Do dementors get cold feet?

Question: Do dementors get cold feet? Quick answer: Sure! Cast a patronus charm at them and away they scurry!

 

Follow-up question: Do dementors even have feet? Good question. I don’t know. When first introduced in Harry Potter and the Prisoner of Azkaban, the dementor is described as “a cloaked figure that towered to the ceiling… face completely hidden beneath its hood… a hand protruding… [was] glistening, grayish, slimy-looking, and scabbed, like something dead that decayed in water.” So we know dementors are tall and have hands. Dementors are never mentioned as walking, though. They glide. But do they have feet?

 

In Harry Potter’s first encounter with a dementor, it draws a “long, slow, rattling breath, as though it were trying to suck something more than air from its surroundings. An intense cold swept over… Harry felt his own breath catch in his chest. The cold went deeper than his skin… He was drowning in cold…” That’s the book’s description. There’s also a darkness about them and they can create a chill mist. Dark and cold. In the movie, dementors seem to be patterned after the Black Riders in Peter Jackson’s Lord of the Rings rendition. They are more skeletal than decaying. And the Harry Potter movies significantly accentuate the cold. Window panes frost up. Water freezes. The thermal energy of the surroundings goes down significantly when a dementor is present. That’s interesting, thermodynamically speaking.

 

Thermodynamic Analysis: If the environment is getting much colder, then thermal energy is being transferred (“heat”) from the environment to the dementor. Is it because the dementor is colder to begin with? If so, you’d expect spontaneous transfer (Zeroth Law of Thermodynamics) until thermal equilibrium is reached. But perhaps the coldness isn’t felt unless the dementor is actively trying to absorb energy (corresponding to human “life-force” in the books), i.e., when it draws its long breath which somehow doubles as a sensor. The dementors were supposed to be searching the Hogwarts Express for Sirius Black. So maybe it’s an active absorption of thermal energy, like when a vacuum cleaner is turned on and sucks in anything in its immediate environment.

 

Does the internal energy of a dementor increase when it takes in thermal energy from the environment? Thermodynamically the answer should be yes. Will its temperature increase? Will the dementor get hotter? Not necessarily. When ice absorbs energy at its melting point, there is no change in temperature. The absorbed energy goes to breaking some of the hydrogen bonds in water. There is no mention of anyone touching a dementor to see if it feels warmer. One might wonder whether it would feel colder to the touch, but it’s unclear why that should be. If a dementor was constantly keeping itself at a temperature much lower than the environment, it would have to actively pump out energy that would otherwise flow in because of the temperature gradient. Humph, this is all sounding contradictory!

 

What if a dementor had feet? And these feet touch the ground. If the ground gets cold in the presence of a dementor, would the dementor then get cold feet? Or are its feet well-insulated from the ground to reduce any transfer of thermal energy? If dementors glide above the ground, then a cushion of air could insulate it to some extent, but it might still get cold feet because thermodynamics doesn’t care who you are, magical or not. Could the energy that the dementor sucks in travel to its feet to keep them warm? Possibly. Dementors might be thermodynamic-heat-engines of a different sort.

 

Final answer: I don’t know, but I would guess, partly yes. And I wouldn’t have thought about this if not for the movies accentuating the coldness of the environment in the presence of a dementor. Who would’ve thought that dementors could be interesting thermodynamically?