My rail gun

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CycloWizard

Lifer
Originally posted by: FrankSchwab
Originally posted by: CycloWizard

Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.

And grant2 was trying to explain that you are wrong.

In your world, the gun would attempt to follow the projectile, because friction on the contacts will cause a force on the gun in the direction of travel of the projectile (it tries to drag it along). This has all kinds of interesting ramifications that are, umm, "not supported by experimental evidence".

/frank
You're only considering mechanical forces. You're right about the direction of friction (forces the gun forward rather than backward), though if these forces are of the order that I'm imagining, then it would appear as recoil when the setup returned to equilibrium. In essence, for a railgun with any sort of efficiency, the friction would have to be very low, so there should be very little 'recoil' in either direction.

RelaxTheMind

Platinum Member
Originally posted by: CycloWizard
Originally posted by: grant2
I don't like it either.

If you accelerate an object, using ANY force (mechanical, electrical, gravitational, magnetic), then an equal &amp; opposite force must be applied to some other object(s).

In the case of a projectile launcher, there are 2 practical possibilities for what that opposite force is applied to: propellant, or the launching mechanism. Since a rail gun does not use propellants, the mechanism must receive the force. This is what I believe is called recoil.

At least that's my very basic understanding of physics.
I was trying to explain that there are more than only mechanical forces. There are mechanical forces (the kind that you can easily see and understand) and electrical forces that are not really visible or tangible. As I said, the mechanical force imparted on the projectile counteracts the electromotive force. Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.

Force is force if you are trying to bend the lawas of physics good luck. If an object is being pushed one way with any type of force the object is pushing back equal and oppisite force. the mechanism is the propellant in this case. If you want o move an object without any type of propellant you may be talking about magic. Relative to the propellant the mechanism is actually moving backwards. Other factors would be air (yes air has friction), gravity trying to pull it to the bottom of the mechanism the "forces" holding the propellant in the mechanism.

Put it this way to put it in better perspective. If you raise the weight of the projectile (say twice the weight of the mechanism) the tendency of the mechanism to push itself backwards "recoil".

there really wouldnt have to be a propellant but would need to be there to make it worth while. The idea of a rail gun is an object getting propelled and accelerated along the rail. a bullet being shot into a tube at 400m/s thats going to accerate it to say 800m/s

agrrements/disagreements?

CycloWizard

Lifer
Originally posted by: RelaxTheMind
Force is force if you are trying to bend the lawas of physics good luck. If an object is being pushed one way with any type of force the object is pushing back equal and oppisite force. the mechanism is the propellant in this case. If you want o move an object without any type of propellant you may be talking about magic. Relative to the propellant the mechanism is actually moving backwards. Other factors would be air (yes air has friction), gravity trying to pull it to the bottom of the mechanism the "forces" holding the propellant in the mechanism.

Put it this way to put it in better perspective. If you raise the weight of the projectile (say twice the weight of the mechanism) the tendency of the mechanism to push itself backwards "recoil".

there really wouldnt have to be a propellant but would need to be there to make it worth while. The idea of a rail gun is an object getting propelled and accelerated along the rail. a bullet being shot into a tube at 400m/s thats going to accerate it to say 800m/s

agrrements/disagreements?

FrankSchwab

Senior member
CycloWizard - your ignorance is daunting.

Basically, magnetism exerts a body force (similar to gravity - no contact required) on the projectile. Since there is no contact between the projectile and the barrel, the projectile exerts no reactive force on the barrel itself.

OK, quick question - say you are holding a 1 kg. magnet in the palm of your hand, north up. Suspended above your hand is another 1 kg. magnet, north down. The opposing magnetic fields maintain a seperation between the two magnets.

How much weight do you feel in your palm? It appears that you are arguing that you would feel only 1 kg., since there is no contact between the floating magnet and your palm. If your friend pushed down on the floating magnet, would you feel it?

/frank

CycloWizard

Lifer
Originally posted by: FrankSchwab
CycloWizard - your ignorance is daunting.
I didn't know asshats would bother coming into this forum. If you want to tell me I'm wrong, you could do it nicely. Why be an ass? Case of penis envy?

Basically, magnetism exerts a body force (similar to gravity - no contact required) on the projectile. Since there is no contact between the projectile and the barrel, the projectile exerts no reactive force on the barrel itself.

OK, quick question - say you are holding a 1 kg. magnet in the palm of your hand, north up. Suspended above your hand is another 1 kg. magnet, north down. The opposing magnetic fields maintain a seperation between the two magnets.

How much weight do you feel in your palm? It appears that you are arguing that you would feel only 1 kg., since there is no contact between the floating magnet and your palm. If your friend pushed down on the floating magnet, would you feel it? [/quote]
In reality, I'm arguing the exact opposite. I'm stating that you do not require contact between bodies for there to be an interaction of forces. Numerous forces of this type exist and are called body forces. They act on the volume of the affected object, whereas more tangible forces act on surface areas.

The apparent 'weight' of the 1 kg magnet in my hand would depend on the magnetic field strength of both magnets. The net downward force would be equal to the gravitational force plus the magnetic repulsion force. In the case of the suspended magnet, the repulsion force acts in the upward direction, so it would be the gravitational force minus the repulsive magnetic force. The repulsion force would grow the closer the magnets become, so if someone pushed down on the top magnet, the downard force on the magnet I am holding would increase accordingly.

Now, if you could enlighten us on the more difficult ideas that are relevant to this thread, which is what I was trying to address in the post you quoted, it would be appreciated. As I've mentioned, I'm no expert in electricity and magnetism. I'm more used to dealing with fluids on a molecular level where electrical interactions are a result of polarity of molecules. Feel free to correct me if you can do so without unnecessary insults.

bobsmith1492

Diamond Member
Whoa now.... I'm just trying to ask a simple question here

By the way, a rail gun firing will exert force on both the bullet and an equal and opposite force on the launcher. I see no arguments; this is basic physics and no one has said otherwise.

So anyway, how does an electromagnet exert force on a nonmagnetic ferrous material? I'm looking for equations. So far, I know the force due to a current i through a wire of length l in a field of strength b is given by F = i lxb where x is crossing vectors l and b where vector l has the magnitude of the length l and direction of current i. The unknown is the field b which would be exerted by the ferrous material in question; I heard something about induced charges/induced currents which I suppose would create a magnetic field. Anyone know about induced currents and (mathematically) how they are formed in a solid object (cylinder) ?

CycloWizard

Lifer
Originally posted by: bobsmith1492
Whoa now.... I'm just trying to ask a simple question here

By the way, a rail gun firing will exert force on both the bullet and an equal and opposite force on the launcher. I see no arguments; this is basic physics and no one has said otherwise.

So anyway, how does an electromagnet exert force on a nonmagnetic ferrous material? I'm looking for equations. So far, I know the force due to a current i through a wire of length l in a field of strength b is given by F = i lxb where x is crossing vectors l and b where vector l has the magnitude of the length l and direction of current i. The unknown is the field b which would be exerted by the ferrous material in question; I heard something about induced charges/induced currents which I suppose would create a magnetic field. Anyone know about induced currents and (mathematically) how they are formed in a solid object (cylinder) ?
Might want to do a search for roller coasters. I know several of the newest, biggest, fastest use similar technology for their initial launch.

Found a simple way to calculate the exit velocity:
V=(L'*I^2*t)/(2*m)
V=velocity
L' = inductance of rods
I = current
t = time length of current pulse
m = mass of projectile

skene

Member
Figuring out the induced current on the projectile isn't just one simple equation. The changing electric field through the wire, ie turning the current on and off, creates the magnetic field. The creating and destroying of the magnetic field creates the induced current in the cylinder.

Honestly, the best thing is to go to a university or possibly high school library and check out some electricity and magnatisim books if you want get into the nitty gritty bits. Another great place to find info is http://scienceworld.wolfram.com/physics/ Check out the mangnetic field section in electromagnatisim. That site has TONS of info on it, and unless you've got a decent knowlege base to begin with it might be overwhelming. There really is no replacement for a good text book or teacher explaining theory.

Just out of curiosity, you say you just started magnatisim in class, are you in high school, college, what? In high school I thought of making a rail gun, but figured out a tesla coil was much easier, made just as much noise, and still scared half the judges in the science fair without shooting things at them. Just be VERY CAREFUL when playing with that much power. Once you get shocked by a car battery you'll learn that quickly...better to do that then with a microwave transformer.

edit: fixed a garbled sentence

bobsmith1492

Diamond Member
Skene:
I'm in my second year (college) studying electrical engineering and physics. I've had experience with building circuits and whatnot; I built a Tesla coil a couple of years ago but couldn't ever get it to run - probably something to do with the power supply and my crappy saltwater capacitors (Jones' Soda)
By the way, a car battery can't shock you - 12 volts?? come on now Mebbe if you put your tongue on it....

CycloWizard:
Well, we're doing inductance and capacitance calculations in circuits class now, so I'll see if I can calculate pulse length of an electromagnet coil which should be an inductor/capacitor circuit... that'll be hard without measuring its inductance first, but guesses can be made at least, heh. Also, when you said inductance of rods, what rods was it talking about and where did the equation come from?

- Thanks

skene

Member
probably something to do with the power supply and my crappy saltwater capacitors (Jones' Soda)
bob, there's your problem, you've got to use winebottles for your leiden jars!
And it was a 24V cell. I picked it up from a wreaking yard and have no idea what it came out of...probably a mac truck or something.

CycloWizard

Lifer
Originally posted by: skene
Figuring out the induced current on the projectile isn't just one simple equation. The changing electric field through the wire, ie turning the current on and off, creates the magnetic field. The creating and destroying of the magnetic field creates the induced current in the cylinder.

Honestly, the best thing is to go to a university or possibly high school library and check out some electricity and magnatisim books if you want get into the nitty gritty bits. Another great place to find info is http://scienceworld.wolfram.com/physics/ Check out the mangnetic field section in electromagnatisim. That site has TONS of info on it, and unless you've got a decent knowlege base to begin with it might be overwhelming. There really is no replacement for a good text book or teacher explaining theory.

Just out of curiosity, you say you just started magnatisim in class, are you in high school, college, what? In high school I thought of making a rail gun, but figured out a tesla coil was much easier, made just as much noise, and still scared half the judges in the science fair without shooting things at them. Just be VERY CAREFUL when playing with that much power. Once you get shocked by a car battery you'll learn that quickly...better to do that then with a microwave transformer.

edit: fixed a garbled sentence
The equation I posted doesn't include induced current. It's the current in the rails rather than in the projectile. Got it here.

Bob: I think the rods it's talking about are the rail circuits, but I could be wrong. The page isn't entirely technical. It's either the rails or the projectile, as I don't see how the inductance of anything else would really come into play. Of course, if you're an electrical enginerd you'd better figure this out and tell me! I'm a chemical superfreak, not a spark-chaser.

FrankSchwab

Senior member
Originally posted by: CycloWizard
I didn't know asshats would bother coming into this forum. If you want to tell me I'm wrong, you could do it nicely. Why be an ass? Case of penis envy?

Asshat? Ass? Penis envy? Frankly, I thought I had very nicely told you you were wrong. Perhaps I misunderstood your argument; if so I apologize. No reason for you to show us the seamier side of your vocabulary. BTW, if you want to be EFFECTIVELY offensive, you really need to put a bit more thought into your insults; what you've shown is pretty banal.

As an aside, I'm not envious of my penis; we're actually very good friends. Your preoccupation with the biological organs of excretion is a bit puzzling, however.

Since there is no contact between the projectile and the barrel, the projectile exerts no reactive force on the barrel itself. For this particular system, there is some small friction between the projectile and the barrel, so this will impart some recoil, though how much depends heavily on the specifics of the system. Ideally, you would impart all of the electrical energy on the projectile as kinetic energy and there would be no recoil. Obviously this isn't the case in reality.
Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.
In essence, for a railgun with any sort of efficiency, the friction would have to be very low, so there should be very little 'recoil' in either direction.

you seem to be arguing that some magical property of electromagnetism accelerates the projectile off the end of the rail without a corresponding force on the rail/barrel. This is ludicrous.

You appear to recognize that non-"visible or tangible" forces can still be intermediaries between two physical objects when you say:
The repulsion force would grow the closer the magnets become, so if someone pushed down on the top magnet, the downard force on the magnet I am holding would increase accordingly.
I have no idea how to reconcile this recognition of reality with your statements above. The truth of the matter is that every newton of force imparted to the projectile will also be imparted to the barrel or rail, directed in the opposite direction. This is recoil, and will exist in any scenario you can imagine, real-world or the frictionless world of freshman physics. I'm hoping that we can agree on this.

Once again, if I misinterpret your statements, I apologize; but I see no other way to interpret what you have written. This is the Highly Technical forum, and I have to believe that we can agree on the basics of freshman physics.

/frank

CycloWizard

Lifer
Asshat? Ass? Penis envy? Frankly, I thought I had very nicely told you you were wrong. Perhaps I misunderstood your argument; if so I apologize. No reason for you to show us the seamier side of your vocabulary. BTW, if you want to be EFFECTIVELY offensive, you really need to put a bit more thought into your insults; what you've shown is pretty banal.[/quote]
Yes, 'your ignorance is daunting' is a very nice way to put it.

As for the recoil argument, you've completely ignored every point that I've made. Maybe that's why you can't reconcile my arguments. I've repeatedly stated that there is a balance between electromotive forces and mechanical forces. Turns out I'm wrong, but not for the reasons you've mentioned. As I thought, it's much more complicated than anyone here thought. Link

DrPizza

I skimmed through the entire post... I didn't see anyone mention this, but it's important: eddy currents. IIRC, that's one of the big problems once you up the current and magnetic field. Resulting heat results in some massive melting and welding.

Anyway, I've seen something fairly similar to what you're describing in a physics demonstration 2 years ago. Instead of a cylinder, we launched magnetic washers in an induced magnetic field. It was pretty amusing - we could shoot them 20 feet or so into the air. However, we realized that any attempt to increase the current or voltage was going to ruin the launching device long before we reached the ability to shoot the magnet a more useful distance.

Hmmm... now that I think about it, I don't even remember if they were magnetic washers... they may have been plain washers. It was a cool demonstration, but at the time I thought the device would cost far more money and take far more time to build than I was interested in investing... I may have the plans around here somewhere.

edit: yes, I just researched it for a few minutes.. It's a ring launcher, very similar to rail gun afaik. Non-magnetic rings.

bobsmith1492

Diamond Member
Yes Dr. Pizza I think that sounds like what I was thinking of, but mine would be multi-stage which would provide for more overall power while keeping indivdual stage powers low. Of course I'm sure the launcher wouldn't do much more than one launch if it did work, so I'm hoping to figure out how to make it work and work well the first try.

Generic Moniker

Member

Rail guns require a return current path to produce the B-field and consequent lorentz force imparted on the projectile.

Projectile need not be magnetic, just highly conductive.

Calculating the force gets a little tricky because a lot of energy is lost through resistance and heating (otherwise you could take conservation of energy approach).

Essentially, you calculate the b-field with ampheres law, and then integrate over the area then multiply by the charge (current) passing through the projectile. [check you units to make sure that result is in Newtons]

bobsmith1492

Diamond Member
Ah, good; Ampere's Law is the topic of physics class today I guess it's about the same as Gauss' Law only with magnetic flux.

What do you mean, can't use a cylinder? The cylinder would be part of the current path - it actually connects the contacts that activate the electromagnet. I hadn't thought of it, but I suppose the current through the cylinder would create a field by itself. I guess I'd have to check that it flowed the right way to help propell the thing rather than hold it back.

Also, as I said before, it isn't technically a 'rail' gun; a cylinder closes two contacts which power up an electromagnet to pull the cylinder forward. Multiple stages would be used.

Generic Moniker

Member
LOL! You're making a big solenoid....not at all a rail gun.

Let me give that idea some thought.

bobsmith1492

Diamond Member
Exactly; multiple stage solenoidal launcher (R).

We discussed solenoids in physics class today, so I know how to find the field in the center of a solenoid, but how does that translate to a force on a metal object? That is the question of the hour.

grant2

Golden Member
Originally posted by: CycloWizard
I was trying to explain that there are more than only mechanical forces. There are mechanical forces (the kind that you can easily see and understand) and electrical forces that are not really visible or tangible. As I said, the mechanical force imparted on the projectile counteracts the electromotive force. Thus, this interaction of forces will not result in mechanical force (i.e. recoil) being imparted on the gun itself except due to friction at the contacts.

Let's say, hypothetically, you build a perfect rail gun. 100% of used energy is used to accellerate the projectile, and there is 0 friction or electrical resistance between any parts of this machine. This machine is mounted on frictionless coasters and placed in the middle of a flat surface.

Are you suggesting that when a projectile is fired from this hypothetical railgun, the gun itself will not move at all?? If so, please explain how this railgun/projectile system evades Newton's laws.

If this is a question of semantics (you seem to define "recoil" as "mechanical force"), then I shall defer to Mr.s Merriam &amp; Webster, who define recoil as: "to fall back under pressure".

Certainly I have never heard it implied that recoil cannot be caused by magnetic or "electrical" forces.

PlasmaBomb

Lifer
The induced voltage is given by the equation -

EMF = BLvsin(THETA)

Where B is field strength, L is length, v is the velocity of the object that you are moving through the field and sin(theta) is the angle you are moving through the field.

You only get an induced voltage if either the object is moving through (cutting) the magnetic field.