ENGINEERING, ART, AND SCIENCE ECA.590: 9-29. Lenz's Law; the Art of soldering; Aurora Bear fabrication

Lenz's Law:

A spherical neodymium magnet is dropped down a copper tube,

with surprising results:

A magnet falls slowly through a copper tube because its moving magnetic field creates circular electric currents in the metal, which produce an opposing magnetic field that acts as an electromagnetic brake. [1, 2, 3]

This phenomenon involves a few key steps:

1. Creating Eddy Currents

Moving magnetic field: As the magnet drops, its magnetic field moves along with it.

Swirling electrons: Copper is a non-magnetic metal, but it is an excellent electrical conductor.

The induction effect: The changing magnetic field passing through the walls of the copper tube forces the free electrons in the copper to swirl in circular loops. Scientists call these circular paths eddy currents. [1, 2, 3, 4, 5]

2. Lenz's Law and Resistance

Opposing fields: According to Lenz’s Law (a fundamental rule of electromagnetism), an induced electric current always flows in a direction that opposes the change that created it. [1, 2]

Magnetic braking: The swirling eddy currents turn the copper tube into a temporary electromagnet. This temporary magnetic field pushes back against the falling magnet, creating an upward retarding force that counteracts gravity. [1, 2, 3, 4, 5]

Energy conversion: The kinetic energy of the falling magnet is converted into electrical energy (the eddy currents) and then dissipated as heat in the copper walls. [1, 2]

A non-magnetic object (like an aluminum or copper ball of the same weight) does not have a magnetic field, so it induces no eddy currents and falls straight through at normal speed. [1, 2]

Soldering and Wire Stripping Workshop

This workshop provides instruction on the technical fundamentals of soldering, wire stripping, and assembling Aurora Bears.

Soldering Fundamentals

Ed explained that effective soldering is a chemical bonding process achieved by heating the metal components rather than just dripping molten solder onto them. Heating only the solder can lead to a "cold solder joint," where the connection lacks structural integrity.

Key technical points include:

To increase heat transfer between two cylindrical wires, apply a small amount of solder to create a meniscus at the contact point.
Ensure stability by using blue masking tape to secure wires against a hard surface; floating wires prevent proper heat transfer.
Solder types discussed include rosin core (which aids attachment) and 64/60 leaded solder, noting that lead-free alternatives have higher melting points and may be harder to use.

Wire Stripping Techniques

The instructors demonstrate how to use wire strippers without damaging the internal conductor. Ed emphasized maintaining a perpendicular angle relative to the wire to avoid nicking the metal.

Best practices for wire management:

Avoid repetitive bending or "nicking" near the joint, as this creates weak points that cause wires to break during presentation.
When using strippers, hold them closer to the cutting hole rather than by the handles for better control.
Select an appropriate gauge size; for example, 26 gauge wire requires specific handling to ensure insulation is removed without exposing excessive bare wire or damaging the core.

Assembly Guidance

Participants are instructed on how to assemble their Aurora Bears projects while gaining hands-on experience with previously discussed skills.

Project assembly notes:

For visual optimization, place the red LED in the middle of others because it is naturally dimmer; this allows blue and cream lights to blend more aesthetically through total internal reflection.
When working with materials, use cast acrylic rather than extruded acrylic for better results when laser cutting due to differences in material stress and finish.
A demonstration of a laser cutter is provided, highlighting its ability to create lensing effects on edges for light diffusion.

Soldering Fundamentals and Techniques

Soldering is a chemical bonding process that requires heating the metal itself rather than just dripping molten solder onto it.
Applying solder to cold metal results in a "cold solder joint," which lacks a proper bond and appears smeared.

To improve heat transfer between two cylindrical wires, a small amount of solder should be applied to the contact point first to create a meniscus.
Using blue painter's tape helps stabilize components and provides a hard surface to press against, ensuring efficient heat transfer.

Wire Stripping Best Practices

Wire strippers should be held perpendicular to the wire to avoid nicking or damaging the internal conductor.
Nicking the wire creates weak points that can cause the wire to break during use or presentation.
When using strippers, it is more effective to hold them closer to the cutting hole rather than by the handles for better control.
The thickness of a wire is measured by gauge, where a larger number indicates a smaller diameter wire. 22 gauge solid is a commonly used type of wire for hookup connections.

Solder Types and Safety

Rosin-core solder is preferred over acid-core plumber's solder because the rosin helps the solder adhere to the metal.
60/40 lead/tin solder is commonly used, though users must be aware of potential lead fumes.
Lead-free solder is an alternative but has a higher melting point and can be more difficult to work with.

Aurora Bearialis

https://edgerton.mit.edu/aurora-beaveralis

Building Aurora Bears

Participants will practice their skills by building "Aurora Bears" using various components and LEDs.
For optimal color blending, it is recommended to place the red LED in the middle because it is the dimmest color.
The project utilizes total internal reflection to keep light inside the material so it shoots out through the top.

Laser Cutting Demonstration

A demonstration of laser cutting is provided, including how to locate specific files like "beaveralis"
Users are cautioned to monitor laser cutting closely as the process can pose a fire risk.
When working with acrylic, cast acrylic is recommended over extruded acrylic to avoid stress issues and cracking.