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Help with a coil gun

  1. Mar 10, 2012 #1
    I am making a coil gun for my high school physics project and I am running into some issues.

    I am using 400 turns of 36 gauge wire wrapped around a 1.25in diameter PVC pipe to propel a 1 inch diameter steel ball.

    I started with a 6v battery but that didn't move the ball at all even with the help of a slight incline.

    Then I taped together around 40 AAA batteries together in series but that only caused the ball to roll 2 inches down the pipe and I need to launch the ball 3 meters.

    The next step up I am considering is just plugging the coils directly into the wall but I am not sure how the coils would work if the magnetic field were constantly switching.

    How would the math work out with an AC current through the solenoid and any pointers on power sources or increasing the efficiency of the coils?
     
  2. jcsd
  3. Mar 10, 2012 #2

    vk6kro

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    A solenoid like that can only attract a steel ball, so you would have to put the ball just outside the solenoid and then remove the power when the ball reaches the center of the solenoid.

    Or, you could attract a piece of soft iron which then hits the ball.
    This may be better than using a steel ball which will tend to get magnetized.

    DO NOT plug the coils into the mains. That is extremely dangerous.

    Also, the resistance of that wire may be a problem. I estimate that the coil will have a resistance of about 54 ohms, so, with a 6 volt supply, the current will only be 110 mA.
     
    Last edited: Mar 10, 2012
  4. Mar 10, 2012 #3
    What I am doing is attracting the steel ball until it reaches the coil and then turning it off.

    Why would magnetization be a bad thing on the steel ball and why would using mains power be dangerous when I already used 60 v dc without incident?

    Also, on Wikipedia I read that field is proportional to N*I so if i use a thin wire with high resistance but also a high number of turns, wouldn't that produce the same field while saving energy according to P=VI?

    Additionally, how do I strip 36 gauge wire? I have been burning it off with a barbecue lighter but I'm not sure that is the best or the most healthy way to go about it.
     
    Last edited: Mar 10, 2012
  5. Mar 10, 2012 #4

    vk6kro

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    If the ball was magnetised, the effect next time you used it would depend on the orientation of the ball.

    If the ball was positioned so that, say the north pole was pointed towards the coil and the coil was producing a north pole in that direction, then the ball would be repelled instead of being attracted.

    why would using mains power be dangerous when I already used 60 v dc without incident?

    Because 120 volts AC can kill you.
    Anything over 40 volts is considered hazardous, even from dry cells.

    Yes, the field depends on the product of current and turns, but 400 turns is not a big coil and your coil has so much resistance that you can get very little current.

    You could try a half inch ball. This would have about 1/16th of the weight of the one you are using now so it would be easier to accellerate..

    You could get more effect if your wire turns were wound on top of each other (ie multilayers) instead of wound along the pipe.
    If the wire is wound along a pipe, the more distant turns will have very little effect on the ball.
     
  6. Mar 10, 2012 #5
    The projectile is fixed because it is provided by my teacher when we run our devices in class so I cant change anything about that although the class ball looks darker than mine so it could be made of iron.

    I am already using multilayers of wires and my coils are about an inch long with 400 turns.

    If I add more turns, I get more resistance so adding or subtracting turns should do nothing to the field strength at this point right?

    I only need to get a bit less than 1 joule into the ball which seems like it would be a miniscule amount of energy to my 40 battery chain.

    Is there any other way to proceed other than to increase the voltage further into the danger zone?

    Also, does voltage matter in any of this or can I make 40 1Ω loops bunched together to get the same results?

    And on what dimension is the magnetic field acting on the steel? the length, area, volume, mass or something else?

    And would AC wall power be safe if I carefully heatshrinked or otherwise insulated every single connection and what would the results on the magnetic fields? Would it counteract the magnetization of the steel by by rapidly switching?
     
    Last edited: Mar 10, 2012
  7. Mar 10, 2012 #6

    vk6kro

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    I think you would gain a bit if you could find the center of the coil and split the coil into two coils and connect them in parallel. You would have to phase them correctly, but the exact center wouldn't be essential.

    Assume the present coil is 400 turns and has a resistance of 50 ohms and you only have a 6 volt supply.

    The current into the full coil will be 6 / 50 ohms or 120 mA so the product of turns and current will be 400 * 0.12 or 48 ampere turns

    If you split the coil in half, each coil will have 25 ohms resistance so the current from 6 volts would be 240 mA.
    So, the product will be 200 turns * 0.24 or 48 ampere turns.
    But there are two such coils, so the product will be 96 ampere turns. A good improvement.

    If you could borrow a set of digital scales (from a Chemistry Dept perhaps?) you can get a bit Scientific about this.

    You put a piece of wood or polystyrene foam on the scales. (this is to lift the ball well away from the scales which also work magnetically.).
    Put the ball on the polystyrene.
    Mount the coil above the ball.
    Zero the scales then apply current to the coil.
    There should be a negative change in the weight of the ball as it gets lifted up by the coil's magnetic field.

    Now, any changes you make to the coil will produce a measurable effect on the scale reading.

    So, you could try putting soft iron nails around the outside of the coil, for example to see if this helps. Just one nail should make a difference to the scale reading.

    We don't discuss dangerous practices here, but I invite you to calculate the power dissipated in that coil if you were to apply mains power to it. (power = 120 * 120 / 50 watts). Just don't do it.
     
  8. Mar 10, 2012 #7
    Ok, now I'm thinking of using a 12v pc power supply as it is more "renewable" than batteries.

    My question now is if field is constant given with a given voltage and wire thickness, how does conservation of energy kick in and prevent me from using a coil with millions of coils and minimal drain on my source while providing the same force?

    What is a good number of coils at 12v and 36 gauge so that I don't blow out my power supply and not run into any issues regarding conservation of energy?

    Thanks for all the guidance, you have been really helpful and informative today :D
     
  9. Mar 10, 2012 #8

    vk6kro

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    We didn't get anythng for nothing in the splitting of one coil into two.

    The current started at 120 mA for the whole coil and it went to 480 mA for two coils.

    So, the Law of Conservation of Energy is safe. (phew!)

    I notice that the maximum current for 36 gauge wire is 210 mA, so you could use this as a guide for winding coils.
    See this chart:
    http://www.powerstream.com/Wire_Size.htm
    the resistance is given as 414.8 ohms /1000 ft or 0.4148 ohms per foot.
    So, if you had a 12 volt supply, you could work out the resistance needed to limit the current to 210 mA and then from the 0.4148 ohms per foot figure, work out the total length of wire per coil to get that resistance.

    Keep a note of these calculations and put them in your report, even if the ball doesn't fire very far.
     
  10. Mar 11, 2012 #9
    No I mean how do the fields conserve energy.
    For simple math, if i had a pipe with circumference of 1000 ft and a source of 414.8v, if i have 1 turn, the current will be 1 amp and i will have a field of 1 amp-turn.
    If i have a coil with 2 turns, the resistance will be doubled and the current will be halved so i will have 1/2 amp and 2 turns for the same 1amp-turn of field.

    The second coil woulds seem to produce the same field with half the energy. Maybe this is because of less waste heat but if i get a million coils with 1/million amps, this obviously does not have the power to accelerate a ball.

    Does the field collapse when it comes in contact with an object to pull in this case or will it act like a motor under load and increase current draw untill the energy requirements are met?
     
  11. Mar 11, 2012 #10

    The Electrician

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    Have you done any searching on the web for info about coil guns? Such as:
    http://wiki.4hv.org/index.php/Coil_gun

    I built a simple coil gun that can launch a metal object across the room. It works by using an SCR to apply a single half cycle of the power line across a coil of about 40 turns of 12 gauge wire. The current is about 300 amps for a magnetic field of about 12,000 amp-turns. That's about what you will have to achieve to meet the launch objective.

    If you connect a coil to the line without some kind of mechanism to terminate the current after a single half cycle, when the current reverses the ball will be attracted backwards. Also, by only conducting a current from the line for only a single half-cycle, the current pulse is over quickly enough that the coil will not be overheated and damaged. I don't recommend this method for your project.

    A better way for you would be to charge a large electrolytic capacitor to a fairly high voltage and discharge it through the coil with a hefty switch. Your coil would need far fewer turns of heavier wire.

    You can find details about some of these considerations on the web.
     
  12. Mar 11, 2012 #11

    vk6kro

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    I don't see any conflict. It is just amps times turns.
    You can have 500 amps and one turn but you have to find 500 amps somewhere and wire thick enough to carry it..
    Or you can have 500 turns and 1 amp.
    The limit may be how much wire you can fit in the space available or how much current the wire can carry or what current the power supply can deliver

    The ball gets a temporary magnet induced in it and this is attracted to the magnet formed by the current in the coil.
    A steel ball is not a good choice for inducing a temporary magnet and a piece of soft iron would be better.
     
  13. Mar 11, 2012 #12
    But the second case uses 1/500 of the energy from the power source to get the same effect?
     
  14. Mar 11, 2012 #13

    berkeman

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    Although this discussion is about a low-power coil gun, I'm not comfortable discussingt coil guns on the PF. It comes too close to violating the PF rule concerning dangerous activities. I'm going to close this thread for now.
     
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