Energy From Atoms I

***Energy From Atoms I*** is the sixteenth lecture within the **Properties of Matter** subtopic of PH1011. It covers an introduction to the four fundamental forces, binding energy, and the processes of fission and fusion in terms of energy.

Previous: Radioactivity II

Next: Energy From Atoms II

## Content

### Fundamental Forces

* Gravity

Gravity is the force which causes masses to be attracted to one another. It has the weakest relative strength of the forces but acts over the longest distance (infinity), and is carried by the graviton.

* Electromagnetic

The electromagnetic force is responsible for holding atoms and molecules together. Its relative strength is between that of the weak and strong forces, and it is carried by photons.

* Weak

The weak force is actually stronger than gravity - but acts over a far shorter distance, and controls how protons and neutrons behae in quark conversions, enabling β decay. It is carried by intermediate vector bosons.

* **Strong**

The **strong** force is what prevents the positive charges of protons in the nucleus from repelling one another. It is the strongest force but only acts over the distance of the diameter of a nucleus (~10<sup-15m), and is carried by gluons.

### Binding Energy

Binding energy refers to the energy stored by a stable nucleus. Due to ***E=mc2***, any change in mass must cause a change in energy - an in the event of fusion or fission of atoms, there is almost always a change in overall mass (or in other words - nucleii are observed to weigh less than the sum of their components). This energy is called the nuclear binding enercy **ΔEbe** and is given by:

***ΔEbe = (Σmic2) - Mc2***

The binding energy per nucleon is given by ***BEN = ΔEbe/A***, A being the number of nucleons present. The energy released in a reaction can be given by ***Q = BENfinal x (nfinal) - BENinitial x (ninitial)***.

In reference to the graph - the higher an atoms' BEN is, the lower the mass of each nucleon. Nuclei such as iron are more tightly bound than those at either side; they exist in the most engergetically favourable form, with great stability. Nuclei on either side would achieve greater stability by being closer to this peak - and therefore will release energy if they move toward it via fission or fusion.

### Fission

Nuclear fission is the splitting of one heavy nucleus - usually uranium - via an incident neutron. This neutron causes the uranium to become a larger, more unstable isotope, which quickly decomposes into two smaller atoms alongside several neutrons and energy.

* **Example question:** How much energy would fission of 1g of a material with A = 240 and BEN ~=7.6 MeV yield?
* if the material splits into two equal smaller atoms (240E => 2x 120Q), then -
* Q = 2x 120x 8.5MeV - 240x 7.6MeV
* = 216 MeV
* For 1g -
* 1/(240g mol-1) x 216x106eV x 6x1023mol-1 x 1.6x10-19J eV-1
* = 8.6x1010J

(values of MeV are read roughly from graph of BEN)

### Fusion

Fusion is the opposite end of the curve; energy is created by the joining of two atoms to form one heavier atom. This is much more difficult to achieve, as both individual nucleii must have enough energy to overcome their coulomb barriers to allow the strong force to take effect. In order to get this much energy, very high temperatures and pressures must be present - in the range of >100x10<sup6K. Fusion could in theory allow much cheaper power than fission (as hydrogen can be produced easily from water, whereas uranium doesn't exist in large quantities) and with lower accident rates than fission, however is currently not feasible.

* **Example question:**
* 2H + 3H => 4He + n
* BEN 2H = 1.1MeV; BEN 3H = 2.8MeV; BEN 4He = 7.1MeV
* Q = 4x 7.1 - 2x 1.1 - 3x 2.8 = 18MeV
* 1g of a mixture of the H isotopes would therefore yield:

1/(4g mol-1)x 6x1023 x 18x106eV x 1.6x10-19J eV-1

* = 4.3x1011J

In comparison to fission - per atom fusion releases less energy, but per given weight it releases far more.

## Summary

There exist four fundamental forces; **strong**, *weak*, EM and gravitational. These hold everything in the universe together, and are particularly important here for being responsible for holding nucleii together. Splitting or combining nucleii can release large amounts of energy as the binding energy changes; a good power source if used correctly.