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How do Microwaves Work?

A transcript of Aaron Slepkov's "Why can't you put metal in a microwave?"

By Stefan SchoemanPublished 2 months ago 4 min read
Image by: Ferdinandona

During World War II, Percy Spencer, an American engineer, played a crucial role in the development of RADAR technology, which proved instrumental in detecting Nazi aeroplanes. However, the significance of his work extended beyond military applications. In the year 1945, while standing in proximity to a RADAR instrument known as a magnetron—a device designed to generate high-intensity microwaves capable of reflecting off aircraft—Spencer would uncover unexpected and diverse applications for this technology.

In a sudden revelation, he observed the melting of the candy bar in his pocket. Intrigued, he subjected various items to the magnetron's influence, confirming that popcorn kernels popped, and, unexpectedly, an egg explosively met its fate, splattering onto a colleague. This serendipitous experimentation led to the development of the inaugural microwave oven, utilising the identical technology that had initially been designed for military RADAR purposes.

So, how do they function?

Light energy propagates through oscillating electric and magnetic fields in the form of waves. These oscillations encompass a spectrum of frequencies known as the electromagnetic spectrum. The energy of the light increases with higher frequencies, with gamma rays and X-rays having the highest frequencies, and microwaves and radio waves having the lowest.

Primarily, the oscillating electric field of light exerts forces on charged particles, such as the electrons within a molecule. When light encounters polar molecules, like water, it induces rotational movement as the positive and negative regions experience forces in different directions. The frequency of light also dictates its interaction with matter.

Microwaves, for instance, strongly interact with water molecules present in most foods. This interaction causes the molecules to jostle against each other, generating frictional heat. Household microwave ovens incorporate cavity magnetrons, where activating the oven prompts a heated element within the magnetron to release electrons. A strong magnet compels these electrons to spiral outward, inducing an oscillating charge as they pass over the magnetron's metallic cavities, producing a continuous stream of electromagnetic microwaves.

Directed by a metal pipe, these microwaves enter the main food compartment, bouncing off metal walls and penetrating a few centimetres into the food. When microwaves encounter polar molecules, like water in the food, they induce high-frequency vibrations. The effects vary based on the food's composition; for instance, oil and sugar absorb fewer microwaves, resulting in minimal changes when microwaved alone. However, in the case of a marshmallow, microwaves heat the moisture within its gelatin-sugar matrix, causing the marshmallow to puff. Butter, being a mixture of water droplets in fat, rapidly vaporizes water when microwaved, leading to quick melting—sometimes, a bit explosively.

Indeed, microwaves heat food molecules through mechanical means, specifically friction, without inducing any chemical alterations. Soup heated in a microwave remains molecularly identical to soup heated using a conventional stove or oven. Despite the potentially alarming term "microwave radiation," it's important to note that in physics, radiation simply denotes any transfer of energy across a gap. The use of microwaves in heating does not involve the introduction of harmful chemical changes to the molecular composition of the food.

High-frequency ionising radiation poses potential harm as it can strip electrons from molecules, including DNA, leading to adverse effects. However, it's crucial to recognize that microwaves, with their lower energy levels, lack the capacity to alter chemical bonds. Microwave ovens are meticulously designed to prevent any leakage, prioritising both safety and efficiency. Nevertheless, to further minimise exposure, experts recommend maintaining a distance of a few feet when a microwave oven is in operation. This precautionary measure ensures an additional layer of safety despite the inherent design features aimed at preventing any potential harm.

Isn't microwaving metal bad?

The interaction between metals and microwaves depends on the conductive nature of metals. Being conductors, metals have electrons that are loosely bound to their atoms, allowing them to move freely in response to electric fields. Instead of absorbing microwave radiation, the electrons in metals concentrate on the surface, resulting in high voltages at sharp edges, corners, and small gaps. This phenomenon is notable in areas such as the creases of aluminium foil, the prongs of a fork, or the interface between a metal object and the microwave oven's metal walls.

Under certain conditions, the voltages generated can become high enough to strip electrons from the surrounding air molecules, creating an electrically charged gas or plasma. This plasma may manifest as lightning-like sparks and continue to grow as it absorbs more microwaves. However, once the microwave oven is turned off, the plasma dissipates. It's worth noting that not all metal objects spark in the microwave, though they may cause uneven cooking.

Interestingly, many microwavable packaging materials leverage this phenomenon by incorporating a thin metal coating to crisp the food's surface during microwaving. Generally, leaving a metal spoon in a bowl of soup while microwaving, as long as it doesn't approach the oven's walls, should be uneventful. This aspect showcases an interesting and beneficial aspect of cooking with microwave technology, which has roots in RADAR development.

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