1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Earth's magnetic field ten times superior to what's expected

  1. May 14, 2009 #1

    1. The problem statement, all variables and given/known data
    In a laboratory we want to measure the value Bt which is Earth's magnetic field.

    We got a magnetic needle which is held by a wire, when the needle points out Earth's magnetic field, it creates a 64.5 degree angle in comparison to the horizontal Bh which is the table. The axis of the needle is Earth's magnetic field called Bt, it can be divided in Bv, the vertical line which is perpendicular to the horizontal and Bh which is the horizontal.

    First we want to know the horizontal Bh so we put the needle in a solenoid, the needle is perpendicular to the magnetic field line Bs of the solenoid when there's no current, then we turn on the solenoid which creates a 5,0*10^-4 Tesla field called Bs, the needle rotates by 68 degrees.
    Questions : We want the value of Bh which is the horizontal and Bt which is the Earth's magnetic field at Paris. ( the value is made at Paris)

    2. Relevant equations
    With trigonometry calculus I found Bh = Bs/tan(68) = 5.0*10^-4/tan(68) = 2.0*10^-4 T

    3. The attempt at a solution

    Here's the problem, when I look for Bt I got those calculus : Bt = Bh/cos(64.5) = 2.0*10^-4/cos(64.5) = 4.7*10^-4 T

    I checked up on the web for Earth's magnetic field in Paris, it's 47 microtesla but the micro order of magnitude is 10^-6 so my result is 470 microtesla, ten times greater than what should be expected!

    Which mistake did I do?
  2. jcsd
  3. May 14, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to Physics Forums :smile:

    Here it looks like you are saying Bh is the horizontal component of the solenoid field ...

    ... but here you are saying that Bh is the horizontal component of the Earth's field.

    The solenoid field is different than the Earth's field, and about 10 times stronger. It appears that you are not distinguishing between the two fields.

    Also, without seeing a diagram it is difficult to see exactly what is going on in the experiment.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook