How a cup of water could save your microwave

How a cup of water could save your microwave

Mizu Nishikawa-Toomey is Pi’s new Scientific Agony Aunt – ask anything!

Mizu is a scientific super-star and will be answering  your everyday science queries, dilemmas and downright silly questions in her new regular feature for Pi Science. Have one of your own? Send it to

Why can’t I put a spoon in the microwave?

First, we first need to understand how EM (electromagnetic waves) waves work. We encounter EM waves in everyday life in visible light, x-rays, ultraviolet, infrared and microwaves.

Richard Hammond explains the electromagnetic spectrum with … a piano

As well as being able to heat up your veggie lasagna, EM waves have the ability to build up a current in materials with a free flow of charge such as metals, which have a sea of free-roaming charged electrons. When an EM wave – such as a microwave – strikes an electron, a current flows through the metal and the electrons are accelerated to one end of the metal. This is where the problems start. Large charge build-up tend to discharge onto their closest uncharged neighbour – and BANG! Just as when lightning strikes the ground after a cloud has been discharged of its electrons, there is a spark that might damage the microwave.

However, if the metallic substance has a shape which does not permit the build up of charge, such as a circle, there will be no spark. The electrons will not cluster to one end of the shape, but keep circling around it. That is why we can put a bag of popcorn in the microwave even though it has a thin layer of aluminum coating on the inside. As the aluminum is roughly in the shape of a sphere there is no charge build up.

You can also put a spoon in the microwave if it is submerged in a cup of water. The electrons will discharge in to the water, rather then building up on the end of the spoon. Nevertheless, you can’t be 100% sure which shape is safe so do not try this at home! Another thing you shouldn’t try at home is to dissolve the skin of a grapefruit with acid…

How do they peel the skin off tinned grapefruit?

image credit: raeky

Global production of grapefruit is set to rise to 6.2 metric tonnes in 2015, 70 to 80% of which will come from the U.S.

First, the workers hand-peel the hard outer skin of the grapefruit – the exocarp – and immerse the separated segments in a diluted bath of hydrochloric acid. The inner skin –named the endocarp, unsurprisingly – dissolves in to the acid and melts away from the surface of the grapefruit.

The acid submerged grapefruit is then doused in caustic soda, an alkali. This neutralizes the acid present on the grapefruit flesh. The grapefruit segments are then put into tins filled with a syrupy solution.

Tinned grapefruit never gives you the same citrusy kick to the back of your tongue that fresh grapefruit does. The syrup in the tin is only partially to blame for sweetening and removing the sourness from the fruit. Caustic soda is present in slight excess to ensure all the hydrochloric acid is neutralised. Once all of the hydrochloric acid is used up, it starts to neutralise the naturally occurring acids in grapefruits such as pantothenic acid, making the grapefruit less sour.

Pantothenic acid is an essential acid in all forms of life, which helps to support an enzyme called Coenzyme A. This molecule plays a key role in energy metabolism, metabolising fats to use as ATP, the energy currency of the body. As well as fresh grapefruit, cauliflower and broccoli are great sources of pantothenic acid if you want to boost your energy levels to keep you going through these cold winter months.

Email or tweet @pi_media if you have any science questions of your own that you want answering (and can’t be bothered to google to google it for yourself – its much easier asking us, trust.) 

Featured image credit: Vinod Shankar via flickr


Leave a Reply

Your email address will not be published.