Lab 13
Induction

An induction flashlight does not require a flashlight bulb or batteries. The flashlight contains a cylindrical coil of wire called a solenoid, a permanent magnet, a solid state light bulb called a light emitting diode, or LED, and a circuit to control their operation. When the flashlight is shaken the magnet is moved through the solenoid generating voltage pulses by Faraday’s Law of Induction. The circuit converts the pulses into a DC voltage that lights the LED.

The goal today is to explore Faraday’s Law of Induction by investigating the operation of an induction flashlight. See Figure 1.

Figure 1. Induction Flashlight

Review Theory

In 1831 Michael Faraday and Joseph Henry independently found that an electromotive force (emf or voltage), is generated between the ends of a coil of wire placed in a changing magnetic field. Faraday found that the emf, ε, generated with a coil having N turns of wire is proportional to the rate of change of the magnetic flux, ΦB, through the coil, where:

This equation is known as Faraday's Law of Induction where ΦB depends on the strength of the magnetic field, B (measured in Tesla), the area of the coil, A (measured in m2) and the cosine of the angle between them.

If a resistance, R, is placed across the ends of a coil of negligible resistance the induced emf, ε, will result in a current, i, in the resistor where (from Ohm’s law):

The direction of the current is such as to oppose the change that caused it (Lenz's Law) and the fingers of your right hand will curl in the direction of the current if your right hand thumb is pointed in the direction of the changing magnetic field. See Figure 2.

Figure 2. The Right Hand Rule