Eddy’s Brake—Lenz’s Law

Introduction

How can a spinning wheel be rapidly slowed down without any contact? Yes, by using magnetic forces—the same principle is used to slow down many roller coaster cars.

Concepts

• Electromagnetism
• Eddy currents
• Lenz’s law

Background

During the 19th century, scientists like Michael Faraday (1791–1867) and Heinrich Lenz (1804–1865) used observation and experimentation to determine that an electric current in a wire creates a magnetic field around the wire. They also discovered that a changing magnetic field can generate, or induce, an electric current in a wire—known as an eddy current. The eddy current, in turn, will also produce its own magnetic field. Faraday determined that the strength of an induced eddy current is equal to the rate of change of the magnetic field. This is known as Faraday’s law. Lenz further investigated Faraday’s law and discovered that the induced eddy currents travel in the direction that satisfies the conservation of energy principle. Thus, since the eddy currents produced by the changing magnetic field generate their own magnetic field, the eddy currents travel in the direction that maintains the original magnetic field strength. This is known as Lenz’s law. As an example, consider that the initial magnetic field surrounding a closed loop of wire is zero. As a magnet is moved close to the wire, it changes the external magnetic field surrounding the wire. This changing magnetic field induces a current in the wire. The current travels in the wire in a direction that will generate a magnetic field that tends to cancel out the external magnetic field, leaving the net magnetic field surrounding the wire at zero. Eventually, magnetic equilibrium is reached and the wires adjust to the new stable, nonzero magnetic environment and the eddy currents stop. When the magnetic field changes again, eddy currents will be generated again according to Faraday’s law, and the current will travel in a specific direction following Lenz’s law.

By convention, current traveling counterclockwise in a loop of wire (when looking at the loop) produces a magnetic field with a north pole pointing towards you. Current traveling clockwise produces a magnetic field with a north pole pointing away from you. For the spinning metal disk, virtual closed loops of current can be generated throughout the solid disk (see Figure 1). These loops of eddy currents will travel in such as way as to keep the magnetic field constant. As a section of the metal disk approaches the external magnet, the eddy currents will travel in a way to generate an opposite-polarity magnetic field (Figure 1), in an attempt to cancel the increasing magnetic field. Because the pole of the induced magnetic field is the same as the pole of the external magnet, the two magnets repel each other, causing the spinning disk to slow down. As a section of the metal disk travels away from the external magnet, the eddy currents will change directions in order to produce a net, stable, nonzero magnetic field. Because the magnetic poles are now opposite, the metal sections that have moved past the external magnet are now attracted to the magnet and again cause the metal disk to slow down (see Figure 1). 

Materials

Safety Precautions

Use caution when handling the neodymium magnet. Neodymium magnets are very strong and may quickly snap together and pinch skin. The magnets are fragile and may shatter if dropped or if they hit another object too hard. Keep the magnets away from computer disks, computer monitors and TVs. The edge of the disk is smooth, but use caution when spinning the disk. Wear safety glasses when performing this demonstration.

Prelab Preparation

Assembly

1. Use a pushpin to carefully poke a hole through the center of the plastic lid. It may help to heat the tip of the pin with a match flame for a few seconds before poking it through the lid. The hole should be a little smaller than the axle of the motor of the Bracken’s Demonstration Spinner.
2. Place the plastic lid, top-side down, onto the exposed motor axle of the demonstration spinner, as shown in Figure 2. The hole in the lid should fit snugly around the motor axle.
3. Turn on the motor to test the placement of the lid. The lid should rotate quickly and evenly, with little wobble or slipping.

Procedure

1. Place the metal disk, nut-side down, onto the motor axle. The disk should remain centered and balanced on the axle (see Figure 2).
2. Bring the neodymium magnet near the metal disk to show students the disk is not magnetic (it is composed of aluminum). Students should record their answer to the Question 1 on the worksheet. Note: The weld nut and screw attached to the disk are composed of steel and are therefore magnetic. Keep the magnet near the edge of the disk.
3. Turn on the Bracken’s Demonstration Spinner and allow time for the metal disk to rotate quickly and smoothly. (Although the metal disk is not physically attached to the motor axle or plastic lid, the combination of the friction between the spinning motor axle and the spinning plastic lid will eventually cause the metal disk to spin very quickly.)
4. With students observing the spinning disk, bring the magnet near the edge of the disk, without touching it. Students should record their observations on the worksheet.
5. Remove the magnet and watch the disk begin to spin quickly again. Students should record their observations in the worksheet.
6. Repeat steps 4 and 5 as often as necessary until all students have observed the action of the magnet on the spinning metal disk.

Student Worksheet PDF

12622_Student1.pdf

Teacher Tips

• The materials in this demonstration are completely reusable and should be saved and stored for future use.
• Use the ½" screw if the Bracken’s Demonstration Spinner (or its equivalent) is not available. Insert the ½" screw provided with the kit into the weld nut attached to the disk. Then spin the disk on the head of the screw (like a top) so that the disk spins horizontally and remains balanced for a few seconds. Bring the magnet near the spinning disk to show the braking action caused by the magnet. Perform this alternative method on a large sheet of paper to protect the surface of the tabletop, if desired.
• Use a much stronger Mega-Magnet (available from Flinn Scientific, Catalog No. AP1738) to slow down the spinning metal disk much more quickly and dramatically.
• The Eddy’s Brake Worksheet may be copied and used during the discussion or as a post-demonstration assignment, if desired.
• The Eddy Current Demonstration Kit, available from Flinn Scientific (Catalog No. AP4698), is another great kit to show Lenz’s law and electromagnetism.

Answers to Questions

1. Is the metal disk magnetic? How can you tell?

   No, the metal disk is not magnetic. The magnet does not attract or “stick” to the metal disk.

2. How does the spinning metal disk respond to the magnet? Explain in terms of eddy currents and Lenz’s law.

   The spinning disk slows down quickly when the magnet is brought near the edge of the spinning disk. The plastic lid continues to spin however. Eddy currents in the metal disk are generated by the changing magnetic field caused by the motion of the metal disk under the magnet. These eddy currents create magnetic fields that repel to the magnetic field of the external magnet when the region of the metal disk approaches the magnet and attract the external magnet when the region of the metal disk travels away from the magnet. The magnetic forces cause the disk to slow down.

3. What happens when the magnet is moved away from the disk on the spinning axle?

   The disk begins to spin again due to the force of friction caused by the spinning plastic lid and motor axle.

4. Can you think of a use for this kind of braking system?

   Student answers will vary, but any braking system that requires no moving parts and one that needs to stop objects rotating very quickly. The faster the motion, the greater the braking power.