# Few questions about coil physics and capacitors.

Hello. Hopefully there's some smart electromechanical and physics guys on this board.

Long story short I have been working on a coil gun project for about a month now. Originally I was using disposable flash camera circuit, a simple push button, and a coil I took out of a fan. The coil itself is roughly 42 gauge wire, wound an inch long with about 14-18 layers. I achieved best results from this coil, about 4% efficiency, using a 200v 470uf capacitor and a magnetic projectile. However when I used a more powerful capacitor rated at 400v 820uf efficiencies were much lower. I realized the coil was limited and the capacitors energy wasn't being maximized.

So I bought some 16 gauge magnet wire to wind a new coil. I wound a coil 40mm long, 3 layers thick around a 1 cm pvc pipe. The coil works but once again there is a huge loss of efficiency at higher energies. So I need to wind a new coil. Now here are my questions.

1. I have read that a coil should be about 33% longer than the projectile. Is this accurate?

2a. Is there a magical equation to determine the number of layers a coil should have if all other factors are known?

2b. How many layers of 16 gauge wire should I use for a coil wrapped around a 1/4 inch barrel to maximize efficiency with a 400v 820uf capacitor? What about a 200v 1200uf capacitor?

3. Could someone please demonstrate how to convert farads into amps?

My main objective is an optically triggered multi stage coil gun with kinetic energy transfer of above 30 joules. I've moved away from flash camera circuitry and am now using dc to dc converters to charge the capacitors. I need to find out how to convert farads into amps to figure out what kind of transistors I need for the trigger switch. Any help is truly appreciated.

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