BBC Focus Magazine

[Croatguy]

Why is it that when you place more than one food item in the microwave the longer it takes to heat up? Is this because the other food items block the intensity of some microwaves when it hits the food?

[M Paul Lloyd]

Its a question of the absorption rate of the microwaves for the total mass to be heated Croatguy.

You see the microwaves bounce around inside the 'oven' until they encounter the food you wish to cook which is one reason why it is unwise to run a microwave oven with nothing in it as the build up of said radiation will begin to heat the inside walls to quite dangerous levels.

So if you double the amount of food it will absorb twice as many micrwaves so it will take twice as long to heat that amount of food to the same point as just one portion.

Hope that helps?

[Doonhamer]

Also though you have to be careful if you put items in one after the other because the magnetron in domestic ovens is less efficient the longer it runs. That is why cafe's are not supposed to use domestic microwave ovens as they're only rated to a certain amount of running time per hour. They often get round this by having 2 and alternating between them. Commercial microwaves are much more expensive and often have multiple magnetrons rather than a single one with a turntable, and they can be used almost constantly.

[Croatguy]

So if you double the amount of food it will absorb twice as many micrwaves so it will take twice as long to heat that amount of food to the same point as just one portion.

Hope that helps?

Yeah, the answer I was looking for. Are there any microwaves that exist then that emit enough microwaves where absorption amount is unaffected, regardless of the amount of food items inside it?

[M Paul Lloyd]

None that I am aware of Croatguy, unless of course you went for a professional set-up as Mr.A describes whereby doubling the magnatron output would mean that twice the mass could be heated in the same time as a single portion, but they are very much more expensive than a standard microwave oven.

[Jamie]

One of our microwaves at work had it's cord cut off recently. There was a sticker on it telling us it emitted radiation. I thought that's how it worked!

[M Paul Lloyd]

You'll probably find that the door seal is not working properly Jamie, I don't wish to cast aspersions on your workplace hygiene but it could probably do with a good clean?

Either way mocrowave ovens can sometimes 'leak' although you have to be damned close to them for it to have any harmful effects. Personally I tend not to stand right next to them when they are operating or risk watching food going around inside them too closely just in case, even with new ones.