|LECTURE DEMONSTRATION MANUAL | Instructional Research Lab : ucla physics|
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E.2.8 Coin Shrinker, Can Cutter
This apparatus is based on a fusion energy device called a theta pinch. A large capacitor is charged and a high current switch discharges it through a coil with just a few turns. The "theta" current in the coil produces a radial pinching effect on any conductor inside the coil.
In the can cutter variation, an empty aluminum can is pinched in the center and shoots out both ends. In the coin shrinker version, a coin is placed inside a sacrificial coil and a blast shield is used. The coil blows apart into pieces of molten wire, leaving behind a shrunken quarter.
There are a couple ways to explain what happens. The first is to look at the eddy currents induced in the conductor, and then noting that the I x B forces are radially inward. Or, it can be treated as the force between antiparallel wires. Another way is to look at the magnetic energy which goes as B^2 and the magnetic pressure which is the gradient of B^2. The discharge happens on an RC timescale but the magnetic field penetrates the conductor in an L/R time. Before the field can penetrate there is a large gradient in the magnetic energy trapped between the conductor and the coil and this results in a radially inward force.
The current and instantaneous power in this device is enormous. The coin shrinking happens in about 40 microsec. The current is about 60,000 A and the power (during the discharge) is 360 MW, as much power as a small city uses for that instant.
Magnetic pinch technology is used for industrial metal forming. Thin tubes are crimped
or welded and metal sheet can be magnetically pressed into a form.