# Rate of change of the magnetic field

1. Jul 26, 2009

### talaroue

1. The problem statement, all variables and given/known data
The resistance of the loop is 0.55 Ω. At what rate the magnetic field strength increasing or decreasing? (a=8.4 cm, I=148 mA. Give answer in T/s.)
The loop is a square, so all the distances around are a=8.4 cm

2. Relevant equations
dI=rate of change of the flux
A=area
B=magnetic field

3. The attempt at a solution

I found V by using Ohm's Law V=IR, which is the same as emf

but now I am lost, I feel like I need to find B, obvious reasons. Do I just use the equation

B=Uo*N*I/2R

N=1 because only 1 loop
Uo= constant
I=current

2. Jul 26, 2009

### talaroue

I am still lost, no matter how I go about solving this I seem to be missing 1 or 2 bits of information..... usually B is one of those.

3. Jul 26, 2009

### diazona

Okay, let me restate this:
Does that accurately describe what the problem is asking?

If so, you do have enough information. Figuring out the EMF was a good first step, but now you need an equation that relates that EMF to magnetic flux. Do you know what it is?

(your B=Uo*N*I/2R is not the right equation for this problem)

4. Jul 27, 2009

### talaroue

EMF=N(dI/dt)

dI is delta flux?

5. Jul 27, 2009

### diazona

Yep, that's it. But note that nobody really uses I for flux. Normally the formula would be written
$$\varepsilon = N \frac{\mathrm{d}\Phi}{\mathrm{d}t}$$

6. Jul 27, 2009

### talaroue

right, I just didnt know how to right Phi on the computer

7. Jul 27, 2009

### diazona

Some people have signatures with the Greek letters that you can copy and paste from. There are other ways too, depending on what operating system you're using.