An empty aluminum soft drink can, placed inside a coil of wire, is made to implode when a capacitor is discharged through the coil.
What It Shows:
A dramatic demonstration of Faraday induction and of the repulsion of oppositely directed electric currents. An empty aluminum soft drink can is placed inside a coil of wire. When a large capacitor is discharged through the coil, the sudden increase in magnetic flux induces a large current in the wall of the can (Faraday's Law). This current is oppositely directed to the current in the coil (Lenz's Law) and experiences a huge Lorentz force directed radially inward. Since the current is confined to the wall of the can, the portion of the can inside the coil is driven violently inward, pinching the can down to form a waist. With a large enough charge on the capacitor, the can is actually torn in half and the radial component of the magnetic field propels the two halves to opposite walls of the lecture hall. Shown in the photograph are three cans subjected to discharges of capacitor charged to 3.8, 4.4, and 4.8 kV (left to right).
How It Works:
The coil consists of 3½ turns of #10 magnet wire and is embedded in a 1/2"-thick 10"×10" piece of fabric-based phenolic (a.k.a. garolite) for support and strength. It's inductance is 1.7 μH and resistance 1 mΩ.
An oil-filled 100 μF capacitor is charged by a H.V. power supply to approximately 5 kV. It is then disconnected from the power supply and discharged through the coil by way of a high-voltage relay switch. The current in the coil increases rapidly with a time constant determined primarily by L/R, equal approximately to 1½ ms.
Setting It Up:
The coil and capacitor are mounted on a dedicated dolly and are best positioned in the middle of the floor space, close to the lecture benches. The H.V. power supply1 is on a relay rack with wheels and can be located between the lecture bench and blackboard.
A 250 kΩ resistor, in series with the output of the H.V. supply, limits the charging current to 20 mA. With an RC time constant of approximately 25 seconds, it takes about 2½ minutes to fully charge the 100 μF capacitor. Increase the voltage to 5 kV in 1 kV increments, pausing ½ to 1 second at each setting. When the desired charge has been reached (as indicated by the voltmeter), disconnect the H.V. supply from the capacitor with the H.V. vacuum relay2 switch. Reduce the supply voltage to zero and switch it to the standby mode. When ready to crush the can, connect the capacitor to the coil by energizing the H.V. relay3 switch.
The experiment provides a memorable demonstration of the force between current carying conductors. It was modeled after an apparatus at the University of Maryland.4
1 Fluke model 408B (0-6 kV, 0-20 mA)
2 Jennings Radio Corp model E02P (60 kV, 50 A)
3 Ross Engineering Corp E Series high voltage relay (model E15-NO-15)
4 A.W. DeSilva, "Magnetically imploded soft drink can," Am J Phys 62(1), 41-45 (1994). DeSilva also describes the implosion as a reaction to the "pinch effect" as follows: For a rapidly rising current in the driver coil, the induced current in the can wall excludes the magnetic field from inside the can, and thus the field is confined to the narrow annulus between the can and coil. The pressure the field exerts is not balanced by a corresponding pressure from the inside, causing the can to implode. Larger versions of the pinch effect (utilizing considerably higher voltages on the capacitor) are used in plasma physics to create hot dense plasmas. In that case, the can is replaced by a glass tube filled with some test gas at a low pressure. The induced electric field causes the gas to break down into a plasma, which is pinched to the center of the tube and heated intensely by the compression.