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Magnet and ring

PRESENTATION: A light aluminum ring is suspended from 2 threads to form a bifilar pendulum. When a potent magnet is approached, currents are induced in the ring and make it be repelled by the magnet. When the magnet is withdrawn the ring tends to approach. The same effect of opposition to movement is observed changing the polarity of the magnet. If there does not exist the movement relative to the ring keeps its balance.

• Another Way to Demonstrate Lenz’s Law, Chang Geng Zhang and Shi Feng Hu, Phys. Teach. 40, 249 (2002)
• A Lenz’s law experiment revisited, Charles A. Sawicki, Phys. Teach. 38, 439 (2000)

INTRODUCTION: At the beginning of the XIX^th century, Michael Faraday, a British physicist, and chemist, carried out a series of observations and experiments which showed that under certain conditions, the magnetic fields could generate electric currents in conductors. It was then when he discovered in 1831, the electromagnetic induction. Thus, for instance, connecting a conductor turn to a galvanometer in the presence of a magnet it could be observed (on some given conditions), without the necessity of a generator, which provided electromotive force (e.m.f.).

Heinrich Friedrich Emil Lenz, a German from the Baltic sea, is known for formulating Lenz’s Law in 1833 which says that “the sense of the currents or the induced electromotive force is such that it always opposes the cause that produces it, that is, the variation of the magnetic flux”. Thanks to this law, Faraday’s Law was completed. This is why it is also usual to call it Faraday-Lenz’s law to honor his efforts towards this problem.

OBJECTIVE: To study the behavior produced by the interaction between the magnetic and the electric field.

MATERIALS: copper wire (coil), horizontal bar, rigid stand, bar magnet, crocodile pliers or a flat cable.

SETUP: Surround a coil with 5 turns of wire or copper cable leaving 2 loose coil ends to suspend them from a horizontal bar, which must b fixed to a rigid stand and connect them between themselves by crocodile pliers or a flat cable. The other fundamental part is the magnet, preferably a bar magnet.

EXPLANATION: According to Faraday’s law the e.f.m. induced in a closed coil equals minus the variation with the magnetic flux time though that coil

ε_ind = −d [Φ_B (t) / dt]

where the negative sign is a consequence of the Conservation of Energy principle and is reflected in Lenz’s law. In this project the consequence of Lenz’s law is verified, that is, the induced e.f.m. always opposes the change in the magnetic flux, so  the coil behaves like an electromagnet which attracts or repels the bar magnet (producing a rocking motion of the coil) in order to always oppose to its movement.

CONCEPTS: Lenz’s law, electromotive law, electromagnetic induction, magnetic field, Faraday-Lenz’s law, electromagnetism.

MORE INFORMATION:

• MAGNETIC MATERIALS
• WIKIPEDIA
• YOUTUBE 1
• YOUTUBE 2
• YOUTUBE 3
• YOUTUBE 4
• YOUTUBE 5
• FISICANET
• ALL ABOUT ELECTRONICS
• INTERACTIVE COURSE
• PHYSLET

TEXTS:

• R. Serway, Física, Mac Graw Hill, 2010.
• P. Tipler, Física para la Ciencia y la tecnología, Reverté, 2012.
• R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.

STUDENTS 2011-2012: Cristian Rueda, Álvaro Santiago, Carlos Saso

LINK pdf STUDENTS (in Spanish):

ANILLO E IMAN

STUDENTS 2010-2011: Rosalía Carrillo, Xeila Casais

LINK pdf STUDENTS (in Spanish):

ANILLO E IMAN 2010 2011