Optional Unit VII: Electromagnetism
D. Electromagnetic Induction

Key Concepts

Faraday and Henry discovered that a voltage can be induced in a conductor which is moving relative to an external magnetic field. A current will flow if a complete circuit is present.

Whenever the magnetic field in the region of a conductor is moving, or changing in magnitude, such that magnetic field lines are moving across the conductor, an electric current is induced in the conductor, if the conductor is part of a complete circuit.

The current induced is a cross product of the directions of movement of the conductor and the magnetic field. The current is proportional to sin , where is the angle between the direction of the conductor and the direction of the field. The maximum current will be induced when = 90° (perpendicular), but will still be produced at other angles, diminishing from a maximum at = 90° to 0 for = 0°.

Electromagnetic induction results in a temporary change in the external magnetic field. An interaction occurs between the induced field (formed when the current is induced in the conductor) and the inducing field.

The induced magnetic field opposes the change in the inducing field. They have the same direction if the inducing field is decreasing. (Lenz's Law.)

Lenz's Law is consistent with the Law of Conservation of Energy. Mechanical energy must be supplied externally to produce the induced electrical energy.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: induce, induced field, inducing field.

2. Identify the conditions which must occur before a current can be induced within a conductor.

3. Recognize that maximum current is induced when the direction of movement of the conductor is perpendicular to that of the external magnetic field.

4. Explain that the interaction between the induced and inducing magnetic fields produces a temporary change in the external magnetic field.

5. State Lenz's Law.

6. Recognize that Lenz's Law is consistent with the Law of Conservation of Energy.

7. Apply Lenz's Law to investigate electromagnetic induction.

Teaching Strategies, Activities and Demonstrations

1. Use Faraday's iron ring, or some other apparatus, to demonstrate electromagnetic induction. Moving a conductor through the jaws of a horseshoe magnet, or plunging a bar magnet into and out of a solenoid connected to a galvanometer, are interesting ways in which to observe this.

2. Connect a hollow coil to a galvanometer. Plunge a bar magnet into and out of the coil and observe the readings on the galvanometer. Reverse the direction of the magnetic field on the bar magnet and repeat.

   Connect a galvanometer to a secondary coil. Place a primary coil electromagnet inside the secondary coil. Complete the circuit on the primary coil. Observe the reading obtained on the galvanometer on the secondary coil.

3. Demonstrate the use of a piezoelectric lighter without identifying what it is or how it works. Have the students work in groups. Each group is to discuss the gadget and try to develop a hypothesis of how it works. Groups can then share their hypothesis in a full class discussion, seeing which group has the most interesting hypothesis. Of course, the group that proposes a hypothesis would also have to think of experiments that could be developed to test it.

   The point of the exercise is not just to investigate the piezoelectric effect, although that might be a secondary outcome. Instead, it is to take some phenomenon which students may be unfamiliar with, such as the piezoelectric effect, and begin some explorations which reveal how curiosity and an interest in discovering the underlying causes of natural phenomena are crucial elements in the pursuit of physics.