I’m heating
up my food using a microwave. I’m heating some sweet potatoes and I figured
since I mash it with milk later, I can immerse them in milk during the heating.
But, unlike the sweet potato alone which heats up really fast, the milk + sweet
potato doesn’t heat up that fast… That got me thinking… Why?
Here’s my
hypothesis. The increase in temperature is associated with increase in the
AVERAGE thermal energy. In thermodynamics, this is an intensive property (does
not scale with size). As opposed to the average, think about heat which is a
measure of thermal energy. As the total amount increases, assuming the same
temperature, the total heat increases. This is an extensive propery.
Now, let’s
think about how much heat we need to put in to change the temperature. For
example, we know (from experience) that metal heats up really fast. Metal has
low heat capacity compared to say water. (We shouldn’t confuse between heat
capacity and thermal conductivity. Metals have high thermal conductivity which
means the heat propagates faster. It doesn’t say how much energy is needed to
raise its temperature.)
Now, I
think the answer is somewhat simple… The sweet potato has lower heat capacity
and thus needs less energy to heat it. Assuming that the heat input is
constant, this means that the temperature (how I measure “hotness”) is higher
faster. Adding milk is like adding more stuff but also stuff with high heat
capacity… you need more energy to change its temperature by the same amount.
But then
one question remains… is the heat input really constant? I know that the
microwave is operating at the same power (energy per time), but it does not
tell me that the energy is used up as heat… Microwave works by injecting
microwave radiation into the system which should absorb this radiation.
Molecularly, the species that absorbs the radiation enters a higher energy
vibrational mode which is translated as heat. But if you have more radiation
absorbing species, conversion to thermal energy might be greater. I don’t think
this plays a big role in my milk + sweet potato case… but I do wonder for other
food matter… I know that water tends to absorb this radiation or at least a lot of posts online talk about how water absorbs this energy… but I’ll
need to think more about what happens to almost completely dry materials.
