LivePhoto Physics Activity 32 Name: Date:
Exploring Faraday’s Law
In 1820 Hans Christian Oersted observed that electric currents create magnetic fields. Consequently many scientists made unsuccessful attempts to create current in the presence of magnetic fields. After over 10 years of investigation Michael Faraday and Joseph Henry, working independently on different sides of the Atlantic, made the discovery that if a conducting coil has a changing magnetic flux passing through it an electromotive force is created that causes a current to flow through the coil. The equation describing this phenomenon quantitatively is known as Faraday’s Law. a b c In this activity you will be asked to use Faraday’s Law to explain why the current induced in a coil and thus the voltage across it changes the way it does when the north or south pole of rod shaped magnet is moved into and then back out of the center of a coil of wire at various speeds. No calculations are required in this activity, just thinking! Figure 1: The north pole of a cylindrical magnet is: (a) moving into the center of a coil of wire; (b) reversing direction; and then (c) moving out of the coil. The coil’s voltage vs. time is recorded with a computer interfaced voltage probe. Theoretical Review: According to Faraday’s Law, at any moment the induced emf (denoted ) across a conducting loop with N turns is equal to –N times the rate of change of the magnetic flux passing through the area enclosed by the loop as described in Eq. 1. (Faraday’s Law) [Eq. 1] Figure 1 shows our rod shaped magnet moving along the coil’s symmetry axis. The plane bounded by the coil has an area, A, and is perpendicular to the axis along which the magnet is moving. Even if the magnetic flux passing through the coil is not uniform or constant, we only need to keep track of how much flux passes through the