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Time it takes for electron to travel through a solenoid.

  1. Apr 6, 2015 #1
    1. The problem statement, all variables and given/known data
    An electron enters a solenoid at 65 degrees below a solenoid. The solenoid has a 10 A clockwise current is 33.0 m long and has 400 turns. Ignoring end effects what is the shortest amount of time in which the electron pass through the solenoid?

    2. Relevant equations
    Only one I got in my text for solenoids is B=mu_0 NI

    3. The attempt at a solution
    I am able to find the B field causes by the solenoid by plugging in the values I have

    4piE-7 * 400 * 33.0 = 0.17 T.

    However what I'm stuck on is figuring out the time interval of the particle. I've used the text and I've use google and search as I want I can't find a formula to use. Any help with formulas would be appreciated.
  2. jcsd
  3. Apr 6, 2015 #2


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    Is there a typo here?

    This is not correct. What does N stand for in your equation B = μoNI?

    Can you describe the trajectory of the electron?

    Was the radius of the solenoid given?
    Last edited: Apr 6, 2015
  4. Apr 7, 2015 #3
    It enters the solenoid from a 65 degree angle below the solenoid (That's what the equation states) which is also the trajectory. N stands for the number of turns in the solenoid and no radius for the solenoid is given. It's asking without striking the coils what is the minimum amount of time it would take to make it through.

    I'm thinking of trying to find the velocity use that as the average velocity since the magnetic field wouldn't change once inside a solenoid using it as the average velocity, taking the distance and finding the time using a kinematic formula.
  5. Apr 7, 2015 #4


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    I can't visualize "a 65 degree angle below the solenoid". Was a figure provided? Did you state the problem exactly as given (word for word)?

    What equation are you referring to here?

    What is the shape of the trajectory of the electron as the electron travels through the solenoid?

    In the formula B = μoNI, N is not the number of turns in the solenoid. But N is related to the number of turns.
    What does "I" represent in the formula?
  6. Apr 7, 2015 #5
    I meant question not the equation, which gives no radius of the solenoid and only the length. The trajectory would be a line through the solenoid since the magnetic field lines are compressed to that point is my way of thinking. The other thought I have is that it it will continue on it's arcing path and come out the other side at a 65 degree angle from the solenoid.

    I represents the current of the solenoid. What you said with N not being the number of turns of the solenoid, but related to the number of turns doesn't make sense. Don't I want to compute the magnetic field inside of the solenoid than bring the electron in after knowing the value of B?

    The other part I'm questioning is when it says "The solenoid is made from a 33.0m length of wire, and has 400 turns along the .2m that is represented if that means through out the whole thing if means 400 turns per .2m or if the 33.0m wire is compressed down to 400 turns that span .2m.
  7. Apr 7, 2015 #6


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    As you can see, your original statement of the problem did not include the crucial information that there are 400 turns in a distance of 0.2 m.

    I would like to request that you state the entire problem word-for-word as it was given to you. Are you having to translate it from another language?
  8. Apr 7, 2015 #7
    An electron enters one end of a solenoid at a 65 degree angle to the horizontal. The solenoid carries a 10-a Counterclockwise current. The Solenoid is made from a 33.0m length of wire and as 400 turns in 0.2m of length. (a) ignoring end effects, what is the smallest time interval required for the electron to pass trough the solenoid (b) if the electron follows the quickest path through the solenoid how many revolutions does the electron make through the solenoid?
  9. Apr 7, 2015 #8


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    OK, thank you. Was a figure provided? It still seems to me that you need more information regarding either the length of the solenoid or the radius of the solenoid, unless the 0.2 m is to be interpreted as the overall length of the solenoid.

    [Edit: If the length of the solenoid is 0.2 m then you should be able to determine the radius of the solenoid. The key to this problem is knowing what the shape of the trajectory of the electron looks like. You have a charged particle moving in a uniform magnetic field and the velocity of the particle is not perpendicular to B.]
    Last edited: Apr 7, 2015
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