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PURPOSE: To blast a cola can into two pieces using electromagnetism.
DESCRIPTION: A 400 microfarad capacitor is charged to 3000 volts (1.8 kilojoules) and discharged through a three-turn coil into which an aluminum soft drink can has been positioned, as shown in the detail photo at the center. With the circular windows open, as in the photograph at the left, the two pieces of the can will be blasted over thirty feet to the sides of the large lecture hall. Charging the capacitor to less voltage results in a can with a "waist," as seen in the photograph at the right.
This device can be explained in two distinct ways:
(1) The rapidly rising current creates a rapidly rising magnetic field along the axis of the coil, which in turn induces an electric field going in circles inside the coil. The induced electric field causes an electron current in the can which experiences a vxB force in the magnetic field of the coil, causing the can to break into two pieces which are blown to the opposite sides of the lecture hall.
(2) The plasma "theta pinch" phenomenon occurs when the capacitor is discharged. This effect causes radiation pressure on the can which breaks it in two and blasts apart the two halves.
This is an UNFORGETTABLE DEMONSTRATION. A must when you cover electromagnetism.
This video, from the Video Encyclopedia of Physics Demonstrations, shows the operation of the can crusher with an animation illustrating (1) the electron current in the coil, (2) the vector magnetic field that it creates, (3) the induced electric field within the coil created as the coil current rapidly rises, (4) the electron current circling in the can created by that induced electric field, (5) and the vxB force on the electrons moving around the can.
Following a description of the crusher electronic components, the animation is displayed. The animation may be stopped so that the directions can be studied in detail for the five (5) quantities listed above. Using the left hand rule (for electrons) the directions can be verified; note that according to Lenz's law the direction of the electron current induced in the can must be in the opposite direction to the electron current in the coil.
Note that the magnetic field at either end of the coil possesses both an axial and a radial component; the electron current in the can is entirely azimuthal. Using the left hand rule to determine the direction of the cross product of the electron velocity and the magnetic field, it can be seen that the axial component of the magnetic field leads to an inward force, crushing the can, while the radial field component leads to an axial force, away from the plane of the coil at both ends of the can, causing the two parts of the can to move rapidly away from the coil. (In the large lecture hall the two parts of the can will be blown to the sides of the lecture hall.)
SUGGESTIONS: This device may be DANGEROUS if not operated properly. You must obtain permission from a Lecture-Demonstration staff member before attempting to operate it by yourself.
REFERENCES: Available.
See Demonstration Reference File for a very nice article by Prof. Alan Da Silva of the University of Maryland Plasma Physics research area, who designed this device and describes its theory.To see the operation of the can crusher select larger view at 600 frames per second or closer view at 1200 frames per second slow motion.
Click here for a nice slow-motion video of the can crusher at Wesleyan University, compliments of Vacek Miglus.
The web site http://hibp.ecse.rpi.edu/Can_Crusher/home.html contains a drawing and animation showing how the RPI electromagnetic can crusher works.
EQUIPMENT: Electromagnetic can crusher.
SETUP TIME: None, if you know how to operate it.
