If we have a volume of hydrogen gas, when not heated, only collisions without much change in heat (say it is in room temp.)
If the volume is heated a bit, the collisions increase and the temperature of hydrogens in it also increase.
If even MORE heat is supplied, the electrons in the hydrogen leave the atom and get excited.
If WAY MORE energy is supplied, we might get nuclear fusion, (like what happens inside the sun),
the volume is constant and the pressure increases with temperature.
The question is:- What is the minimum energy required for an atom (say carbon (or) hydrogen) to undergo fusion and below the energy, it undergoes excitation and collision
heating is basically giving the system energy