# Homework Help: Electromagnetism - Simple question

1. Jan 8, 2008

### kalo

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?

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. Jan 8, 2008

### blochwave

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?

3. Jan 9, 2008

### kalo

thanks, ill take a look around the notes again

4. Jan 11, 2008

### kalo

still stuck

5. Jan 11, 2008

### blochwave

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

6. Jan 11, 2008

### kalo

thanks again!

7. Jan 13, 2008

### kalo

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....

8. Jan 13, 2008

### kalo

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!!!