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Author Topic: Why do explosives produce a bigger bang when bound up?  (Read 3582 times)

Randal

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Randal asked the Naked Scientists:
   Firstly, thanks for saving my sanity. I'm an Aussie in China and your
podcasts
are the highlight of my week.  It is great to hear English but even better to hear about interesting science stuff.

Can you tell me why an explosive will produce a bigger bang if it is bound up with tape or something similar please? This seems a bit counter intuitive to me but I am a beginner with physics but keen to know more.

Thanks again for a fabulous program....

Randal

What do you think?


 

Offline Soul Surfer

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Why do explosives produce a bigger bang when bound up?
« Reply #1 on: 06/02/2009 18:23:37 »
Because they build up a lot of pressure (that is energy) before the container disrupts.  Gunpowder on its own just burns very quickly without an explosion but on a cardridge propelling shot it builds up pressure and accelerates the shot down the barrel of the gun.

So called High explosives are different.  Their burn rate once detonated is extremely quick and even without containment these can damage materials with the shock waves of the detonation.  Cutting charges used for demolition are like this.
 

Offline lightarrow

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Why do explosives produce a bigger bang when bound up?
« Reply #2 on: 07/02/2009 10:57:08 »
In some (maybe most) cases there is also another reason: the increased pressure (and the increased temperature because of the increased pressure) generated inside the chemicals by virtue of the confinement, increases the reaction's speed.

The "Bang" that you hear does not depend only on the energy released by the reaction, but also on the speed at which such energy is released: if you move 10cm of air for 1 m in 10 seconds, you won't hear anything (the air's pressure is low); if you move it in 0.01 seconds, you will hear a loud "Bang" (the air pressure is very high).
« Last Edit: 07/02/2009 11:06:11 by lightarrow »
 

Offline randalf

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Why do explosives produce a bigger bang when bound up?
« Reply #3 on: 14/02/2009 07:47:07 »
Thanks for helping me understand this.....but further to my question, how come you can burn a small quantity or C4 and it will burn with a bright light like magnesium but if you try to burn a big quantity you will have a huge expolosion?
 

Offline Chemistry4me

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Why do explosives produce a bigger bang when bound up?
« Reply #4 on: 14/02/2009 07:51:05 »
C4 does not explode without a primer explosion. But it does burn rather well.
 

Offline Chemistry4me

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Why do explosives produce a bigger bang when bound up?
« Reply #5 on: 14/02/2009 07:56:19 »
Quote
C-4 is also well known for its durability and reliability. It will not explode even if shot, punched, cut, or thrown into a fire. The only method for detonation is a detonator or blasting cap.
A detonator is a device used to trigger bombs, shaped charges and other forms of explosive material and explosive devices.
A blasting cap is a small explosive device generally used to detonate a larger, more powerful explosive such as dynamite.

http://www.nationmaster.com/encyclopedia/C4-explosives
 

Offline lightarrow

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Why do explosives produce a bigger bang when bound up?
« Reply #6 on: 15/02/2009 10:49:07 »
Thanks for helping me understand this.....but further to my question, how come you can burn a small quantity or C4 and it will burn with a bright light like magnesium but if you try to burn a big quantity you will have a huge expolosion?
I don't know how the reaction depends on pressure, temperature ecc. in the case of C4, but I assume that its speed is strongly dependent on the pressure and/or the temperature: when you burn it, the reaction proceeds at low speed, probably because the heat/pressure generated is not high enough to increase the temperature/pressude inside all the material at the same time; you could verify if it depends more on the temperature by putting a small quantity inside a microwave oven, which heats up all the points of the material quite at the same time: if it explodes then it's temperature inside, the critic parameter; if not then it's pressure; the trick is always to make the reaction happen (almost) simultaneously in all the points of the material, then you have an explosion.
 

Offline Bored chemist

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Why do explosives produce a bigger bang when bound up?
« Reply #7 on: 15/02/2009 12:47:17 »
"you could verify if it depends more on the temperature by putting a small quantity inside a microwave oven, "
Don't try this at home.
 

Offline lightarrow

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Why do explosives produce a bigger bang when bound up?
« Reply #8 on: 15/02/2009 18:13:10 »
"you could verify if it depends more on the temperature by putting a small quantity inside a microwave oven, "
Don't try this at home.
Of course. However, if one have C4 (a military explosive, they certainly don't sell it on shops), I assume he should be someone who knows well what he does... ;)
 

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Why do explosives produce a bigger bang when bound up?
« Reply #8 on: 15/02/2009 18:13:10 »

 

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