Pressure CookerScience brain teasers require understanding of the physical or biological world and the laws that govern it.
Most people know that it takes longer to boil an egg at higher altitudes. If instead of boiling an egg in the traditional manner, however, you used a pressure cooker at the higher and lower altitudes, would it:
a) still take longer to cook the egg at higher altitudes,
b) take the same amount of time no matter what the altitude is,
c) take longer to cook the egg at lower altitudes, or
d) the question can't be answered without knowing what the pressure setting on the cooker is.
Ignore the fact that the egg might be crushed by the pressure inside the cooker.
If you're not sure why it takes longer for an egg to boil at higher altitudes, see the hint.
HintThe lower the pressure over a liquid is, the lower the boiling temperature of the liquid is. In other words, at sea level water boils at 100 degrees Celsius. At high elevations, where there is less air pressure, water might boil at 90 degrees Celsius. Since at higher altitudes you're cooking an egg at a lower temperature, the egg will take longer to cook.
AnswerThe answer is a) it would still take longer to cook the egg at higher altitudes. This is because a pressure cooker doesn't control the absolute pressure inside the pot. It controls the pressure DIFFERENCE between the inside of the pot and the outside.
In other words, if the atmospheric pressure is 15 psi and a pressure cooker is set to run at 15 psi, then the absolute pressure inside the pot will be 15 + 15 = 30 psi. At higher altitudes where the atmospheric pressure is say, 13 psi, the pressure cooker will run at 13 + 15 = 28 psi. Since the pressure inside the cooker is lower (28 is less than 30), the water will boil at a lower temperature and therefore it will take longer to cook the egg.
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