The fireball spell is clearly described as a sphere with a 20' radius.
In most case, that is applied at ground level since that's where most creatures are. (We can actually see an example of the area in: 20 foot square versus a 20 foot circle on a battle mat.)
If I think in terms of the law of physics, I would assume that for a fireball cast with its center on the ground, all the energy which is expected to go down has to go somewhere else. Similarly, if casting that same fireball in a small room or a corridor, wouldn't that affect people further than 20' radius around?
How does this actually work according to the rules of the game though?
Are we expected to just lose all the extra area of effect, or does the fireball expand outward to fill its full normal volume? So in a corridor, would you only affect 40' diameter and the extra energy that would create the rest of the sphere just never materializes (the effect is blocked by the walls of the corridor), or do I calculate the volume and fill that much space in the nearby corridors?
Specifically, I'm interested in understanding how to interpret the rules in this case.