# An atomic explosion - more energy out than in?

1. Dec 5, 2007

### johnfullerroot

A nuclear warhead for example doesnt seem to take a lot of energy to make, however it outputs massive amounts of energy in an instant. Why isn't this more energy out than in and why doesn't it break the law of conservation of energy?

I think the answer is going to have something to do with the energy required to make the small amount of matter used in the reaction in the first place. I was hoping someone could explain it to me.

2. Dec 5, 2007

### Astronuc

Staff Emeritus
The fission reaction releases the nuclear energy within the nucleus of particularly unstable nuclides, usually U-235 or U-239. In the fission process U-235 + n -> U-236* and Pu-239 + n -> Pu-240*. In nuclear weapon, the neutrons have relatively fast energies on the order of 1 MeV. The fission reaction releases about 200 MeV of energy as the fission nucleus breaks into two more stable nuclei.

See http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2

3. Dec 5, 2007

### pixel01

The law of energy conservation is still valid. Of all elements, Fe is the most stable atom, all heavier or lighter atoms have a tendency toward Fe, because they have more energy than Fe atom. In a star evolving cycle, there are times different elements are created ranging from He to U or whatever (the last in Mendelev table). So a nuclear warhead gives of massive amount of energy, yes, it all comes form the star long before.

If you ask: where did the energy of a star come from, may be it's from the BB, before that nobody knows.

4. Dec 5, 2007

### dst

Same as with burning petrol. Energy out is far greater than energy in - or is it? The energy comes from what has been already stored in the nucleus' bonds. With fission, that energy is released.

An analogy - imagine you had a 40km high wall on Earth. Place a ball on top of that, so it is barely balanced and needs 0.0001J to let it roll off. Next, a toddler comes and pushes that ball off with just that amount of energy. But, the ball releases its stored potential energy which is obviously a ridiculous amount in comparison to the original.. Some things just need a little push, like enriched uranium.

I think people on here overdo simple questions. As seen above...

Last edited: Dec 5, 2007
5. Dec 5, 2007

### rbj

i just want to point out, that's 200 MeV, is per atom, right Astronuc?

"an MeV here, another MeV there, ... pretty soon that adds up to real money.. errr, energy."