23 Mar, 2013
Through a tube
PRESENTATION: When a powerful Neodymium magnet is dropped inside a copper tube a partial slowdown in its fall is perceptible due to the induced currents, which oppose the causes that produce them.
- More on Faraday’s and Lenz’s laws ‐ Qualitative demonstrations, Roberto Hessel, Phys. Teach. 49, 184 (2011)
- Lenz’s Law Magic Trick, Michael J. Ruiz, Phys. Teach. 44, 96 (2006)
- Action-reaction at a distance, Howard Brand, Phys. Teach. 40, 136 (2002)
INTRODUCTION: Faraday’s law is a basic law of electromagnetism that establishes that the voltage induced in a circuit is directly proportional to the speed at which the magnetic flow which goes through the circuit section changes.
Lenz’s Law postulates that an electromagnetic force (e.m.f.) induced always generates a current that provokes a variable magnetic field which opposes the change of the magnetic flow that induced it. The negative sign in Faraday’s Law is due to this law.
Foucault’s currents constitute an electric phenomenon which is produced when a conductor goes through a variable magnetic field, or vice versa. The relative movement induces currents inside the conductor. These Foucault’s currents behave like magnets which oppose the effect of the applied magnetic field.
OBJECTIVE: To observe what happens when a magnet is dropped through a conductor tube.
MATERIALS: copper tube (l=1m Ø=5cm), aluminum tube (l=1m Ø=5cm), Neodymium magnets, chronometer, non-magnetic weight (same mass as the magnets).
SETUP: A tube is held and the non-magnetic weight is dropped (approximately the same mass than the magnets). The same procedure is followed with the magnets and their speed is measured. These two steps are followed for both tubes, copper and aluminum; finally, both are compared dropping them at the same time.
EXPLANATION: When a magnet is dropped through a conductor tube, it makes the magnetic flow vary in such a way that it induces currents which generate fields opposed to those of their fall. If the magnet falls with the South Pole facing down and we see it for a fixed moment. The flow in the South Pole zone rises (more field lines come in from bottom to top), which creates a clockwise current which generates a magnetic field downwards. In the South Pole zone, the magnetic flow diminishes (many field lines go inside from bottom to top), and this is the reason why a counterclockwise current is induced, which creates a magnetic field from bottom to top.
If the position of the magnet were the opposite, a counterclockwise current in the North Pole part would appear, and a clockwise one in the South Pole.
In both cases, the current which is placed under the magnet repels the magnet, while the one placed above attracts it. So, a magnetic field appears which compensates for the weight. The terminal velocity depends of the resistivity, diameter and thickness of the walls of the conductor tube and of the weight and power of the magnet. That’s why, under the same conditions, for copper and aluminum tubes, the velocity is 1.6 times higher in the aluminum (copper has less resistivity).
CONCEPTS: Faraday’s law, Lenz’s law, Foucault’s currents.
- WIKIPEDIA 1
- WIKIPEDIA 2
- WIKIPEDIA 3
- INTERACTIVE COURSE
- YOUTUBE /ORIGINAL
- YOUTUBE 1
- YOUTUBE 2
- YOUTUBE 3
- OTHER 1
- OTHER 2
- OTHER 3
- EXTRAS 1
- EXTRAS 2
- 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: Enrique Braña, Adrián Regueira, Guillermo Rial, Aday Rivera
LINK pdf STUDENTS (in Galician):
STUDENTS 2012-2013: Daniel Díaz, Agustín Moreira, Fabián Valera, Carlos Vázquez
LINK pdf STUDENTS (in Spanish):