Why does it take longer to cook two things in a microwave?

11 November 2007

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Question

Why is it that if you want to cook two items in the microwave, you’ve got to cook them for twice as long when you don’t have to do the same with a normal cooker? And do microwaves kill bacteria in food?

Answer

Because air is a good insulator the thing that limits how fast a normal oven can heat something up is basically how fast the energy is transferred from the hot air to your food. The oven is powerful enough to keep the air hot, so adding another cake will not slow it down much. A microwave on the other hand is very efficient at transferring energy from the magnetron (the thing which produces the microwaves) to the food. So the thing that limits the amount of energy getting to your food every second is basically how much energy you are converting to microwaves. If you have twice as much food the same amount of energy will be transferred every second but it will be shared out over twice as much food, so it will heat up slower.

In terms of microwaves killing bacteria, if you don't heat food to the right extent there's a danger of getting hotspots and colds spots. That's why there's a turntable in a microwave. Without a hot enough area in the food any bugs could persist so you could end up getting food poisoning. Microwaves are just radio waves and the reason they make food cook is that it absorbs the energy of the microwave, the molecules vibrate and this raises the temperature. They're not doing anything special or zapping bugs by gamma irradiation or blasting DNA. They're just heating them up. If you don't make the food hot enough then you're not going to kill anything. It's all down to heat and nothing else.

Comments

What is the relationship of energy transfer to mass via the MW.
If one frozen hash brown heats to a set temp, say 140dF, after 1 min.
How long does it take to heat Two hash brown to 140dF.
That is really the concern.

who made this article

This answer was provided by the Naked Scientists team.

the answer is just very poorly formed, I was banned from school at age 13 and so missed out on English writing yet my self taught efforts are far more refined than this blunder bus of an author. I recall from junior school to avoid using 'because' and then there is the part about an area being hot enough to kill the bacteria, surely the whole item should be cooked to the appropriate temperature. Sorry but the person answering sounds illiterate

And the person making the above comment sounds like a pedantic jerk.

I bet you're a real treasure to work with. Your co-workers must love you, assuming anyone would actually give you a job.

The answer provided to this question is accurate and communicates well the physics, which is what was asked.

Grow up.

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