# Rate of change of the magnetic field

talaroue

## Homework Statement

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

## Homework Equations

dI=rate of change of the flux
A=area
B=magnetic field

## 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

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.

Homework Helper
Okay, let me restate this:
A square loop of wire has sides that are 8.4 cm long. The wire in the loop has a total resistance of 0.55 Ω. There is a magnetic field perpendicular to the plane of the loop which is spatially uniform, but is changing in time. If the changing magnetic field induces a current of 148 mA in the wire loop, at what rate, in T/s, the magnetic field strength through the loop increasing or decreasing?
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)

talaroue
EMF=N(dI/dt)

dI is delta flux?

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

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

Homework Helper
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.