Fun fact: if you make two cups of coffee, one with milk and one black, which will cool faster?
Counterintuitively, the black coffee cools off FASTER.
(Yes adding cold milk 🥛 to hot coffee ☕️ does cool it off somewhat but the question is about the RATE of cooling. Also the ☕️ will reach room temperature first.)
But WHY is this?
Buckle up for some cool morning physics!
(Yes adding cold milk 🥛 to hot coffee ☕️ does cool it off somewhat but the question is about the RATE of cooling. Also the ☕️ will reach room temperature first.)
But WHY is this?
Buckle up for some cool morning physics!
There are two reasons:
1. Dark objects radiate heat faster (this has to do with the Stefan Boltzmann law & black body radiation)
2. Viscosity slows down evaporation (milk/cream thickens the coffee; evaporation is an important mechanism of cooling)
We’ll talk more about each…
1. Dark objects radiate heat faster (this has to do with the Stefan Boltzmann law & black body radiation)
2. Viscosity slows down evaporation (milk/cream thickens the coffee; evaporation is an important mechanism of cooling)
We’ll talk more about each…
1. Darker objects absorb & radiate heat faster than lighter objects.
This is why a black asphalt road is hotter than a concrete sidewalk during the day, and why the same road is cooler than the sidewalk after a cold night.
It’s also why a Crookes radiometer spins
This is why a black asphalt road is hotter than a concrete sidewalk during the day, and why the same road is cooler than the sidewalk after a cold night.
It’s also why a Crookes radiometer spins
The Stefan–Boltzmann law tells us that the power radiated by an object depends on its
A surface area
ε emissivity
T temperature
(σ is a constant)
Emissivity (ε) depends on the color of the object. A perfect black body has ε = 1 & a perfect reflector has ε = 0.
A surface area
ε emissivity
T temperature
(σ is a constant)
Emissivity (ε) depends on the color of the object. A perfect black body has ε = 1 & a perfect reflector has ε = 0.
Adding milk/cream 🥛to coffee ☕️ changes the color and alters the emissivity. This slows down radiation of heat.
2. The other factor is viscosity.
More viscous fluids evaporate slower. Evaporation is an important mechanism of cooling (that’s why we sweat btw).
Adding milk/cream to coffee increases viscosity of the coffee and slows evaporative cooling. This makes the coffee cool slower.
More viscous fluids evaporate slower. Evaporation is an important mechanism of cooling (that’s why we sweat btw).
Adding milk/cream to coffee increases viscosity of the coffee and slows evaporative cooling. This makes the coffee cool slower.
If you ever look inside electronic devices you may see another great example of this: heat sinks.
These have a large surface area and are black to optimize radiation of waste heat.
These have a large surface area and are black to optimize radiation of waste heat.
If you like this stuff, I highly recommend reading @ModCuisine.
Great cookbook and blog with fun stuff like this:
modernistcuisine.com/mc/the-physics…
Great cookbook and blog with fun stuff like this:
modernistcuisine.com/mc/the-physics…
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