# Homework Help: Question about magnetism/induced emf in small coil

1. Apr 7, 2010

### nwyatt

1. The problem statement, all variables and given/known data
A solenoid with a length of 25.0 cm, radius 1.60 cm and 5500 turns carries a current of 1.50 A. A small coil of radius 1.05 cm and 130 turns is placed at the center of the solenoid with its axis in the same direction as that of the solenoid. A 470 ohms resistor is connected to the ends of the small coil. The current in the solenoid is switched off within 25 ms.

Determine the magnitude of the induced emf in the small coil.

2. Relevant equations

Induced EMF= -N delta $$\Phi$$ / delta t

3. The attempt at a solution

The main problem I am having is figuring out the change of the magnetic flux. I figured out the magnetic flux through the small coil at 14.4 x10^-6 WB (from the equation magnetic flux=BA cos theta) but I don't know what it changes to or from.

2. Apr 7, 2010

### kerrick

Since the current in the solenoid is switched off, it no longer produces a magnetic field. So the flux changes to 0 over the course of 25 ms.

3. Apr 7, 2010

### nwyatt

so for delta $$\Phi$$ it would be (14.4x10^-6)-(0)?