The electromagnetic can crusher demonstrates Lenz's law. Eddy currents are created in the wall of the can thus causing a repulsion between the current in the coil and the eddy currents in the can. When the capacitor is charged to greater than about 3 Kv the force on the surface of the can is so great that it compresses the can quickly enough to cause it to fail. Computer modeling done by A. W. De Silva [1] showed that the surface of the can reaches a speed of approximately 250 m/s when the capacitor is discharged. Safety Issues: Always use an EMPTY aluminum can. Always turn the power supply off before discharging the capacitors. Keep the coil at least 10ft from your audience. The pieces of the can could go flying and have sharp edges. Always unplug the can crusher when done. Operation: Insert an empty aluminum can into the coil. Plug in the Can crusher. Press the 'on' button on the power supply to charge the capacitor to the desired voltage to bring the can to a nice waist (3 kV) or to cut the can in half (3.5 kV). Once at the desired voltage press the 'off' button on the power supply. Warn your audience that you are about to discharge the Can crusher. Gently depress the plunger on the top of the Can crusher to bring the electrodes together and fire the capacitors. Pontificate on the physics involved in this demo. Unplug the Can crusher when done.
Comments
Only a simple LRC circuit is needed to crush a can. In order to make the can crusher safer and quieter we have added some other components. Power Supply. A power supply is used to charge the capacitors. R1 is a Resistor through which the capacitors are charged. C1 the capacitor bank that is used to crush the can. The capacitor bank consists of two capacitors connected in parallel. They can be charged to a maximum of about (the shorting resistor R2 prevents going higher than this). A four turn coil made of flattened copper tubing and encased in fiberglass and epoxy resin. The Ignitron tube is the last component in the basic LRC circuit used to crush the can. It is a tube that contains a small amount of mercury with the cathode connected to the mercury and the anode at the other end of the tube. When the mercury is excited by passing a current through it some of it becomes a gas. This gas provides the charge carriers needed for breakdown to occur between the cathode and anode thus completing the circuit. In the diagram the cathode is at the bottom of the tube, the anode comes through the top and the 'tickler electrode' (the one used to excite the mercury) goes through the right side of the tube. We used an Ignitron tube as a switch because shorting the capacitor bank with a spark gap switch would be painfully loud in a small classroom. R2 is a resistor. It is used to short the capacitor bank C1 so that C1 will never remain charged indefinitely and can never be over charged (over charging could cause C1 to explode with enough force to kill someone). To prevent the oscillating current, created when the capacitor is discharged, from flowing back into the power supply and damaging it, the diode D1 was added to the circuit. C2 is a capacitor used to excite the mercury in the Ignitron tube. R3 is a resistor used for charging the small capacitor C2. R4 is a resistor used for discharging C2 through the spark gap. This is the firing switch. When this circuit is closed the capacitor C2 is discharged into the Ignitron tickler thus closing the circuit and discharging the capacitor bank C1 through the coil. The shorting wand is not shown in the circuit diagram. It consists of a resistor on the end of a non-conducting rod connected to the chaise ground. It can be used to discharge the capacitor in case of circuit failure.
Apparatus
Can crusher
Soda cans
References
A. W. Desilva, Magnetically imploded soft drink can, AMERICAN JOURNAL OF PHYSICS, 1994.