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: 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

    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

    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.
    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 = ?


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