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: Homework help -- magnetic field from a wire causing compass deflection...

  1. Apr 11, 2016 #1
    1. The problem statement, all variables and given/known data
    A current 2A flows in a wire. Value of horizontal component of earth magnetic field in the region is 2x10^-5T. Small compass placed 5cm due north of the wire. Calculate the angle through which the needle deflects due to the current flowing in the wire. Assume u0= 4pi x 10^-7 Hm^-1

    2. Relevant equations
    Pythagoras a2+b2=c2

    3. The attempt at a solution
    Tried B=uI/2piR -> 2x10^-5=u2/2pi(0.05)
    But that didn't make sense so I tried to solve for R instead and got R equal to 0.02 but didn't know what to do with R.
    Then I tried using F=BILsinx and assumed sinx=90 assuming it was perpendicular but the question wants the angle through so I don't think F=BILsinx is relevant?

    Please help thanks
  2. jcsd
  3. Apr 11, 2016 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF.

    It will help you to make a drawing of the setup. Have the wire coming out of the page toward you, with the current coming up at you. So the B-field circulates counter-clockwise around the wire, right? (by the Right Hand Rule).

    Draw a vertical axis and a horizontal axis on your figure -- the vertical axis points up toward north, and the horizontal axis points to the right towards the east. The wire is coming up at you from the origin of those x-y axes.

    Draw where the compass is. When there is no current in the wire, you are given what the Earth's B-field is, and it makes the compass point due north (at least in this simple problem). When the current is switched on, the B-fields from the Earth and from the wire add up vectorially, giving a new resultant B-field vector at the compass. This causes the compass needle to deflect one way or the other. Your task is to add up those two B-fields to see what how much the resultant B-field vector rotates compared to true north.

    Have at it! :smile:
  4. Apr 11, 2016 #3
    hi thanks for that i tried it and i added the two B-field vectors together, i just want to know what do i do with this and if my sketch is correct
    thanks again
    i got B-field for the wire= 2.55x10^-6= u0(2)/2pi(0.05)
    added both B-fields together= 5.1x10^-11T

    Attached Files:

  5. Apr 11, 2016 #4


    User Avatar

    Staff: Mentor

    How did you add those and get:
    Also, you need to add them vectorially. They do not point in the same direction...

    As a tip, you might re-draw the diagram to more closely match what I suggested. It should be orthogonal x and y axes, with the wire coming out of the page vertically at the origin. Then the compass sits some distance up the y-axis, as described in the problem statement.
  6. Apr 11, 2016 #5
    sorry i forgot to mention the wire is vertical, also this isnt homework its a question from a past paper. my physics tutor is stumped on it, he said that you if you add them vectorially then the B-fields move in the same direction. im really finding this hard its only the second question in the paper. thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted