English

A magnet ball will fall slowly when made to travel along an aluminum L-shaped bar.

Keywords

Electromagnetic induction, eddy current*, Joule heat*

Objective

We help the students to understand qualitatively and quantitatively that an electric current can flow by electromagnetic induction, when a magnetic field fluctuates not only in coils, but also in plates and pipes. In addition, the students will be led to understand that mechanical energy is lost by Joule heating.

Things to Prepare

Fig. 1. From the left, a thin-walled stainless steel pipe, a thick-walled copper pipe, and an aluminum L-shaped bar.

Fig. 2. A slit copper pipe (a tube wherein the thin copper plates are insulated from each other and arranged in a pipe shape) on the left, and a thin-walled copper pipe (a pipe with a rounded copper plate, which is the same plate used for the slit copper pipe) on the right. 

Experiment Time

20 minutes

Preparation

• Procure the bar and the pipes, and trim them to the appropriate length.
• Make your own slit copper pipe.

Procedure

Before performing each experiment, make the students predict the results of the experiment.

1. Bring the magnetic ball close to the aluminum L-shaped bar, and verify that the magnet does not stick to the aluminum.
2. [Comparison of the magnetic ball and the metallic ball] Drop a metallic ball and magnetic ball respectively, along the inside of the aluminum L-shaped bar which stands nearly upright. (Place the magnetic ball on top of one aluminum L-shaped bar inside and place the metallic ball on top of another aluminum L-shaped bar inside, as both bars stand nearly upright, and then drop the balls at the same time along the plates.) (Result): The magnetic ball descends more slowly compared to the metallic ball. (Explanation by the instructor): Even if the aluminum does not stick to the magnet of the ball, the magnet, as the magnetic ball moves down on the aluminum plate, affects the speed of its fall.
3. [Comparison of the aluminum L-shaped bar and the plastic ruler] Place the magnetic balls on top of the aluminum L-shaped bar inside and on top of the plastic ruler, as they stand nearly upright, and then drop the magnetic balls along he planes. (Result): The magnetic ball descends more slowly in the aluminum L-shaped bar than in the plastic ruler. (Explanation of the instructor): The bar is a conductor so it affects the speed of the fall.
4. [Comparison of the inside and the outside of the aluminum L-shaped bar] Place the magnetic balls on the inside and the outside of the aluminum L-shaped bar, as it stands nearly upright, and then drop the magnetic balls. (Result): The magnetic ball descends more slowly inside the aluminum L-shaped bar than outside. (Explanation of the instructor): The entire area of the metal plate feels the change in the magnetic field and affects the speed of the fall.
5. Bring the magnetic ball close to the copper pipe, and verify that the magnet does not stick to the copper.
6. [Comparison of the magnetic ball and the metallic ball] Place one of the balls at the opening of the thin-walled copper pipe, as it stands upright, and then drop it. Do the same with the other ball. (Result): The magnetic ball descends more slowly compared to the metallic ball. (Explanation by the instructor): Even if the copper does not stick to the magnet of the ball, the magnet, as the magnetic ball moves through the copper pipe, affects the speed of its fall.
7. [Comparison of the thin-walled copper pipe and the thick-walled copper pipe] Put the magnetic balls at the openings of the thin-walled copper pipe and the thick-walled copper pipe, as they stand upright, and then drop the balls. (Result): The magnetic ball descends more slowly inside the thick-walled copper pipe than the thin-walled copper pipe. (Explanation of the instructor): The instructor explains that the electrical resistance of the thick-walled copper pipe is smaller than that of the thin-walled copper pipe. And then he/she will explain that the electrical resistance of the pipe and the electric current that flows through the pipe affects the speed of the fall.
8. [Comparison of the stainless steel pipe and the thin-walled copper pipe] Place the magnetic balls at the openings of the thin-walled stainless steel pipe and the thin-walled copper pipe, as they stand upright, and then drop the balls. (Result): The magnetic ball descends more slowly in the thin-walled copper pipe than in the thin-walled stainless steel pipe. (Explanation of the instructor): The instructor explains that the electrical resistance is larger in stainless steel than in copper. From this result, we find that the electrical resistance of a material that the pipe is made of affects the speed of the magnetic ball’s fall through it.
9. Now the students must analyze, in accordance with the law of electromagnetic induction, the direction of the electric current flow inside the pipe when the magnetic ball travels through it. They must verify whether the direction of the induced current and the direction of the force on the magnetic ball are indeed the direction to impede the ball’s movement. Then, based on the results of Experiments 7. and 8., they must explain the relationship between the magnitude of the Joule heating and the magnitude of the electrical resistance.
10. [Comparison of the copper pipe and the slit copper pipe] Place the magnetic balls at the openings of the copper pipe and the slit copper pipe, as they stand upright, and then drop the balls. (Result): Even though they are made with the same material and have the same thickness, the magnetic ball descends more slowly in the copper pipe than in the slit copper pipe. (Explanation by the instructor): The instructor confirms the directions of the electric currents flowing inside the pipes with the students.

Cautionary Note and Remarks

• This experiment is for students who have already studied electromagnetic induction.
• If the students are going to run the experiment on their desks, it would be wise to place a soft cloth under the bar and the pipes where the balls fall, so that they will not bounce and roll off. In contrast, if the instructor is going to demonstrate the experiment from his/her teaching platform, no cloth should be laid in place, since the sound produced when the balls land will make it easier for all the students to understand the difference in the speeds of the falls, no matter where they are sitting in the lecture room.
• Since the magnetic force of the neodymium magnet is strong, do not to bring it close to magnetic cards.
• The instructor can also divide the class into groups of ~ 5 students, and prepare a set of materials for each group so that the students can perform the experiment themselves.
• The instructor may also suggest the students to analyze the magnitudes of the electrical resistances of the pure copper and the alloy stainless steel, and compare them with the lengths of the mean free paths.

Video

Related Topics

The eddy current effect can be used in non-contact type brakes. In Japan, a magnetic field was added to the metal disk attached to the axle, to serve as brakes for the first Shinkansen (0 series). In German railways, a magnetic field is added to the rails instead of a disk, and the rail is heated by eddy currents. Moreover, in Shinkansen, the current generated by a motor flows through a resistor, resulting in Joule heating. In these methods, energy is discarded as heat. Since we are currently trying to save energy, the electric current generated by a motor in conventional railroad lines is recovered by overhead wires (regenerative brake).

Author of the article

Yuichi Miura (Graduate School of Science, Nagoya University)