Why can't you put metal in a microwave? - Aaron Slepkov
American engineer Percy Spencer developed
World War II RADAR technology
that helped detect Nazi airplanes—
but it would soon have other
surprising applications.
One day in 1945, Spencer was standing
near a RADAR instrument
called a magnetron,
a device that produced high-intensity
microwaves that could reflect off planes.
Suddenly, he noticed that the
candy bar in his pocket had melted.
He exposed other things to the magnetron
and, sure enough,
popcorn kernels popped,
and an egg—well— exploded
onto a colleague.
Soon after, the first microwave oven
became available,
operating using the very same technology.
So, how does it work?
All light energy travels in waves of
oscillating electric and magnetic fields.
These oscillations span a range
of frequencies
comprising the electromagnetic spectrum.
The higher the frequency,
the more energetic.
Gamma rays and X-rays
have the highest frequencies;
microwaves and radio waves,
the lowest.
Generally, light’s oscillating electric
field exerts forces on charged particles,
like the electrons in a molecule.
When light encounters polar molecules,
like water,
it can make them rotate,
as their positive and negative regions are
pushed and pulled in different directions.
The frequency the light is traveling at
also determines how it
interacts with matter.
Microwaves interact strongly with the
water molecules found in most foods.
Essentially, they make the molecules
jostle against each other,
creating frictional heat.
Household microwave ovens are
fitted with cavity magnetrons.
When you activate
a microwave oven,
a heated element within the magnetron
ejects electrons,
and a strong magnet forces them
to spiral outwards.
As they pass over the magnetron’s
metallic cavities,
the electrons induce
an oscillating charge,
generating a continuous stream
of electromagnetic microwaves.
A metal pipe directs the microwaves
into the main food compartment,
where they bounce off the metal walls
and penetrate a few centimeters
into the food inside.
When the microwaves encounter
polar molecules in the food, like water,
they make them vibrate
at high frequencies.
This can have interesting effects
depending on the food's composition.
Oil and sugar absorb fewer
microwaves than water,
so if you microwave them alone,
not much happens.
But when microwaves encounter
a marshmallow,
they heat the moisture trapped
within its gelatin-sugar matrix,
making the hot air expand
and the marshmallow puff.
Butter is essentially a suspension
of water droplets in fat.
When microwaved,
the water rapidly vaporizes,
making the butter melt quickly—
and sometimes, a bit violently.
So microwaves heat food molecules
mechanically, through friction—
but they don't alter them chemically.
Soup heated in the microwave
is molecularly indistinguishable
from soup heated using a stove or oven.
The term “microwave radiation”
can be alarming.
But in physics, radiation simply describes
any transfer of energy across a gap.
High frequency, ionizing radiation
may be harmful
because it can strip electrons
from molecules, including DNA.
However, microwaves aren’t energetic
enough to alter chemical bonds.
And microwave ovens are designed
to prevent leakage—
for safety and efficiency’s sake.
Nonetheless, to totally limit exposure,
experts recommend simply standing a
few feet away when a microwave oven is on.
Microwaving metal is dangerous,
though, right?
Well, it depends.
Metals are conductors,
meaning their electrons are loosely bound
to their atoms
and move freely in response
to electric fields.
Instead of absorbing microwave radiation,
the metal’s electrons concentrate
on the surface,
leading to high voltages at sharp edges,
corners, and small gaps.
This includes areas between the creases
on a sheet of aluminum foil,
the prongs of a fork,
or a metal object
and the microwave oven’s metal walls.
Sometimes, voltages get high enough
to strip electrons
from the surrounding air molecules.
This electrically charged gas, or plasma,
may then form lightning-like sparks
and grow as it absorbs more microwaves.
Once the oven is turned off,
the plasma dissipates.
But not all metal objects
spark in the microwave—
though they might make things
cook a little unevenly.
In fact, a lot of microwavable packaging
takes advantage of this,
using a thin metal coating
to crisp the food’s surface.
And overall, as long as it doesn't
approach the oven's walls,
leaving a metal spoon
in a microwaving bowl of soup
should be a pretty uneventful affair.
That’s just another neat benefit
of cooking with RADAR.
