Two objects each carry positive charge and are brought near. What happens?
Kilogram for kilogram, fusion releases even more energy than fission: in the last lesson's cases, about 0.7% of the mass converted against about 0.1%. So why is there no fusion in everyday life? A struck match starts a chemical change in an instant. Nothing in everyday life starts a fusion.
In the fields topic you saw that like charges repel. In the particles topic you saw that the strong nuclear force acts only at very short range. Put those two facts side by side and they explain why fusion is so hard to start.
Every nucleus is positively charged, because of its protons.
So two nuclei brought near each other repel.
The repulsion is not fixed: it grows stronger and stronger as the nuclei approach.
The strong nuclear force can bind two nuclei into one, but it acts only at very short range: the nuclei must nearly touch.
In a hot material, the particles move fast. In a hotter material, they move faster. Enormous collision speeds therefore mean enormous temperatures.
Hydrogen nuclei fuse in practice only above tens of millions of kelvin, the scientific temperature unit.
Tens of millions of kelvin is far beyond anything in everyday experience; no flame comes anywhere near it.
So fusion's condition is set: collision speeds that need temperatures of tens of millions of kelvin. Who, or what, can afford temperatures like these? That question opens the next topic.
Two deuterium nuclei drift toward each other at low speed. Will they fuse?
Both nuclei are positively charged, so they repel each other.
As they approach, the repulsion grows stronger and stronger.
At low speed they turn back before the strong nuclear force, which acts only at very short range, can reach them.
No. The two slow nuclei turn back before they get near enough to merge.
The same two nuclei now collide at enormous speed. Why can they fuse this time?
The repulsion still grows as they approach; the charges have not changed.
But the enormous speed carries the nuclei closer and closer in spite of the growing repulsion.
Within the strong nuclear force's very short range, that force binds the two nuclei into one, and the fusion releases energy.
The speed carries them near enough for the strong nuclear force to bind them, so they can merge.
Two slow-moving hydrogen nuclei drift toward each other. What happens?
Two nuclei approach each other. What happens to the repulsion between them as the gap shrinks?
Ranya says: 'Two nuclei can merge only if they collide fast enough to get nearly touching.' Is Ranya correct?
Why does fusion happen only at extremely high temperatures?
In a sample of deuterium and tritium at a few thousand kelvin, the nuclei keep colliding gently. Why do these collisions produce no fusion?
Under which conditions do hydrogen nuclei fuse in practice?