does K.E.=∫ F*dx from xi=0 to xf=d to find out F works? is this F the same with force due to magnitude field? or actually I need to consider about gravity?
    thank you for your help!
  2. jcsd
  3. Oct 12, 2015 #2


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    Homework Helper

    What path does the charged particle follow due to the presence of the magnetic field?
  4. Oct 12, 2015 #3


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    Staff: Mentor

    A magnetic field produces a force at right angles to the direction of motion of a moving charge. The particulars are determined by a vector cross product of the velocity and field vectors, or through the application of the right- or left-hand rule.

    As is the case for circular motion, which you must have studied previously, the acceleration is at right angles to the motion and so changes the direction of the velocity vector but not its magnitude (speed). So, think about centripetal force and the types of calculations that pertain to it.
  5. Oct 12, 2015 #4
    thank you for the explanation. So magnetic field will not change the magnitude of the velocity generally? or this question just a particular case since acceleration is at right angles to the motion?
  6. Oct 12, 2015 #5


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    Staff: Mentor

    In general a magnetic field won't change the speed of a charged particle moving through it. Any force that a moving charge "feels" due to its interaction with a magnetic field is always perpendicular to its direction of travel. It can change it's direction of travel, but not its speed.

    To investigate further it would be helpful to know about the properties of the vector cross product. That's the mathematical approach. You can also investigate the "right hand rule" or "left hand rule" practical embodiments of the cross product properties that are used in practice to determine the direction of the effects.
  7. Oct 12, 2015 #6
    thanks again for the explanation.
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