Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Lorentz Force

  1. May 10, 2006 #1
    A diagram I attached (you can view the website) shows a uniform magnetic field of magnetic flux density B[Wb/m^2] parallel to the x-axis. On the xy-plane, an electron of electric charge -e[C] and mass m[kg] departed from the x-axis in the direction of [tex]\theta[/tex][rad] with a speed of v[m/s]. Then the electron carried out a spiral movement on a cylinder whose side contains the x-axis.

    This is the question: What is the magnitude of the Lorentz force acting on the electron?

    My idea: F=qvBcos[tex]\theta[/tex]
    so the magnitude of the force is:

    Am I correct?
    What would be the relationship of the mass on this question?

    This is the website: www.geocities.com/willydavidjr/emf.html
  2. jcsd
  3. May 10, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Almost.... it's [itex] sin(\theta) [/itex] (remember, it's a cross product)
    I am not sure what you mean here...The mass does not enter the calculation of the force. If you woulsd then calculate the *acceleration* then the mass would enter, following Newton's second law.
  4. May 11, 2006 #3
    I see. I was wrong with the cosine instead of sine. I found out also that if we will calculate the radius of the circle, we will need the newtons law of motion together with the formula of magnetic force.

    This is the last question: Find he time between the electron leaving its starting point and returning to the x-axis again. Let phi denote the circular constant.
  5. May 12, 2006 #4
    Be sure that you keep in mind the sings here. You are using vectors !!! As has been stated before the cosine should be a sine.

    To get the circular motion : do you know how to acquire the equation for the orbit (the path) if you know the force ? Since, you already know what this orbit is, i suggest you look at the formula's that caracterize a circular motion. Do you know these ?

  6. May 13, 2006 #5
    I don't have any idea about the orbit but I know the magnetic force. Can you give me details on how to get this one?Thank you!
  7. May 13, 2006 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The motion is a sum of a motion at constant velocity in the same direction as the B field plus a circular motion at constant speed in the plane perpendicular to the B field.

    So you must decompose the initial velocity in a component parallel to the B field, v_par and a component perpendicular, v_perp. For the perpendicular component just apply [itex] m {v_{perp}^2 \over r} = |q| v B sin \theta = |q| v_{perp} B [/itex] since "v sin(theta)" is the component of the velocity perpendicular to the B field (at the condition that theta is the angle between the B field and the velocity vector). Using this you can find the radius of the circle. Not only that but you can find how long it takes to complete a circle which is just the period of circular motion, [itex] T = {2 \pi r \over v_{perp}}[/itex] (of course, by that time the charge will have move along the x axis so it won't get back to its initial position, it willhave moved a distance [itex] v_{par} T[/itex])
  8. Jun 23, 2006 #7
    Your formula for the perpendicular component [tex]m \frac {v_perp}{r}[/tex] doesn't match with mass x acceleration. How is that so?
  9. Jun 23, 2006 #8


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The equation I entered was
    [tex] m { v_{perp}^2 \over r} [/tex]
    (as you can check by left clicking on my equation to see the Latex code). It's just that the exponent "2" is so small that it is hard to see (I should have used tex anchors instead of itex anchors).

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook