Why Do Some Coffee Cups Get Hot In The Microwave?

on September 22, 2022

Have you noticed that if you have ever reheated coffee or water in the microwave that the coffee cup warms up as well. However, there were certain cups that get scalding hot extremely quickly and seem to heat up much much faster. Is this really the case? And why does this happen?

All coffee cups will warm up in the microwave if you heat a liquid in them such as water due to the transfer of heat through conduction (direct contact). However, there are some cups that will get disproportionately hot to the point where you cannot touch them due to the presence of metal in the ceramic glaze.

The reason for this is that metal objects heat much faster than many other materials including traditional ceramics because they are able to absorb the energy rapidly creating currents which can transfer heat within the object quickly.

Is It Safe To Put Metal Objects Into A Microwave?

There is a persistent myth that the addition of any medals to a microwave will cause significant issues and cause a fire and therefore it is an extremely dangerous thing to do.

However, the reality is that many of the containers which are designed to go into microwaves such as preheated soups for one actually contain metal which is designed to help the food heat up more quickly. So to say that any metal object that goes into the microwave is going to cause a fire is actually quite incorrect.

Microwaves can cause metals to generate sparks which obviously are potentially dangerous if combined with flammable foods such as oils. However, creating the spark in a microwave required specific conditions.

There have been a number of videos created on the subject, the one that I found the most effective at showing how difficult it is to actually create sparks is provided below. This video showed that they are only a couple of situations such as putting a scrunched-up aluminium foil that would actually generate sparks.

Additional, the sparks that were generated did not do any long-term damage to the microwave but obviously, any situation where that is beginning to occur is likely to be potentially dangerous over time/

Is It Safe To Use Aluminium Foil In A Microwave?

Aluminium foil is generally not recommended to add to a microwave simply because it can shield the food from the microwaves preventing them from heating up. Aluminium that is scrunched up has the potential to create sparks as the energy can jump from one part of the aluminium to the other part. So generally it is best not to use aluminium foil in the microwave for those reasons.

How Does A Microwave Actually Heat Food?

Microwaves heat food in a distinctly different manner to conventional sources of heat such as hot plates. Microwaves are energy waves that are part of the electromagnetic spectrum which includes a whole heap of other energy waves that we utilize for different purposes, examples of this include radio waves, ultraviolet, infrared and X-rays.

These energy waves are magnetic in nature which is a critical property that allows food to be heated particularly materials with significant amounts of water in them. Water molecules have a negative end and a positive end that can act to some degree like a magnet which gives water many of its unique properties.

When a magnetic wave is passed through a material the vibrating waves cause the molecules to move which creates kinetic energy which we perceive as heat. Due to the magnetic nature of water, it tends to heat up faster than many other materials.

The other significant difference between how microwaves heat food is that they will heat them in the centre as well as on the outsides of the object which means that you can heat an object up much faster.

However, the distribution of these waves is not even so you can get extremely hot sections of food and other sections that are cool which is why all microwaves have a rotating plate in them to more evenly distribute the heat within the food.

Why Do The Walls Of The Microwave Not Get Hot?

You may have noticed, however, that in the box in a microwave that houses the food is in fact made of metal so you may be wondering why don’t the walls of the microwave get hot like some metal objects.

The reason for this is because the metals used are coated in a specific reflective coating which is designed to ensure that the microwaves bounce around the oven until they are absorbed by something within the oven such as your food.

However, if you don’t put anything into the microwave oven and allow it to run on an ongoing basis the energy within it can, over time, build up to the point where it can actually cause damage to the circuits within the microwave because the energy pushed into the box has nowhere to go.