The Physics of Cooking: What's Actually Happening When You Fry an Egg

Your Kitchen Is a Physics Lab

I’m going to ruin cooking for you. Or maybe make it better — depends on how you feel about thermodynamics.

Because that’s what cooking is: thermodynamics. Every time you boil water, sear a steak, or bake bread, you’re running a heat transfer experiment. The three modes of heat transfer — conduction, convection, and radiation — are all happening simultaneously in most cooking scenarios. And the chemical transformations that turn raw ingredients into food are temperature-dependent reactions governed by the same Arrhenius equation that describes every other thermally activated process in nature.

None of this makes the food taste different, obviously. But once you see the physics, you can’t unsee it.

Conduction: Why Pans Matter

Put a pan on a hot burner. Heat flows from the burner into the pan by conduction — molecular vibrations passing energy from one atom to the next through direct contact. The rate of heat flow depends on the thermal conductivity of the pan material, and this is where things get practical.

Copper conducts heat about 25 times better than stainless steel. An aluminium pan falls somewhere in between — roughly 10 times better than steel. Cast iron? About 4 times better than steel, but much worse than aluminium or copper.

So why don’t we all cook with copper? Cost, mostly. And reactivity — copper reacts with acidic foods. That’s why good copper pans are lined with tin or stainless steel. The copper provides fast, even heat distribution; the lining provides a food-safe cooking surface.

Cast iron is interesting from a physics perspective. Its thermal conductivity is mediocre, but its heat capacity is enormous. A heavy cast iron skillet stores so much thermal energy that when you throw a cold steak on it, the temperature barely drops. A thin aluminium pan, despite conducting heat faster, stores less energy and cools down sharply when cold food hits it. For searing — where you need sustained high temperature — mass matters more than conductivity. Cast iron wins not because it conducts well, but because it’s a massive thermal reservoir.

This is just specific heat capacity and thermal mass at work. The physics predicts exactly what every experienced cook knows: heavy pans sear better.

Convection: What Happens in a Pot of Water

Boiling water on the stove is a masterclass in convection. Water at the bottom of the pot, heated by conduction from the burner, becomes less dense than the cooler water above it. It rises. The cooler water sinks to replace it, gets heated, and rises in turn. This sets up a convection current — a continuous loop of rising hot water and sinking cool water that distributes heat throughout the pot.

Before the water actually boils, these convection currents are doing most of the work. You can see them if you look carefully: wavering patterns near the bottom of the pot, like heat shimmer on a road.

When the water reaches 100 °C (at sea level), something dramatic happens. Water undergoes a phase transition — it vaporises. Bubbles of steam form at the bottom, rise, and burst at the surface. This is vigorous, turbulent convection, and it transfers heat much faster than the gentle currents that preceded it.

The temperature of boiling water doesn’t increase past 100 °C no matter how high you turn the burner. All the extra energy goes into converting liquid water to steam — that’s the latent heat of vaporisation, about 2,260 joules per gram. This is why a watched pot does eventually boil but never gets hotter than boiling. Turning the burner from medium to high after the water boils just makes steam faster. It doesn’t cook your pasta faster. The water temperature stays at 100 °C either way.

A lot of home cooks don’t realise this, actually. A gentle boil cooks at exactly the same temperature as a rolling boil. The only difference is energy waste and a steamy kitchen.

Radiation: The Grill and the Broiler

The third mode of heat transfer is radiation — electromagnetic waves carrying energy. Every object above absolute zero radiates energy, with the spectrum and intensity determined by its temperature (the Stefan-Boltzmann law and Planck’s law, if you want the details).

A charcoal grill at 300 °C radiates primarily in the infrared. The coals glow red because a small fraction of their thermal radiation is in the visible spectrum — the tail end of the Planck distribution extending into red wavelengths. At 600 °C, they glow brighter red. At 1,000 °C, they’d glow orange-yellow. The colour of the coals is literally a thermometer — blackbody radiation that Planck explained in 1900.

A broiler in an oven works almost entirely by radiation. The heating element at the top glows red-hot and bathes the food surface in infrared. Unlike conduction (which requires contact) or convection (which heats from below, typically), radiation comes from above and heats the top surface directly. That’s why broiling is so effective for browning — the intense radiant heat drives surface temperatures well above 150 °C, triggering the Maillard reaction.

The Maillard Reaction: Chemistry Meets Heat Transfer

Speaking of the Maillard reaction — this is where cooking goes from physics to chemistry, but the physics sets the conditions.

The Maillard reaction is not one reaction. It’s a cascade of hundreds of chemical reactions between amino acids and reducing sugars that begins around 140–165 °C. These reactions produce the brown colour and complex flavours of seared meat, toasted bread, roasted coffee, and dark beer. Without it, food would taste flat and pale.

Here’s the thing, though. The Maillard reaction requires surface temperatures above 140 °C. Water boils at 100 °C. So if there’s moisture on the food surface, the temperature is stuck at 100 °C — all the energy goes into evaporating water rather than raising the temperature. The surface can’t get hot enough for browning until the water is gone.

This is why you pat a steak dry before searing it. This is why crowding a pan with too many pieces of meat makes everything steam instead of brown — too much moisture, not enough surface heat. This is why deep-fried food browns beautifully — oil transfers heat so fast that surface moisture evaporates almost instantly, and the temperature rockets past 140 °C.

The physics of phase transitions — specifically, the latent heat barrier at 100 °C — is literally what determines whether your dinner is golden-brown or pale and soggy. Every chef knows the rule “dry the surface, get the pan hot.” Now you know the thermodynamics behind it.

The Microwave: Electromagnetic Cooking

A microwave oven doesn’t use conduction, convection, or thermal radiation — at least not directly. It uses electromagnetic radiation at a frequency of 2.45 GHz, which corresponds to a wavelength of about 12.2 cm.

Water molecules are polar — they have a positive end and a negative end. In an oscillating electric field, they try to align with the field direction, flipping back and forth 2.45 billion times per second. This molecular rotation generates friction between neighbouring molecules, which converts electromagnetic energy into thermal energy. The food heats up from the inside.

Well, sort of from the inside. Microwaves actually penetrate only about 1–2 cm into most foods before being absorbed. The interior of a thick piece of food is heated mainly by conduction from the outer layers — which is why microwaved food often has a hot exterior and a cold centre (the reverse of oven-cooked food, where the outside heats first by convection and radiation).

There’s a persistent myth that microwaves “heat from the inside out.” For thin foods, this is roughly true. For anything thicker than a couple of centimetres, it’s not. The physics of electromagnetic absorption depth contradicts the myth, and anyone who’s bitten into a microwaved burrito with a frozen centre can confirm it experimentally.

Altitude and Boiling Points

One last thing that ties cooking back to fundamental physics. At higher altitudes, atmospheric pressure is lower, and water boils at a lower temperature. In Denver (altitude ~1,600 m), water boils at about 95 °C. In La Paz, Bolivia (~3,640 m), it boils at about 87 °C. On top of Everest, water boils at roughly 70 °C.

This matters. Cooking times increase because the water temperature is lower. Baking recipes need adjustment because leavening gases expand more readily at lower pressure (bread rises faster and can collapse). Candy making — which depends critically on the boiling point of sugar solutions — requires recalibration at altitude.

It’s all the same physics: the Clausius-Clapeyron relation, which connects pressure, temperature, and phase equilibrium. The fact that you have to adjust a cake recipe in Denver and a mountaineering expedition can’t make proper tea on Everest — those are the same equation, applied to different kitchens.

Next Time You Cook

I won’t pretend that knowing the thermal conductivity of copper will make you a better cook. Technique, practice, and tasting your food will do that. But understanding the physics — why the heavy pan sears better, why the dry surface browns faster, why the rolling boil doesn’t cook faster — gives you a framework. Cooking stops being a set of arbitrary rules and becomes a set of logical consequences. Follow the heat, and the recipe starts to make sense.