Changing Fields
According to Ampere's law, current (yellow) running in the z direction is causing a B field in the - θ direction (purple).Copyright 2006 Dave Burke, Wikipedia Commons http://commons.wikimedia.org/wiki/Image:Z_pinch.png
To begin, students use the associated activity to induce EMF in a coil of wire using magnetic fields. Then, demonstrations on eddy currents show how a magnetic field can slow magnets just as eddy currents are used to slow large trains. Then students observe a demonstration in which a loop "jumps" because of a changing magnetic field. Finally, a lecture reviews the cross product with respect to magnetic force and introduces magnetic flux, Faraday's law of Induction, Lenz's law, eddy currents, motional EMF and Induced EMF.
Engineers must find ways to reduce the effects of eddy currents in an MRI machine as energy dissipation must be carefully controlled for personnel safety. Currently, engineers use the physics behind eddy currents to create brakes for large trains, a concept shown in lesson demos 1 and 2. During the train's braking, the metal wheels are exposed to a magnetic field from an electromagnet creating eddy currents in the wheels. The eddy currents meet resistance throughout the wheel and dissipate their energy as heat, which essentially slows down the wheels and causes the train to stop. In this lesson's homework assessment, students apply the concept of magnetic flux to a changing circuit, as electrical engineers would.
After this lesson, students should be able to:
- Describe the effects of moving a current loop in and out of a magnetic field.
- Calculate the magnetic flux through a solenoid or loop of current.
- Use Farraday's law to calculate electric and magnetic fields.
- Describe eddy currents and their properties.
- Calculate the EMF formed from changing a magnetic field