Why do eggs explode when microwaved?

Why do eggs explode when microwaved but not when boiled?

There are two fields of science involved thermodynamics (heat transfer) and electromagnetics (radio waves).

Let's deal with the thermodynamics first. Heat transfer happens in three ways by conduction, convection and radiation.

Conduction happens when heat is transferred from one part of a substance to another because of a temperature gradient.

Radiation happens when electromagnetic waves excite molecules inside a substance, causing the temperature to rise.

Convection happens when hot fluid, gas or liquid, moves from one area to another taking heat with it. Convection may be natural, such as hot air rising or forced such as an airconditioner blowing air into a space.

Conduction happens in a solid when there is a temperature gradient between two points. i.e. the heat energy is gradually transferred from the hot point to the cooler point. If there is no outside source of energy the hotter surface gets cooler and the cooler surface gets hotter until they are at equal temperature.

If you apply a constant temperature to the hot surface (through an external heat source) the cooler surface gets hotter until it almost reaches the temperature of the hot surface. It will not exceed that temperature.

When you are boiling an egg, you are heating it by conduction. First you are applying heat to the water. They water then boils at a constant boiling temperature. That temperature is applied to the outside of the egg. The heat of the water is then conducted into the egg cooking it. The temperature inside the egg will never exceed the boiling point of water. You won't get steam inside the egg. It won't explode.

Remember when boiling you are transferring heat to the egg by conduction by maintaining its outside surface at the boiling temperature of water.

When you are microwaving an egg, you are transferring heat directly inside it by RADIATION. The microwaves are heating the water molecules directly inside the egg. Therefore it is possible to heat the molecules to a temperature ABOVE the boiling point of water. Remember the boiling point of water is dependent on pressure (it is 100°C at sea level atmospheric pressure).

Let us also look at the pressure cooker. The pressure cooker cooks food faster by trapping steam inside a confined space thus raising the pressure inside. That raises the boiling temperature of the water inside that confined space. If your pressure cooker does not have very strong walls, it will explode or crack because inside it there is more pressure than the atmosphere outside.

When you are microwaving an egg, you are heating it by radiation from inside. The eggshell acts as a pressure cooker but a rather flimsy one. It will hold some steam inside making the temperature inside rise slightly above the boiling temperature of water. Once the pressure rises high enough it will crack the eggshell. When it cracks because the water inside is slightly above the boiling point of water at atmospheric pressure, it will instantly evaporate into steam. That steam explodes the egg.

Also keep in mind that the egg-white is firmer and holds less water than the york when boiled. So the temperature may rise higher in the york before the egg-white builds enough pressure to crack the shell and itself.