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Electromagnetism - Simple question

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data
    -------/\/\/\---------
    I X X X 10 Ohm X X XI
    I X X X X X X X X X XI
    I X X X X X X X X X XAmmeter
    I X X X X X X X X X XI
    I--------il-----------I
    ..........5V

    Length=10cm and width =10cm
    R=10 Ohms
    V=5 volts
    decreasing magnetic field at rate 150T/s (into the page)

    What is the reading of the Ammeter?
    (Correct Answer:0.35A)

    2. Relevant equations
    I=e/R
    e=BvL



    3. The attempt at a solution
    well basically i froze.
    all i get is
    s=1m (0.1x0.1)
    L=40 (10x4)

    From there onwards i can not link them. Probably because lacking the correct formula.
    Anyway, ill much appreciate some help.
    Cheers
     
    Last edited: Jan 8, 2008
  2. jcsd
  3. Jan 8, 2008 #2
    Part of your given information, kinda the key part, is a rate of change of a magnetic field. Remember you also have an area you can calculate, what can you find with a magnetic field and an area?

    Do you perchance have an equation among your notes and book that features a rate of change?
     
  4. Jan 9, 2008 #3
    thanks, ill take a look around the notes again
     
  5. Jan 11, 2008 #4
    still stuck
     
  6. Jan 11, 2008 #5
    Magnetic flux? -Rate of change of magnetic flux = ?


    http://en.wikipedia.org/wiki/Electromagnetic_induction

    Stuff like that? Unless I'm badly misinterpreting what the picture is, you've got a current flowing through the ammeter just because of the battery and the resistor, but now you have a changing magnetic field through the loop. A changing magnetic field can induce a voltage, which is what will happen here
     
  7. Jan 11, 2008 #6
    thanks again!
     
  8. Jan 13, 2008 #7
    magnetic flux = B.dS
    where S is the surface area (0.01m^2).
    emf is directly proportional to the rate of change of magnetic flux, so
    e= d(magnetic flux) / dt -> those that mean that 150T/s = e ?
    (1 T= 1Wb.m^-2)
    if so then 150=Bx0.01 which makes B=15000 T
    But that cant be right. im not sure where im going wrong....
     
  9. Jan 13, 2008 #8
    by a reversed method i go:
    1. I=e/R -> 0.35=e/10 -------hence e=3.5 V
    since directly prop then it becomes 3.5 Wb
    2. magnetic flux= B.dS ------ 3.5= B.dS where S (surface area) = 0.1x0.1 = 0.01m^2
    it comes out B= 3.5/0.01 = 350 T

    But how do i do it the right way around rather then back wards :) and what formula do i use to links the initial 5V? Perhaps a formula to derive B using area and voltage (dont think theres such a formula tho)?

    Just a little more help please, blochwave you been great!!!
     
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