[Tony Bunn]

Tony Bunn  asked the Naked Scientists:
   
When heating soup in a soup bowl in a microwave oven the soup begins to heat rapidly on the perimeter of the bowl and remains luke warm in the centre.

I imagine this is a boundary effect caused by reflection of the microwaves from the bowl edges resulting in increased heating at the edges.

This effect will have been observed by almost everyone with a microwave oven and it would be interesting for you to provide a scientific reason for this.

Tony

What do you think?

[Soul Surfer]

I agree with your observations.  Heating a bowl of soup or even making porridge it is usually best to do it in two or three short bursd with physical stirring in between

There are several possibilities  Microwave heating tends to be non uniform and depends on the local match to the radiation this is why you have turntables and/or stirrers in microwave cookers.

It could be the reflection effect that you suggest

Liquid in bowls is usually shallower near the edge it may be a thermal capacity effect

It could also be a matching effect  (very similar to the reflection effect but not just an energy density thing more an energy coupling thing)

The same thing happens with defrosting solid lumps but this can be worse because if it is too violent the edges can be burnt while the centre stays frozen.  This is because the dielectric constant of ice is much lower than that of water and the energy couples much more effectively into water than ice

[Geezer]

I wonder if it's just because the bowl is a relatively good insulator compared with the rest of the soup.

[CZARCAR]

use microwaveable plasticware & it dont heat up anywhere near as much

[damocles]

Microwave radiation is light, with a wavelength of the order of a centimetre. It heats things up because the actual wavelength that is used corresponds to a strong absorption band of the water molecule. When a bulky item is placed in the cavity of a microwave heater, the interior -- that is, anything more than about a centimetre from the edge -- does not heat up because the radiation has already been absorbed by the material on the outside.

In effect what this means is that the outside gets very hot by absorbing the energy from the microwave radiation, while the inside can only be heated by conduction or convection from the material at the hot outside, and the small proportion of the radiation that can get through without being absorbed.

For a bulky liquid sample, like a bowl of soup, stopping and stirring certainly does increase the efficiency of a microwave cooker, and is a good thing to do.

[chris]

The wavelength of a kitchen microwave oven is actually more like 12 centimetres.

[Geezer]

Microwave radiation is light, with a wavelength of the order of a centimetre. It heats things up because the actual wavelength that is used corresponds to a strong absorption band of the water molecule. When a bulky item is placed in the cavity of a microwave heater, the interior -- that is, anything more than about a centimetre from the edge -- does not heat up because the radiation has already been absorbed by the material on the outside.

In effect what this means is that the outside gets very hot by absorbing the energy from the microwave radiation, while the inside can only be heated by conduction or convection from the material at the hot outside, and the small proportion of the radiation that can get through without being absorbed.

Yes, but if that's the case, why does the top surface not start to boil as quickly as it does at the edges?

[JP]

I wonder if it has to do with the way the standing waves are generated in the cavity.  I suspect most of them are directed horizontally, so you have a lot of waves bouncing left/right, but not so many bouncing up/down.

[Geezer]

I suspect there is significant evaporation from the surface which keeps the surface cool, but the liquid in contact with the surface of the bowl is under a certain amount of pressure, and that causes its temperature to rise more rapidly.

There is a similar effect when you heat a pan of water on a stove. The water at the bottom of the pan starts to form bubbles of steam long before the rest of the water starts to boil.