Question

An empty plastic or glass dish being removed from a microwave oven can be cool to the touch, even when food on an adjoining dish is hot. How is this phenom- enon possible?

Answer 1

The coolness of an empty dish removed from a microwave is due to the absence of water molecules in the dish. Only polar molecules like water in food absorb microwave energy and heat up, while materials without them, like plastic or glass, stay relatively cool.

Step-by-step explanation:

The phenomenon of an empty plastic or glass dish being cool to the touch after being in a microwave, while food is hot, is an example of dielectric heating. Microwaves generate electromagnetic waves with a frequency around 2.45 GHz that are absorbed by polar molecules, such as water, within the food. These polar molecules have a slight negative charge at one end and a positive charge at the other. When these molecules try to align with the rapidly alternating electric field of the microwaves, they absorb energy which increases their thermal motion, thus heating the food. Glass and plastic do not contain such polar molecules, so they do not absorb the microwave energy and therefore remain cool.

Additionally, the microwaves reflect off the oven's walls to produce standing waves, and with features like rotating fans and turntables, they spread out the hot spots, ensuring even cooking. However, plates or dishes that do not contain water molecules won't heat up significantly, as they do not interact with the microwaves in the same way food does.

Answer 2

See explanation

Step-by-step explanation:

Microwaves work by flipping water molecules upside down, then right side up again, and so on at very high speeds, and the friction generates heat. Some glass or plastic isn't affected by the microwaves, so the molecules don't get flipped, and so no friction occurs and no heat is produced, leading to the dish not being heated up.