# Emf induced current

1. Jan 10, 2007

### metalmagik

1. The problem statement, all variables and given/known data

The magnetic field perpendicular to a circular loop of wire 23 cm in diameter is changed from +0.52 T to -0.45 T in 175 ms, where + means the field points away from an observer and - toward the observer.

2. Relevant equations

(pardon me with the equations, I am LaTeX challenged.)

emf = N x (delta flux)/(delta time)

emf = Blv

3. The attempt at a solution

emf = (1) (.97)/(.175)

emf = 5.54 V

I have seen this equation with a negative infront...does this need to be put in the answer? This is for a WebAssign. I apologize for the rushed manner in which I posted this, but I have a ton of other work to do. Any help on this is greatly appreciated.

2. Jan 11, 2007

### Staff: Mentor

$$V = -N \frac{d\phi}{dt}$$

So first, yes, there is a negative sign in front. Second, what is the equation that relates the B field to the flux $$\phi$$ ?

3. Jan 12, 2007

### andrevdh

The induced emf in a coil also depends on the area it presents to the changing manetic field through it. So the flux is the product of the magnetic field and the area perpendicular to the magnetic field lines. That is why the induced emf changes as the coils of a generator turns in the constant magnetic field - the area they present to the magnetic field lines changes as they revolve in the field.