# How to calculate capacitance?

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#### oneslowz28

##### New Member
I am trying to figure out how much energy is stored in my capacitor bank I am making for my coil gun. I have 30 330v 80uF photo flash capacitors wired in parallel.

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#### dknguyen

##### Well-Known Member
THe formula for energy stored in a capacitor is:

E=0.5CV^2

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##### Banned
Keep in mind the discharge of that energy is limited by the resistance of the circuit. The timeing get's a little tricky because if the pulses lasts too long it will push the load forward and then pull it back.

#### oneslowz28

##### New Member
I have had it together already. With a coil 1.2" long and 8 layers of 28 gauge magnet wire each insulated with scotch tape, I can shoot a 2" 16guage finishing nail about 50 feet. The barrel the coil is wound on is a normal white bic pen tube. It will also penetrate a soda can @1' away. I am just trying to figure out how much energy I am using for each shot. So by my calculations I have a 330v 2400uF capacitor bank and about 100 joules.

##### Banned
100 joules available, NOT delivered to the load.
Weigh the nail and determine it's impact velocity and you can calculate the EXACT joule value delivered to the projectile. I'm sure you'll find it's much less.

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#### user_88

##### Member
Energy of a Projectile

Another projectile energy measurement technique is to take a reasonable sized block of wood, weigh it, and suspend it as a pendulum in front of the coil gun.
The idea is to have the nail stick in the wood block when it is fired.
When you fire the gun, you should be able to observe, or at least estimate, the angle θ,from a vertical line, when the pendulum is deflected upward. Then use the formula for potential energy:

PE= WR(1-cos θ) units of your convenience ...

Where R is the length of the cord or string, and W is the weight of the object.
A little more accurate version would be to include the weight of the nail, along with the weight of the wood block, for the variable W.

Basically, you are converting the kinetic energy of the moving nail into potential energy ..... by raising the block vertically

n.b.: 1 ft-lbf=1.356 J

Maybe get a piece of balsa wood as the target ... a little easier to stick the nail.

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