Why is a pressure canner needed when canning?

Physics stops you from heating up liquids that consist of mostly water to temperatures above (roughly) 100 C.

The temperature of your heating element can be set higher, but neither the temperature of the water bath nor the liquid in your jars can go higher than the boiling point where water changes from liquid to vapor - which is 100 C at normal pressure ¹. To raise the temperature, you have to increase the pressure, so that the new boiling point is at or above the temperature required for safe canning. A pressure cooker may be suitable for the purpose, provided the model is designed for the specific pressure and allows setting/reading the specific pressure or alternatively, temperature. Pressure canners on the other hand are specifically designed for that purpose and marketed as such.

——

¹ Note that 100 C is correct only at sea level and given a few more constraints. At higher altitudes the boiling point may be significantly lower. For the answer here, 100 C is close enough. (Or far enough from safe canning temperatures for non-acidic items.)

I like to explain the physics this way: heat is a thing, temperature is a place. If you put a hot thing next to a cold thing, heat stuff will flow out of the hot thing into the cold thing. This will cause the cold thing's temperature to rise, and the hot things's temperature to fall, unless other factors interfere. How far and how fast these temperatures move depends on the material: dense things like iron can absorb a lot of heat but temperature rises slowly. Light things like air gain or lose temperature quickly as heat moves.

For a stove burner, you have continuous input of more heat, so the burner's temperature will not fall. The liquid in the pot is mostly water, which can absorb a lot of heat and its temperature will rise--to a point. What interferes is called a phase transition (what you and I call boiling). At this point, further heat energy going into the water is used to turn it into steam, while the remaining water stays in place (this place is about 100C at normal pressure).

Putting the water under pressure changes the place at which this transition happens, and so the water can absorb more heat, going to a higher temperature, before turning into steam.

The physics behind why you can't heat liquid water past its boiling point is defined as when vaporization pressure equals atmospheric pressure. So, putting your canning water in an oven, which is at atmospheric pressure, will result in the water getting no hotter than if it were on the stove, which is also at atmospheric pressure, because the energy required for water to change from liquid to gas is the same for both situations.

Why you need to use a pressure cookers is shown in The Ideal Gas Law: PV = nRT. Rearranging the equation for your situation gives: P = (nR/V)T and noting that (nR/V) is a constant, we can rewrite it as P = kT, which indicates that increasing the temperature will increase the internal pressure of the pressure cooker, thus increasing the energy required for a water molecule to become vapor. Therefore, liquid water can become hotter in a pressure cooker, which is what you need to ensure sterilization!