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Back around the turn of the century -- from the 18th century to the 19th century -- there was a fellow named Count Rumford. Well, his real name was Benjamin Thompson, but he was an Loyalist in Massachusetts and he found it prudent to leave when Gates evacuated Boston, the alternative being not too pleasant.
Count Rumford was a odd fellow. Most people didn't care for him very much, apparently. And evidently he didn't care much for them, either. But he was very good at organizing, which got him his title. He re-organized the Bavarian army and changed it from a demoralized entity into a first-rate army.
Rumford was also good at scientific observations. He was especially interested in heat.One of his major accomplishments was to greatly improve the efficiency of the common fireplace. He also determined that heat is not a fluid.
Rumford observed that if two equivalent cannons were loaded with equal charges of gun powder, but one is loaded with an appropriate cannon ball while the other is loaded with a blank (something to keep the powder in place), that the cannon firing the blank will get hotter than the cannon firing the cannon ball.
The question: Why blasting out nothing would make things hotter than actually doing something like throwing out a heavy cannon ball?The answer: The gunpowder contains a certain amount of energy. If there are equal amounts in each cannon and one cannon uses a lot of that energy to throw the cannon ball, then it has less energy available to be converted to heat. Nearly all of the energy in the gunpowder in the cannon firing the blank goes into heat. Thus the cannon firing the blank gets hotter.
Maybe a factor, I don't think that's the prime reason. Instead, I think the heat was transfered to the cannon ball, which left when fired.
Right?
Thus, if you take the kinetic energy held by the swiftly moving cannon ball, it would make sense that the heat energy would be less.
In other words, you are both right, but your friend is "more" right. The cannon ball took the heat, but only in the conversion to kinetic energy.
When the cannon ball stops suddenly at the end of its flight, that kinetic energy will be converted back into heat energy.
What me strikes about this story is that it is 200 years old when there were no good measurement devices yet and where people had also no understanding of the dynamics of exploding substances. The truth is, that this is still an area that is not fully understood and where testings in laboratories take place on a regular base. Blow-by heat transfer is influenced by many factors, where the absence of a cannon ball is just one of them. With current high-speed high-power cannons, it is even more an issue than it was 200 years ago.
Construction of the cannon tube, lining, wear, venting holes and the shape of the projectile do all highly influence the heat transfer to the gun.
If you want to know more about this subject, you could look at the articles on this authorative site [google.com].
Thanks lammert for backing me up on this. You can count on me sharing your thoughts with my friend.
The assumption that the kinetic energy absorbed by the cannon ball is the reason that the loaded cannon gets less hot than the empty one is a strange one. It assumes that a significant part of the energy of the explosion is transferred to kinetic energy instead of temperature increase of the cannon and ball, where it is much more likely that the largest amount of energy is just present as heat in the gas itself...
If the heat of the expanding gas is transferred to the projectile as kinetic energy at only 10% efficiency (which would be very low indeed) then the law of conservation of energy would suggest that barrel temperatures would be 10% less (ignoring the effects of friction between the barrel and projectile).
I would agree that the largest amount of energy is in the heat in the gas. The function of the barrel is to convert that heat energy into kinetic energy. In effect, the barrel is a transducer. It converts energy in a way analogous to a speaker converting electrical into mechanical energy.
Studies of barrel wear in modern high velocity weapons may, or may not be applicable to the relative low pressures and velocities obtained from black powder. The instantaneous temperatures are much higher, the frictional forces between the projectile and barrel are much higher, and the velocity of the gas flow is much higher.
Fire a couple of rounds out of a modern firearm and a black powder weapon of similar calibre and you will see a marked difference in all the above.
To assert that the conversion of heat into mechanical energy is "strange" strikes me as strange. f=ma2, or e=mc2. It is all the same. The total amount of energy remains the same. Accelerate an 8 pound iron ball to 1000 fps and you have moved 124378.11 foot-pounds of energy (or, 168634.07 joules) down range.
It seems to me that you have to account for that energy somewhere.
eg:
an Loyalist
a odd fellow.
I appreciate the thoughtful replies. Makes the case for why it is always good to build <whatever> to make certain it works, not just buy the <whatever> on specs.
